PIGMENTS, PAINT
AND
PAINTING

PIGMENTS, PAINT
AND
PAINTING

A PRACTICAL BOOK FOR PRACTICAL MEN
BY
GEORGE TERRY

London
E. & F. N. SPON, 125 STRAND
New York
SPON & CHAMBERLAIN, 12 CORTLANDT STREET
1893

INTRODUCTION.

In days gone by, the painter who served the usual term of apprenticeship was deemed to have done all that was required to qualify him for his trade. He may have learned little or much, but he had “served his time,” and that was all that was expected of him. So far as it went, the training was good, because it was nothing if not practical, and practice is an essential element of skill. But nowadays such a training can only be considered partial; mere practice, without any scientific knowledge of the principles which underlie it, is but half a qualification for the workman who aims at being really a master of his trade.

When competition was unknown, and the low prices of raw material offered no inducement for passing off inferior or fraudulent substitutes, there was less need for a high degree of knowledge. But under modern conditions, the painter who is unable to gauge the qualities of the materials he uses, and who is ignorant of the rules which govern those qualities, and of the principles which determine the use of this and the rejection of that article, cannot long survive in the struggle for supremacy or even livelihood.

Hence the need for a handbook such as this volume aims at being. Granted that our technical schools and colleges are affording a liberal and invaluable education to the workman who will avail himself of the opportunities given him, still a man does not remain for ever at school, and he needs a guide-book, handy of reference and accessible in price, to refresh his memory and supplement the information gained in the class-room and workshop.

To fulfil this useful purpose is the aim and object of this unpretending volume.

CONTENTS.

ILLUSTRATIONS.

CHAPTER I.
PRELIMINARY.

Colour.—The term “colour” is inappropriately given by common usage to material substances which convey a sense of colour to the human eye, but is properly restricted to that sense itself. The material colour should be called “pigment” or “dyestuff” in the raw state, and paint when compounded with other substances for application in the form of a coating.

The sense of colour is due to light. In the absence of light there is no colour, only blackness; and black is really no colour, but an absence of colour. Very many conditions combine to cause different colour sensations, some of which are understood, while others we are not able to explain.

For instance, take the action of heat upon a solution of chloride of cobalt. As soon as the liquid becomes warm, the pink colour disappears and gives place to blue; but on pouring water into it, the blue vanishes and the pink reappears. Again, on heating the blue crystals of sulphate of copper they become white, but the blue colour comes back when water is added, and the solution assumes a deeper tint as it dissolves more of the white powder.

If all the rays are cut off from an electric light except those which are in and beyond the violet, and a flask containing a solution of sulphate of quinine is held in that portion of the spectrum, it will become luminous. The same thing will occur even more strikingly on placing a piece of uranium glass in the ultra-violet rays. The explanation of this phenomenon is that beyond those rays which give light there are others which do not give light, i. e. which do not cause us to experience the sensation of light; the reason being that their vibrations are too rapid. But when certain other substances, such as sulphate of quinine, or a thin slip of uranium glass, are placed in the path of the rays, this rapid motion is arrested and modified, and these rays, which in themselves are not luminous, are reflected back to our eyes as luminous rays. The rapidity of the vibrations being moderated, our retinas become sensible to them as rays of blue light.

Colour does not depend only upon chemical composition nor solely upon the aggregation of the particles, but upon these and other things besides not yet explained. All matter is in a state of motion. If you heat a substance you communicate an increased activity of motion to the particles of which it consists. When certain coloured rays of light are falling upon a substance, these coloured rays of light have a motion peculiar to themselves. It may be that the degree of motion in that substance, either existing in it naturally without heating, or communicated to it by artificial heating, is such that these rays of light are precisely those which that substance is not capable of sending back to our eyes. They are then absorbed or destroyed in some way, by the particular state of that substance upon which they fall; and those rays which the substance is capable of reflecting back are mainly sent back to our eyes. Certain colours, such as blue, yellow, and green, absorb certain rays more or less perfectly, and reflect back in the main blue, yellow, and green to our eyes. Hence it is incumbent on those who are studying colour, and who are interested in the purity and permanency of colour, to comprehend at least the principles of that science of light which tells of the action of light upon various bodies that are used as pigments in painting.

If we put together two substances one of which destroys or modifies the chemical condition or state of the other, then certainly one of those substances, and very probably both, will lose the colour which it had before it came into contact with the other. It is therefore most important that all engaged in the preparation and use of colours should make a study of this science of light. Of almost equal value is a study of the science of heat. We have seen what heat can do in changing the conditions of a substance. To give another instance. The black sulphide of mercury, after sublimation by heat, exhibits properties, imparted to it by the heat, which it did not possess before, i. e. it can, by trituration, be brought to display a red colour.

On showing the spectrum on a screen, if some solution of soda or other sodium salt be held in the course of the light, almost all the coloured rays but one will be cut off, and a little band is seen in the yellow part of the spectrum. This is because the sodium flame is almost “monochromatic,” or single-lined: it cuts off all the colours but the yellow. Again, if metallic thallium is held in the flame, the only band remaining in the spectrum will be the green; and if a lithium salt, the only surviving colour will be red.

Pigments.—The term “pigments” is applied to those colouring matters which are mixed in a powdery form with oil or other vehicle for the purpose of painting. They differ in this respect from the dyestuffs, which are always employed in solution. A very large proportion of the pigments in common use are derived from the mineral kingdom, the most notable exceptions being found in the blacks and lakes. All pigments are required to possess “body,” or density and opacity; to be insoluble in water and most other solvents, except the stronger mineral acids; and to be inert, or incapable of exercising chemical or other influence on each other or on the vehicle or drier with which they are mixed prior to use. They may be conveniently classified according to their colours in the first place, reserving the consideration of their preparation for use for a later chapter. The chief classes are Blacks, Blues, Browns, Greens, Reds, Whites, and Yellows.

CHAPTER II.
BLACKS.

All the black pigments in use owe their colour to carbon, and all are produced by artificial means, no natural form of carbon possessing the requisite qualities.

Several manufactured carbonaceous substances are known in commerce under the generic name of “Blacks.” The most important of these are animal-black, bone-black, Frankfort-black, ivory-black, and lamp-black. They are usually obtained by carbonising organic matter, particularly bones, in closed vessels or crucibles, or by collecting the soot formed by the combustion of oily, resinous, and bituminous substances. Other blacks than those enumerated are manufactured, but only on so small a scale as to be of no commercial importance.

Carbon, lamp, and vegetable blacks consist almost entirely of carbon, containing usually from 98 to 99½ per cent. of that substance, the residue consisting of a little ash, water, and occasionally unburnt oil. Bone and ivory blacks, on the other hand, are chiefly composed of mineral matter, which may amount to 65 or 75 per cent. and is mainly represented by phosphate of lime. Their actual colouring matter, the carbon, only constitutes 15 to 30 per cent. of the mass. The balance is water and unburnt animal tissue. Blacks prepared from animal matters other than bone and ivory carry 40 to 80 per cent. of carbon, and their mineral matter is generally in the form of carbonates of lime and of the alkalies.

The principal impurity to be watchful of in the vegetable and lamp blacks is a small quantity of oily matter which may seriously interfere with their drying qualities. They should leave very little ash after being burned in a crucible. Bone and ivory blacks are sometimes valued as much for their mineral matter as for their colouring matter. The proportion of this mineral matter is ascertained by heating a certain weight of the black to red heat in a crucible till every trace of black has disappeared, and then weighing the residue. The residue should next be boiled in strong hydrochloric acid till it is dissolved; if there is any which will not dissolve it is most probably barytes, which has been added as an adulterant and to make the black weigh heavy. When the solution is complete, the addition of ammonia will throw down a precipitate of phosphate of lime, which should equal 60 to 70 per cent. of the original weight of mineral matter. If much less than this, it is likely that whiting or gypsum has been mixed with the pigment. As carbon is not acted upon by acids or alkalies, it follows that all pure carbon blacks are in themselves perfectly stable and permanent pigments, and that they exert no influence on other pigments with which they may be mixed.

Animal-black.—This substance is almost identical with bone-black, but is generally in a more finely divided state. Any animal refuse matter may be used in its preparation, such as albumen, gelatine, horn shavings, &c. These are subjected to dry distillation in an earthenware retort. An inflammable gas is given off, together with much oily matter, ammonia, and water, while a black carbonaceous mass is left behind. This is washed with water and powdered in a mill, the product being animal-black. It is largely used in the manufacture of paint, printing ink, and blacking.

Bone-black.—When bones are heated in a retort or crucible, the organic constituents are decomposed and carbonised. A mixture of combustible gases is given off; some of these do not condense on cooling, others condense in the form of a heavy oil, called bone-oil. Also much water containing tarry water and ammoniacal salts in solution passes over. The residue in the retort or crucible consists of finely divided carbon, in intimate mixture with the inorganic constituents of the bones: this mixture constitutes ordinary bone-black, or animal charcoal, as it is sometimes called. The inorganic portion may, if required, be removed by washing the residue in dilute hydrochloric acid.

The process, as worked on the large scale, is carried on in different ways, according as it is desired to collect the volatile condensable portion of the distillate, or to allow it to escape. In the latter case, when it is required to obtain only bone-black, the apparatus employed is of a very simple nature, and the amount of fuel needed is comparatively small. The carbonisation is effected in fire-clay crucibles, 16 in. high and 12 in. diameter. These are to be preferred to crucibles made of iron, which were much used at one time, since they do not lose their round form when subjected to a high temperature; in consequence of this, they fit more closely together in the furnace, less air can penetrate, and therefore less of the charcoal is consumed by oxidation. The furnace is an ordinary flat hearth, having a superficial area of about 40 square yards, and is covered in with a flat arch, all of brickwork. The fireplace is situate in the middle of the hearth; the crucibles are introduced through doors in the front, which are bricked up when the furnace is filled; each furnace holds eighteen crucibles. The crucibles, filled with the coarsely broken bones, are covered with a lid luted on with clay. To economise fuel, the furnaces should be in a row, and placed back to back.

The arrangement of the furnace and pots is shown in Figs. [1] and [2]. A is the fireplace; B, the crucibles, eighteen in number, spread over the floor of the furnace in a single layer; c, d, e, and f are the flues for conducting away the heated gases arising from the calcination of the bones, as well as the waste heat itself; the last portion of the flue is fitted with a damper g. The furnaces are intended to be built in fours, back to back, the waste heat serving in a great measure to conduct the operation of the revivifying apparatus placed in the centre of the group, and marked C.

Figs. 1 and 2.—Bone-black Furnace.

When the furnace is filled and the doors are bricked up, the heat is slowly raised to redness, at which point it is kept for six or eight hours. The combustible gases are evolved and consumed in the furnace as the bones begin to decompose, and by this means so much heat is produced that only a small quantity of fuel is needed to maintain the required temperature. When the carbonisation is complete, the doors are taken down and the crucibles are removed to cool, their places being immediately filled with fresh ones. The heat must be kept as uniform as possible throughout the process: if it be not sufficiently high, the bone-black will contain a portion of undecomposed organic matter, which renders it quite unfit for use; if, on the other hand, the temperature be raised too high, the bone-black will become dense and compact, whereby its efficacy as a decoloriser is much reduced. When the charcoal in the crucible has become perfectly cool, it is removed and crushed. When required for decolorising or deodorising purposes, it is only roughly broken up into small lumps, in which form it is most readily applicable. The crushing is effected by means of two grooved cylinders, consisting of toothed discs, alternately 10 and 12 in. in diameter. These are so placed that the 10-in. discs of one cylinder are opposite the 12-in. discs of the other, and thus, in revolving, the carbonised bones are crushed to fragments between them, but are not reduced to powder. They are passed successively through six of these mills, the cylinders of each couple being nearer to each other than the last. Finally the crushed bones are carefully sieved; the powder is placed apart from the lumps, again passed through finer sieves, and sorted out into different sizes.

A furnace such as that described above will carbonise four charges of bones in one day, each charge being more than half a ton in weight. With careful work, the bones will yield 60 per cent. of bone-black, or more than one ton daily.

If it be required to condense the volatile gaseous products of the carbonisation, this process is conducted in retorts similar to those used in the manufacture of acetic acid from wood: these are so arranged that the whole of the gaseous products are condensed and collected. The aqueous portion of the distillate is usually evaporated down to obtain salts of ammonia; the uncondensable gases may be employed for illuminating purposes. The manufacture of bone-black is usually carried on in the neighbourhood of large towns, where a good supply of bones may be readily obtained.

Ordinary bone-black has about the following composition: Phosphate and carbonate of lime, and sulphide or oxide of iron, 88 parts; charcoal, containing a small quantity of nitrogenous matter, 10 parts; silicated carbide of iron, 2 parts. The decolorising properties of bone-black are due solely to the presence of the charcoal.

When intended for use as a deodoriser or decoloriser, bone-black should be kept carefully excluded from the air, for by exposure it loses this power to a great extent, and becomes almost inert. That which has been freshly burned is therefore best for these purposes.

The cost of production of bone-black may be calculated as follows:—

Bone-black never has the depth or brilliancy of lamp-black, but it mixes well with either water or oil, and though a slow drier as an oil paint, is permanent and not high priced.

Frankfort-black or Drop-black.—This is a black powder obtained from dried vine-twigs carbonised to a full black and then ground very fine. On a large scale it is prepared from a mixture of vine-twigs, wine-lees, peach-stones, bone-shavings, and ivory refuse. It varies in shade according as the animal or vegetable charcoal is in excess; when the latter predominates, the powder is of a bluish colour; but when there is an excess of animal charcoal, it has a brownish tinge. It is customary to wash the powder well when first made, in order to remove any soluble inorganic impurities. The finest Frankfort-black is probably the soot obtained from the combustion of the materials mentioned above. It makes an excellent pigment, and is extensively used by copperplate engravers in the preparation of their ink. Drop-black is simply Frankfort-black ground exceedingly fine, mixed with a little glue water, and dried in pear-shaped drops for sale.

Ivory-black.—Ivory-black is a beautiful black pigment prepared by carbonising waste fragments and turnings of ivory. These are exposed to a red heat for some hours in crucibles, great care being taken to avoid overheating or burning. When quite cold, the crucibles are opened, and the contents are pulverised, the richest coloured fragments being kept apart for the best quality. The powder is then levigated on a porphyry slab, washed well with hot water on a filter, and dried in an oven at a temperature not exceeding 212° F. The product is of a very beautiful velvety black colour, superior even to that obtained from peach-kernels, and quite free from the reddish tinge which so often characterises bone-black. Ivory-black, like Frankfort-black, is employed by copperplate printers in the preparation of their ink. Mixed with white lead, it affords a rich pearl-grey pigment.

Lamp-black.—Lamp-black is an exceedingly light, dull-black powder, formed by the imperfect combustion of oils, fats, resins, &c. It may be prepared on a small scale by suspending a small tin-plate funnel over the flame of a lamp fed with oil, tallow, or crude naphtha, the wick being so arranged that it shall burn with a large and smoky flame. Dense masses of this light carbonaceous matter gradually collect in the funnel, and may be removed from time to time. The funnel should be furnished with a metal tube to convey the gases away from the room, but no solder must be used in making the connections.

Figs. 3 and 4.—Apparatus for making Lamp-black.

An especially fine quality of lamp-black is obtained from bone-oil, deprived of the ammonia with which it is always contaminated. It is manufactured on a commercial scale by means of the apparatus shown in Figs. [3] and [4]. The oil is contained in the lamp A and kept at a constant level by means of the globular vessel B, which is also filled with oil and inverted over A. The oil flows from the lamp into the tube C, which is bent upwards at the farther extremity on a level with the oil in the lamp. A cotton wick is supplied to the bent end of the tube, as well as a little spout D, for conducting away any oil that may overflow into the receptacle E placed beneath. A conical hood a surrounds the flame of the lamp and terminates in a tube b, through which are conveyed the sooty products of the combustion of the oil into the wide lateral tube c, arranged to accommodate the smoke from about a dozen such lamps placed at intervals of about 6 feet, as indicated in the figures. The effect of this wide tube c is not only to cool the smoke, but also to collect the water and other liquids condensed. The smoke and vapours pass hence into d, the first of a series of sacks made of closely woven linen, about 10 or 12 feet long and 3 feet in diameter, closed at the bottom with a trap or slide e, and formed at the upper and lower ends of sheet-copper tubing made funnel-shaped. The upper one of these is prolonged into an additional pipe f, by means of which the smoke arrives at the second sack g in the series, thence finding its way to the third, and so on till the last sack of the row is reached. In connection with the last sack of each row is placed a horizontal flue F, in which are arranged frames covered with wire gauze and mounted on hinges. Their purpose is to retain the small remaining portions of lamp-black passing out with the smoke from the sacks. The meshes of the gauze are constantly getting filled up with soot, which necessitates a periodical checking of the draught for its removal. This is done by means of the rod G, which, when raised and allowed to fall suddenly, jerks the accumulated mass off the gauze. The current of air passing through the entire apparatus can be regulated by a damper placed at the entrance to the chimney in which the flue F embouches. At regular intervals, the mouthpieces in the lower ends of the sacks are removed, and their contents are shaken out separately and collected according to their various qualities. That gathered from the first sack in each row should always be kept apart from the remainder, as it is much contaminated by the presence of resinous and tarry matters.

Fig. 5.—Apparatus for making Lamp-black.

The old-fashioned method of preparing lamp-black from the incomplete combustion of gas tar is conducted in an apparatus resembling that shown in [Fig. 5]. The furnace a, lined with fire-brick, contains a kettle b, and is surmounted by a large thick cast-iron hood c, communicating with a stone or brick condensing chamber, divided by means of perforated partition walls into three unequal sized compartments d, e, f, wherein the black is deposited. A chimney g delivers uncondensed vapours into the atmosphere. In working, the furnace is first brought to a red heat, then the kettle b, charged with tar, is introduced. As a charge is finished, more tar is added, with occasional stirring, till the kettle becomes inconveniently full of residue, when it is withdrawn and a fresh one replaces it. The residue is chipped out and used as fuel. The black is removed weekly through the door h. It is of good quality and colour so long as the combustion is conducted with a minimum of air, admission of which is controlled at the furnace. The yield is about 25 per cent. of the weight of the tar; and one furnace should treat a ton of tar in a week. One workman can manage several furnaces.

An improved process has been introduced by Martin and Grafton for the preparation of lamp-black from coal-tar, which affords a very good product. The coal-tar is first stirred up energetically with lime-water in any convenient vessel, after which the mixture is allowed to stand until the coal-tar has subsided to the bottom, when the lime-water is drawn off. The tar is then well washed by decantation with hot water, and rectified in the ordinary naphtha still. Afterwards it is run into a long iron cylinder, which is placed over a furnace, and supplied with numerous large burners. Each burner has a metal funnel placed immediately above it, connected with a cast-iron pipe, into which all the fumes from each burner are conducted. The naphtha in the cylinder is heated almost to the boiling point by the furnace beneath. A series of smaller pipes lead away the fumes from the main pipe into a row of chambers, and thence into a series of large canvas bags, placed side by side, and connected alternately at top and bottom. The bags vary in number from fifty to eighty, the last one being left open to allow the smoke to escape, after traversing some 400 yards since leaving the burners. The best quality of lamp-black is found in the last bags, that near the furnace being much coarser and less pure. The bags are emptied whenever they contain a sufficient quantity.

The process employed in Germany for the manufacture of lamp-black is to conduct the products of the combustion of any resinous matter in a furnace into a long flue, at the end of which is placed a loose hood, made of some woollen material, and suspended by a rope and pulley. The lamp-black collects in this hood, and, when a sufficient quantity has accumulated, is shaken down and removed. In this manner about 6 cwt. of lamp-black may be collected in twenty-four hours.

One form of the apparatus is shown in [Fig. 6]. The circular structure a is lined inside with hanging cloths upon which the black can condense, and is covered with a conical roof from which depends a movable sheet-iron cone b, perforated at its apex to give egress to a current of air. This cone b is supported by a rope g passing over a pulley c and accessible from the outside. A fireplace d, containing a small iron dish e for holding the resin, is built against one of the side walls of the structure in such a manner that it can be fired externally. The rate of combustion is regulated by a small sliding damper on the door of the fireplace. When the black has accumulated in the chamber a to such an extent that operations must be suspended, the fire is let out, and the chamber is left to cool entirely, so that the black may not ignite on contact with the air. The cone b is then lowered, and in its descent scrapes the walls of the chamber a and causes the black to collect on the floor, whence it is removed through an iron door f which at other times is kept tightly luted.

Fig. 6.—Apparatus for making Lamp-black.

In England, an inferior variety is sometimes obtained from the flues of coke-ovens. That known as Russian lamp-black is made by burning chips of resinous deal or pine wood, and collecting the soot formed; but it is objectionable, owing to its liability to take fire spontaneously when left for a long time moistened with oil.

A modified form of apparatus has been introduced by Thalwitzer, a German manufacturer, and is shown in Figs. [7] and [8]. A vertical tube is provided at its upper end with a funnel, into which cooling water is poured and flows out through openings in the tube immediately above a circular plate of thin cast or wrought iron arranged horizontally and

Figs. 7 and 8.—Thalwitzer’s Lamp-black Apparatus.

secured at its centre to the tube. Round the periphery of this plate is a vertical rim of tin plate, at the top of which is a pipe through which the cooling water runs into a gutter round the top of the cylindrical casing, the water being carried off from this gutter by a pipe. The vertical tube is carried near its upper end in a bearing, and at that part is attached a worm-wheel geared into it by a worm driven by any suitable power. At the underside of the circular plate is fixed a scraper, the edge of which is formed with a strip of leather in contact with the lower surface of the plate. Opposite the scraper, at the bottom of the casing, is a burning lamp, which sucks up the oil for its consumption by a flat wide wick.

The operation is as follows:—The vertical tube is caused to revolve by the action of the worm-wheel, the circular plate thereby receiving a slow rotary movement; and a small stream of water being poured into the funnel at the top of the tube, this water passes down the latter and through the openings on to the circular plate, which is thus kept cool. The burning lamp filled with paraffin or other oil is brought as near to the circular plate as is necessary for the cooling of the flame and the most perfect extraction of the carbon, which, in the form of soot, attaches itself readily to the plate, owing to its coldness and to the condensation of the steam produced. The revolving plate presents continually to the flames a new and clean surface, in consequence of the lamp-black being scraped away by the scraper as soon as deposited, and brought away through a pipe or shoot into a collecting barrel.

The apparatus as shown in Figs. [7] and [8] consists of a round metal plate A, provided with a flange a, and fixed on a vertical shaft b supported by the bearing B, and carrying at its upper end a worm wheel d set in motion by a worm. The plate A is cooled by water admitted through a pipe g, and the flange a is provided with a discharge pipe h, through which the cooling water runs into the groove D, surrounding the whole apparatus. Underneath the plate A a number of lamps J are applied, which are fed with oil by a common pipe l. H is an oblique scratcher or blade, the working edge of which is formed by a strip of leather, and touches the lower surface of the plate A.

For manufacturing lamp-black, a slow rotary motion is imparted to the apparatus by means of the worm and worm-wheel, and a slight current of water is directed upon the plate A through the pipe g. The lamps J, filled with paraffin oil derived from lignite, or with any other suitable oil, are ignited and approached to the plate A as far as is necessary for cooling the flame, so as to deposit the greatest possible quantity of black. The latter adheres to the cold surface of the plate, which is also kept damp by the aqueous vapour formed during the combustion. The revolution of the plate serves to bring the flame continually into contact with new and clean portions of the plate, the black being continually scraped off by the blade or scraper placed opposite the flames, and conducted through a channel into a collecting trough.

There is a risk of overburning, causing a grey tint and a hard and granular texture.

A variety of lamp-black known as “carbon black” or “gas black,” has of late years assumed an important position among black pigments. It is produced in considerable quantities in the United States by the combustion of the natural gas issuing from the earth in the mineral oil regions. The soot arising from the imperfect combustion of the gaseous hydrocarbon is made to deposit itself on cooled iron surfaces. These at first were made stationary, but now take the form of revolving discs or cylinders, which are automatically cleansed of the black as fast as it is deposited. This type of lamp-black is remarkably free from mineral impurities and unburned oil, and of a full colour.

An improved lamp-black kiln has been introduced in which the use of water is dispensed with. It is shown in Figs. [9] and [10]. The furnace A, which is preferably built double, as shown, is constructed of brick lined with firebrick, with a rear wall a that divides the furnace room from the condensing room, side walls b, front c, and central dividing wall d, that divides the furnace into two long and narrow fire

Figs. 9 and 10.—American Lamp-black Kiln.

spaces. The bottom of the fire spaces e is formed by a sheet iron plate f that is supported by the walls, and the space below plate f serves as an air space through which air circulates by openings g in the front and side walls, this circulation of air tending to keep the plate f cool. The rear of the fire space e extends upward and communicates by an opening h through the wall with the condensing room. In the front wall c is an opening to each fire space e and a door i to each opening. The oil or other liquid is supplied by pipes k that enter from the outside near the rear of the fire spaces. The outer end of each pipe k is fitted with a cup-shaped receptacle l, into which the oil will run from the vertical branch of the main supply pipe m, so that the amount of oil running into each pipe k may be observed, and regulated by a cock. The pipe m feeds the oil to one or more furnaces, the supply of material to each furnace being separately regulated. In the fire spaces beneath pipes k are placed shallow cast iron drip pans o to receive the oil, and the oil running in faster than it will burn will drop while on fire into the pans o, and be spattered into small particles. These pans are changed frequently, access to them being obtained by doors i. A slide p is provided in each door i to allow of ventilation when required. The slides and doors should close air-tight. By constructing the fire space e long and narrow, the plate f is more readily kept cool, and the space in front of the point of combustion renders the smoke less liable to escape by the doors. The products of combustion pass through the opening h to the condensing room, which is lathed and plastered, and if the room is sufficiently large a number of furnaces may be fitted to discharge into the same room. This furnace is especially adapted for burning dead oil; but by using burners of suitable construction other oils may be burned, and a superior quality of lamp-black made from mitigated spirits.

A very large proportion of the lamp-black now made is derived from the combustion of creosote or anthracene oils from coal tar, or of the residues of shale-oil distillation. The form of combustion chamber varies in different works, but is typified by the following rough sketch of that in use at the Stampshaw Chemical Works ([Fig. 11]).

Fig. 11.—Apparatus for making Lamp-black from Creosote or Shale Oil.

At these works a horizontal brick flue a about 18 inches square and about 10 feet long is provided. At one end it enters the black-house b, and is here provided with a damper c to shut it off when not working. The other end opens to the air, and here is a sliding door d which, when shut down, leaves an opening round a small pipe e, which enters in this situation from a main pipe that conveys oil in a similar manner to four burners of this description placed side by side. At the bottom of the flue is an iron tray f to catch any liquid that falls from the tube, and in this tray the oil is burned. The burning of oil in one of these flues is not allowed to go on for more than three hours, and, when the combustion is over, the communication with the black-house is closed, the entrance door of the flue is opened, and the cover is taken off the chimney g so that the flue may become cooled, and another flue is taken into use.

The black-house is a brick chamber into which the smoke passes, and where it deposits its sooty particles. In some works there is only one undivided chamber; in other works there are more than one, and the chambers communicate by flues through which the smoke passes from one to another. At other works the chamber is divided by vertical partitions, springing alternately from the two ends, so as to constitute one high zigzag flue, along which the smoke must travel to its outlet from the black-house. This chamber must needs have an opening somewhere to the outer air. The opening is sometimes a small chimney in the roof, and sometimes a short louvre tower. This is necessary to produce a trifling draught, just enough to carry the smoke into the chamber and no more.

In some works, the black from the black-house is also calcined, the object of the “calcination” being to get rid of all greasiness, a point of great importance when the lamp-black is to be used for making fine pigment. This process is conducted in circular iron pans, usually about 2¼ feet high and 2¼ feet diameter, which are provided with removable iron covers. A pan of this size will hold about 2 lb. of lamp-black. A bowlful is first put in and lighted by a red-hot iron; more and more is added from time to time as the ignition proceeds. When the pan, being full, leaves off smoking, the calcination is known to be complete, and the pan is then covered and its contents are allowed to cool. The loss undergone in this process is about 25 per cent. The smoke which comes off is acrid and very irritating to the eyes, like that proceeding from boiling oil, and it is difficult for a person unaccustomed to it to remain many minutes in the chamber where calcining is going on. This process is sometimes conducted within a chamber, but frequently under a shed or even in a building freely open to the air.

There are three sources from which nuisance may arise in lamp-black making: 1. The smoke which issues from the chimney of the black-house, small as it sometimes is, often constitutes a nuisance to near neighbours; but the nuisance is not a very serious one, and it does not extend very far from the works, never to a greater distance than about 50 yards. The odour, even when but little smoke escapes, is oppressive and suffocating in character, and resembles that diffused in a room by a smoking table-lamp. It occasions headache, but is not otherwise injurious to health. 2. A similar nuisance of suffocating smoke sometimes proceeds from the burners, but this is when they are leaky or when there is a deficiency of draught through the black-house, or when the doors of the burning chambers do not shut closely, and when there is much wind blowing past them. This nuisance chiefly occurs when the burners are open to the air and merely protected by an open shed. 3. The escape of acrolein and other offensive vapours from the calcining house.

The best mode of preventing nuisance from the black-house is so to elongate the chamber as to give abundant opportunity for the soot to deposit in the course of the smoke along it to the outlet, and by taking means to consume by fire what little smoke escapes deposition. A most effectual arrangement for the accomplishment of these ends is to have a black-house 150 feet long, and so divided by partitions within as to cause the smoke to traverse a distance of altogether 500 feet before it finds an exit; the exit from the chamber communicating with a fire, in which the last of the smoke is consumed, and which serves to assist in regulating the draught through the chamber.

The regulation of the draught through the burner and black chamber is of importance in order to avoid the escape of smoke from the burners. If the draught be too great, too much black is lost from the chamber, but if, on the other hand, it be too little, the smoke instead of passing into the chamber will come out into the works and create a nuisance, especially where the burners are erected in the open air, under circumstances in which variation in the force of the wind cannot fail to interfere with due regulation of draught. This part of the manufacture should be conducted within a building of some sort.

The best mode of preventing nuisance from calcination is in operation at Shackell & Edwards’ works, in Hornsey Road, Islington. At these works the black is calcined in a chamber 20 feet square and 25 feet in greatest height, with a paved floor and arched roof. In the centre of the roof is the opening where a fire was formerly placed, but which is now closed by a sky-light, capable of being raised. The calcining pots are ranged round this chamber, and a fan, employed to draw off the vapours from the oil-boiling pans, is further utilised to draw off also, from the upper part of the calcining-house, the vapours arising from the calcination, and to drive them into the boiler fire, where they are consumed. Calcination should always be conducted in a closed building duly ventilated so as not to create nuisance.

The transport of lamp-black is effected in barrels or bags; when in the latter, these should be previously soaked in water containing some clay in suspension, which stops up the pores of the sacking, and thereby prevents loss.

The particular virtue of lamp-black as a pigment lies in its state of extremely fine division, which could not possibly be attained by artificial means; this quality renders it invaluable as the basis of black pigments, all of which contain it in a greater or less quantity. Indian ink and printers’ ink are also composed principally of this substance.

Unimportant Blacks.—In addition to the recognised blacks already noticed there are a number of other sources of black pigment which have been drawn upon to a limited extent, or have been suggested as substitutes for the standard articles. They only merit a short description.

Aniline black is prepared by adding an acidified (sulphuric) solution of bichromate of potash to an aqueous solution of hydrochlorate of aniline, and washing the precipitate. The cost is prohibitive.

Candle black is candle smoke condensed on a cold plate.

Charcoal black is finely-ground wood charcoal.

Coal black has been suggested by grinding coal, but lacks the requisite qualities of a pigment.

Cork black is a very fine pigment prepared by calcining cork refuse. Limited supply.

German black is Frankfort black.

Iron black is ground black sulphide of iron.

Lead black is prepared by boiling lead fume in sulphide of soda solution. It would probably be unstable on account of oxidation.

Manganese black is ground oxide of manganese. It is costly, and dries too quickly.

Prussian black is calcined Prussian blue. It is not of a good colour, nor economical.

Prussiate black is the carbonaceous residue from making yellow prussiate of potash. Used chiefly for decolorising syrups, &c.

Spanish black is cork black.

Tannin black is proposed to be made by exhausting the tannin from refuse leather and tanning agents, and adding alum and sulphate of iron. The colour is blue-black, weak, and unstable.

CHAPTER III.
BLUES.

Cobalt Blues.—Some of the compounds of cobalt with alumina, phosphoric acid, silica, and tin, are remarkable for possessing a fine blue colour of great permanency and indestructibility, and still find a limited application. They are chiefly as follows:—

Cœruleum.—This is a light-blue slightly greenish colour, with no purple tendency in artificial light. It is non-granular, covers well, mixes with water or oil, and is a good artists’ colour for sky effects. It is permanent in strong sunlight and impure atmosphere, and resists acids and alkalies at normal temperatures. Hot hydrochloric acid dissolves it, and addition of water to the pale blue solution produces a violet red; evaporation to dryness restores the original pigment. The green tint of a nitric acid solution is due to iron and nickel impurites. Dilute sulphuric acid causes partial decomposition of cœruleum, but it is proof against caustic potash, acetic acid, and concentrated sulphuric acid. Its composition is given as

There are several methods of preparing cœruleum:—

(1) A solution of stannate of potash is added to one of cobalt. A blue precipitate is thrown down, which, on washing, becomes first light-red and then brown. When calcined at a white heat it assumes a blue colour.

(2) A solution of stannate of soda is mixed with a solution of nitrate of cobalt, and the resulting precipitate calcined to bright redness forms a blue pigment.

(3) Solutions of cobalt and tin are mixed and precipitated by soda, the precipitate washed free from soda being calcined as in all the other cases. The silicate of soda is the most satisfactory sodium salt for a precipitant.

Cobalt Blue.—This rich pure blue pigment is not alone permanent, but actually develops its full intensity only after exposure to the air. With age, however, it acquires a greenish tendency, and in artificial light it inclines to a violet tint. It is proof against acids and alkalies, and mixes particularly well with water. In combination with other pigments it is unaltered and has no effect on them. Its tones are different from those of ultramarine. It slowly decomposes when heated in strong sulphuric acid, yielding a violet solution and a white precipitate, which latter dissolves and affords a blue liquid on dilution with water. It consists principally of about 80 per cent. alumina, and 15 per cent. oxide of cobalt.

There are several ways of preparing it:—

(1) By Thénard’s original process, roasted cobalt, from Tunaberg, Sweden, is dissolved under heat in an excess of nitric acid. The solution, evaporated nearly to dryness, is boiled in water, and the deposit of arseniate of iron is filtered out. Into the filtrate is poured a solution of basic phosphate of soda, which throws down a precipitate of basic phosphate of cobalt, varying in hue from violet to pink. This precipitate is washed on a filter, and, while still gelatinous, 1 lb. of it is intimately mixed with 8 lb. of hydrated alumina, recently precipitated by ammonia from a solution of potash-alum. The mass is first dried to brittleness and then calcined in a covered clay vessel for half an hour at a cherry-red heat. The resultant blue pigment is stored in glass receptacles.

The preparation of the gelatinous alumina is conducted as follows. The potash-alum is dissolved in at least three times as much water as is necessary, and is then precipitated by an abundant excess of ammonia, with frequent stirring. When settled, the supernatant liquor is siphoned off, and the precipitate is thoroughly washed several times on a filter.

Thénard blue will vary in tint according to the proportions of alumina used. A pure colour is obtained with 4-5 parts alumina to 1 of phosphate of cobalt; a greenish hue with equal parts of alumina and cobalt salt; and almost any intermediate tone by varying the proportions between these limits.

(2) To 16 parts of gelatinous alumina add 1 part of arseniate of cobalt, obtained by precipitating the solution of cobalt by arseniate of potash.

(3) A richer and more velvety blue is got by using oxide of cobalt and substituting phosphate of lime for the alumina (Boullai-Marillac).

(4) Binder’s process is as follows:—Dissolve by boiling 6 lb. alum, free from iron, in a leaden or earthenware vessel, and filter it into a vat 5½ ft. high and 3 ft. across, one-third full of clean water. Precipitate the alumina by solution of potash, fill up the vat with water, settle, decant the clear liquor, and wash repeatedly till barium chloride gives no precipitate. Dissolve ½ lb. sesquioxide of cobalt in 1½ lb. hydrochloric acid at 22° B., and evaporate to dryness. Dissolve the residue in 3 lb. hydrochloric acid, and pass a stream of sulphuretted hydrogen through it, to throw down any foreign metals. Filter clear, evaporate again to dryness, and dissolve the residue in enough water to produce 4½-5 lb. of solution. Next precipitate the cobalt solution (3 to 6 lb., according to depth of tint required) by ammonia, avoiding excess. Wash the precipitate, and add it to the water, holding the gelatinous alumina in suspension, stirring thoroughly for ½ an hour. A reddish tint in the supernatant liquor shows that some of the cobalt has been dissolved. Add a little ammonia, and allow the precipitate to settle. Decant and add new waters repeatedly. Finally collect on a fine filter cloth, drain, press, stove dry, and calcine for 2-2½ hours at red heat in clay crucibles: then cool, grind first in a mill and then on a slab, and sift.

Smalts.—This pigment has not maintained its position in competition with artificial ultramarine. Formerly it was very largely used to correct the yellow tone of cottons, papers, and pottery. It has a pale violet-blue tint, which, however, is not constant in artificial light. Being a silicate it is very permanent, and proof against the action of acids, alkalies, and sunlight, besides being inert when mixed with other pigments. It can be used with either water or oil as a medium, but is not a successful paint owing to its weak colouring power. It is virtually a double silicate of cobalt and potash, or a cobalt glass, containing a few impurities, of which the chief are aluminium, iron, and lead oxides. The colour varies somewhat according as these impurities fluctuate, and the finest ground sample is always the palest. It is hardly ever adulterated, and the chief point to secure is that it be ground to the finest possible degree.

Its manufacture is most extensively and successfully carried on in Saxony. The raw materials used are cobalt speiss (an arsenide of cobalt and iron), potash, and sand. The ore is broken up into convenient sized pieces and roasted at red heat in a reverberatory furnace provided with a tall shaft for discharging the sulphurous and arsenical fumes at a high altitude. When the evolution of these fumes has ceased and the mass begins to assume a pasty consistence, the roasted ore is removed from the furnace, cooled, reduced to a fine pulverulent condition (then known as “zaffre”) and passed through a silken sieve. Should it be necessary,

Fig. 12.—Furnace for Roasting Cobalt Ores.

the cobalt ore is first spalled and hand-picked to remove the ores of foreign metals which are associated with it; and then reduced to a very fine state in an edge runner or mortar mill, and freed from earthy impurities by washing. The concentrated ore is then dried and dead-roasted in small charges at a time (about 4 cwt.) in a specially designed reverberatory furnace such as shown in [Fig. 12], of which a is the hearth on which the ore is spread; b, the fireplace, the products of combustion from which pass over the ore on the hearth, and thence into the flues c, which repeatedly circle round the furnace so as to provide abundant opportunity for the arsenious oxide derived from the combustion (oxidation) of the arsenic in the ore to condense; this highly poisonous arsenious oxide is collected in a solid form from the flues at convenient intervals by means of the doors d. The ore is charged and discharged at the door e. The roasting should not be carried to such a point that the whole of the sulphur and arsenic are removed when making smalts, as by leaving a portion of these substances in the ore at this stage, the ultimate purification is better accomplished.

The next stage is to fuse the roasted ore with potash and silica so as to form a blue glass. The proportions in which the ingredients are mixed depend upon the depth of colour in the zaffre operated upon and the tint desired in the finished smalts; hence it is always determined by a preliminary experiment, and is then most carefully adhered to, each material being accurately weighed out. Only the best potash can be used, as it must be quite free from soda, and iron or other metal; the effect of soda is to render the blue greenish tinted. Quartz affords the requisite silica, and is hand-picked to ensure freedom from alumina, iron, and lime, which import dullness into the colour, and then ground to a fine powder in an edge runner mill. The duly weighed quantities of the several ingredients are intimately mixed in wooden or cement lined vessels, so as to preclude the possibility of any metallic iron finding its way in; and as a further protection against this risk a little white arsenic is often added so that the iron may be carried down in the regulus which is formed during the fusion in the crucible.

These crucibles are of refractory earthenware quite free from lime, and measure about 18 inches across at top, gradually diminishing to 14 inches at bottom, so that an ordinary charge is about ¾ cwt. They are placed in rows in a furnace which generally bears a close resemblance to a glass furnace, the operation being very similar. The form of furnace common in Saxony, where most of the smalts is made, is shown in [Fig. 13]. By means of a series of openings a in the walls of the furnace the pots b are introduced on to the hearth of the furnace, whereupon the openings a are bricked up again and remain closed during the operation. The ingredients are charged into the pots b

Fig. 13.—Furnace for making Smalts.

by means of long iron ladles which are introduced through the small square apertures c, which can be temporarily closed by a half-brick or other simple article. The fire is then lit in the fireplace d, and the products of combustion circulate around the pots b, and finally escape at the orifices e at the top of the furnace into flues leading to the chimney f. After about 8 hours’ firing fusion commences in the pots, whereupon the contents are thoroughly stirred by rods inserted through the working holes c. The temperature is then increased till a white heat is attained, this being necessary for the formation of a glass. The fused mass is repeatedly sampled, and when it has become quite homogeneous, and the regulus or speiss containing the iron, antimony, bismuth, arsenic, copper, nickel, sulphur and other impurities has completely separated itself and collected at the bottom of the pots, the blue glass is ladled out and dropped at once into cold water, by which it is disintegrated and rendered very brittle ready for the subsequent grinding. The regulus is then drawn off from the pots through holes provided for the purpose, and removed by the orifices g, after which the pots are ready for another charge. They ordinarily remain serviceable for about six months.

The grinding needs to be done with great thoroughness, and is accomplished partly by stamps and partly by edge-runner mills in the presence of water. The particles as reduced are floated off by the water to a series of settling tanks communicating one with another. The portion which settles in the first of the series is too coarse for use, and is returned to the edge-runner for further grinding; while the portion in the last of the series possesses such a weak colour that it is rejected, or put into the crucible to undergo a second fusion. The selected portions are dried ready for the consumer.

Copper Blues.—These form an unimportant class, being unstable and not endowed with great colouring power. Their tint is pale and greenish, and though opaque in water, they are not particularly so when mixed with oil. Exposed to the action of sulphur or its compounds, whether present as sulphuretted hydrogen in the air, or in combination with a metal, forming another pigment with which they may be mixed, copper blues undergo an important chemical change, the carbonates and oxides of copper being converted into the sulphide, which is black. Under the influence of heat too the blue carbonate will lose its carbonic acid, and be turned into the black oxide. Ammonia and the acids dissolve them, but other alkalies are resisted until heat is applied. The chief kinds of copper blue are Bremen blue, cæruleum, lime blue, mountain blue, Péligot’s blue, and blue verditer.

Bremen Blue.—This is a more or less pure hydrated oxide of copper, varying in its qualities according to the method of preparation. When made by precipitating a neutral salt of copper from solution, it forms a dense and compact mass; whereas a porous and pulverulent pigment results when basic and insoluble copper salts are treated with alkalies.

(1) The foundation of the manufacture of this colour was waste copper scrap, such as ship’s sheathing, from which, in various ways, was prepared a basic chloride or oxy-chloride. Some of the methods adopted were:—(a) 100 lb. scrap copper, 99 lb. powdered sulphate of potash, and 100 lb. salt, moistened with clean water; (b) 100 lb. copper fragments, 60 lb. salt, and 30 lb. diluted sulphuric acid (3 volumes of water to 1 of acid); (c) a solution of copper oxide (scales) in pure hydrochloric acid poured over the scrap copper. The method (a) produces a chloride of copper which, in contact with more metal, becomes a sub-chloride; this, absorbing more oxygen from the air, is converted into the basic green “oxide” of the factories. By the (b) process, the hydrochloric acid set free, and the atmospheric oxygen produce the same result. In the (c) process a similar effect is obtained.

It is of primary importance that no trace of this sub-chloride of copper shall be allowed to remain, as it undergoes decomposition by caustic alkalies, and throws down an orange-yellow sub-oxide of copper. Hence it has sometimes been the practice to prepare the basic oxy-chloride twelve months in advance, and to stir it frequently before use. Complete oxidation, however, can be as satisfactorily accomplished by alternately wetting and completely drying the mass.

An interesting phenomenon takes place during the transformation of this green magma into a hydrated oxide of copper. On this magma being introduced by degrees into a caustic potash or soda lye of about 22° B., the thoroughly washed and dried product is exceedingly fine, with great covering power, and deepens on addition of a little water. When the magma is diluted with an equal volume of water, and the mixture at once poured into an excess of caustic lye, with constant agitation, a few minutes’ rest will suffice for the mass to assume a most compact consistence. The colour thus produced, when washed and dried, is much lighter in colour, and has less body. A blue derived from any of these products is unsatisfactory as regards freshness and intensity of colour; whereas by adding a small quantity of concentrated solution of sulphate of copper to the magma before treating it with the alkaline lye, apparently a highly basic sulphate of copper is produced which deepens the colour.

A pigment with good body can be made in the following manner. To 100 lb. of the thick magma of basic oxychloride add a concentrated solution of 7 lb. sulphate of copper, and then 40 lb. of a concentrated caustic lye (32°-36° B,), with vigorous and rapid stirring, finally adding about 150 lb. of caustic lye at 20° B. When the decomposition is quite complete, the precipitate is carefully washed, passed through a fine hair sieve, and filtered. Drying is effected at a low temperature, to ensure that the hydrated state of the oxide is not changed; and the air of the drying chamber must be free from acid or sulphuretted vapours.

(2) If neutral nitrate of copper be decomposed by an insufficiency of potash carbonate solution, the flocculent precipitate of copper carbonate resulting is by degrees transformed into a sub-nitrate of copper, which goes down as a heavy green powder. On treating this sub-nitrate with a potassic solution of zinc oxide, a dark-blue coloured pigment is formed, which is apparently a zincate of copper mixed with a very small proportion of a highly basic nitrate of copper. Though very light it has great covering power. In practice the manufacture is conducted as follows.

Calcine copper scales in a reverberatory or muffle furnace till the sub-oxide is entirely converted into protoxide, or until it dissolves in nitric acid without evolving red nitrous fumes. Heat is applied to the solution of nitrate of copper, which is decomposed by addition of a clear solution of potash carbonate. After effervescence has subsided, small doses of potash carbonate solution are added till but little undecomposed copper is left. This residue is recovered by decanting the clear liquor, and repeatedly washing the green precipitate with small quantities of clean water, collecting all the washings, and finally precipitating by potash solution. On introducing the green carbonate of copper into a new solution of copper nitrate, it is transformed into a basic salt. Crystals of nitrate of potash are obtained by evaporating the previous liquors.

To obtain an economic solution of zinc oxide, clippings of metallic zinc are treated with a solution of caustic potash or soda in a cast-iron vessel. The immediate result is a disengagement of hydrogen, and saturation of the alkali with zinc oxide, which behaves as an acid. The cleared liquor serves for decomposing the basic nitrate of copper. The pigment produced is a handsome blue, and the potash liquor can be evaporated down till it yields crystals of saltpetre. The economy of this method lies in producing nitric acid cheaply from soda nitrate and obtaining saltpetre as a bye-product.

(3) An inferior and cheaper pigment is made in the following manner. To a solution of copper sulphate add one of barium or calcium chloride till a white precipitate ceases to go down, and from the cleared blue liquor all the copper is precipitated by addition of fresh milk of lime. Usually the weight of quicklime required is 20 per cent. of the copper sulphate. The settled, washed, and dried precipitate is the pigment desired. The cleared barium or calcium chloride solution may be used anew as a precipitant for the next batch.

Cæruleum.—This name has been given to the beautiful blue pigment used in Egyptian and Pompeian mural paintings, and exhibiting the same bright blue after 1000 years’ exposure to the weather as when first used. Its composition has been given by Fouqué as approximately 63½ per cent. silica, 21 per cent. copper oxide, and 14 per cent. calcium oxide, and he regards it as a double silicate of copper and calcium. It is supposed to have been produced by fusing together copper ore, sand and lime, but experiments have not yet resulted in a successful imitation of the pigment, a difficulty being encountered in the fact that if too high a temperature be permitted, destruction of the blue colour ensues, and a green glass results instead. This is unfortunate, as its remarkably bright and stable hue would make it very popular if it could be manufactured at a moderate cost. It withstands sulphuretted hydrogen, and even prolonged boiling with any of the acids or alkalies.

Lime Blue.—This pigment is essentially a mixture of hydrated oxide of copper and calcium sulphate. It resists the action of alkalies in the cold, but turns black when boiled in caustic soda, and is completely soluble in hydrochloric acid. Ultramarine has largely, if not entirely usurped its place. There are several ways of making lime blue:—

1. Any soluble copper salt the acid of which will make a soluble salt with lime is suitable, the only precaution necessary being that if in the decomposition of the copper and lime salts, the combination of the whole of the sulphuric acid with the lime is not attained, there should be an excess of copper sulphate in the liquor rather than of the lime salt. The resulting copper solution, containing very little lime sulphate, is settled in a cool place for 24-36 hours, filtered, and diluted with clean water down to about 18° B. Meantime a milk of lime is prepared with very white and well-burned lime, slaked and mixed with abundance of pure water, and kept stirred for a long time in a lead-lined vat. After a short rest to permit sand, &c., to precipitate itself, the milk is drawn off, and left to settle in lead-lined or copper pans. The deposit is collected, ground in a mill where contact with iron is impossible, and passed through a very fine sieve.

The mixture of lime and copper solution is made in the proportion of 100 lb. dry lime with 175 dry copper salt, if the most intense coloration is desired, but the proportion of lime may be much increased without detriment to the pigment beyond lessening its intensity of colour. After complete settlement of the precipitate, the clear liquor is decanted; the pigment is carefully washed with clean soft water, and drained on filter cloths till it is of a convenient consistence forming a green paste. A definite weight of this paste calculated on the dry pigment is taken for further incorporation, consequently it is first necessary to ascertain how much water is in the paste. Usually it amounts to 75 per cent., and on this basis 5 lb. of the paste are stirred up with 1 gal. clean water in a lead-lined vat, with addition of ½ lb. wet lime under constant agitation. Subsequently ¼ pint of clear solution of best potash at 15° B. is well stirred in, and the mixture is immediately taken to the mill and most thoroughly ground.

Further, for each 10 gal. of green paste is prepared a clear solution of 1 lb. pure salammoniac in 2 gal. water and another solution of 2 lb. copper sulphate in 2 gal. water. The liquid paste is drawn off from the mill into a stoneware vessel, and the two solutions of salammoniac and copper sulphate are immediately added. After complete agitation and combination, the mixture is left for 4 or 5 days to settle, and turned into a lead-lined vat, where it is repeatedly washed with clean waters until turmeric paper is not discoloured.

(2) Precipitation of copper sulphate by excess of thin milk of lime in the cold, followed by washing and drying, will give a lime blue which will dry without turning black. Or 100 lb. of the copper sulphate may be treated with a milk of lime prepared from 30 lb. quicklime and addition of 12½ lb. salammoniac. When the liquor has become colourless, the pigment is prepared from the precipitate; but the lime should be ground after slaking, and the milk of lime left to stand for some days, before use. The salammoniac seems to be essential to the production of a pure full blue. Milk of lime poured drop by drop into the ammoniacal copper solution gives a precipitate which redissolves on agitation, and remains long in solution under heat, but finally throws down a permanent precipitate, while the liquor on standing gives beautiful blue crystals. From experiments it appears that out of seven atoms of copper sulphate in the liquor, five are precipitated by milk of lime and the last two are decomposed by ammonia. A greater proportion of lime will produce a precipitate holding a certain quantity of less valuable pigment. A smaller proportion of lime yields a finer coloured and more crystalline pigment, because it crystallises partly in the excess of solution, so that by incomplete decomposition a smaller yield of superior pigment is obtained. The proportions necessary for formation of the colour are 7 equivalents of copper sulphate, 5 of lime, and 2 of ammonia, and if the 2 equivalents of ammonia be replaced by 2 of lime and 2 of salammoniac, the proportions furnishing the best colour will be 100 lb. copper sulphate, 24 lb. lime, and 22½ lb. salammoniac.

Both caustic soda and caustic potash produce a fine blue precipitate in a solution of ammoniacal copper sulphate with excess of ammonia, but the liquor decolorises only on evaporation of the ammonia. The precipitate becomes lighter-hued the more it is washed, and consists of hydrated oxide of copper with a little carbonic acid; it does not turn brown even when heated in presence of excess of potash or soda. Moreover the presence of ammonia renders the hydrated oxide of copper much more permanent. The composition of this pigment is given by Gentele as 33½ per cent. copper oxide, 23½ sulphuric acid, 16 lime, and the remainder water, &c.

Mountain Blue or Azurite.—This natural blue pigment consists essentially of a basic carbonate of copper, and is found in quartz rocks in England, France, Bohemia, Hesse, Saxony, the Tyrol, and Siberia. It affords a rich sky-blue paint of a permanent character, but being comparatively costly is not largely employed. Its composition is about 69 per cent. copper oxide, 25½ carbonic acid, and 5 water. The only preparation needed is exceedingly fine grinding.

Péligot Blue.—(1) Whereas the hydrated oxide of copper precipitated from a solution of a salt of copper by excess of potash or soda rapidly blackens even though washed with cold water, Péligot obtains a blue hydrated oxide which resists boiling and heating at 212° F. He uses any soluble copper salt, but preferably the sulphate. A very dilute solution of the copper sulphate is treated with ammonia in excess (aqua ammoniæ or an ammoniacal salt) and precipitated by soda or potash.

(2) On adding water in excess to a slightly ammoniacal solution of copper nitrate, the same pigment is obtained.

(3) A mixture of 73 parts silica, 16 oxide of copper, 8 lime, and 3 soda, is fused together at a temperature not much exceeding 800° F. At higher temperatures there is risk of the pigment turning black.

Verditer.—This sky-blue and not very durable pigment, used in water-colour painting, closely resembles Bremen blue (see [p. 34]) in composition and manufacture. It consists chiefly of copper carbonate, mixed with a lesser proportion of hydroxide, sulphate, or oxide, and occasionally a small quantity of sulphate of lime; and is most satisfactorily prepared from copper chloride or nitrate, though almost any salt of copper may be used. The mode of fabrication varies.

(1) To a solution of the nitrate or sulphate is added one of potash or soda carbonate so long as any precipitate is formed, and this precipitate, when filtered and washed, is treated with a weak caustic soda solution.

(2) A hot solution of chloride of lime is added to a hot solution of sulphate of copper at 62½° Tw. till the precipitate ceases to go down. The solution of chloride of copper which constitutes the liquor is filtered off, diluted with water to about 31½° Tw., and treated with repeated small doses of slaked lime ground exceedingly fine in water till no more copper is precipitated. The resulting green paste is drained, filtered, washed, and put into wooden vats; here 8 lb. of lime paste and 5 pints of potash carbonate solution at 25½° Tw. are added for every 70 lb. of dry colour contained in the green paste, the whole mass being thoroughly agitated, then allowed to rest till the development of the required shade is accomplished, when it is filtered, washed, and dried.

(3) In some German works the final green paste as prepared in (2) is put into air-tight vessels, and a solution of 3 lb. ammonium chloride and 4 lb. sulphate of copper in 7 gal. of water is introduced for each 70 lb. of dry colour in the green paste. After complete admixture of all the ingredients, the receptacles are fastened up for several days so that the reactions may proceed out of contact with the air, and finally the pigment is removed, washed, and dried for use.

Indigo.—The well-known blue colouring matter termed indigo is produced by a great number and variety of plants, distributed throughout all the tropical countries of the globe. Commercially, it is obtained chiefly from species of Indigofera, as I. tinctoria, the cultivated species of India, furnishing the chief article of commerce, found also in Madagascar, St. Domingo, &c.; and I. Anil, in the Punjab, W. Indies, and on the Gambia river. Some is also obtained from I. argentea, in Africa and America: I. Caroliniana; I. disperma, the cultivated plant of Spain, America, and some of the E. Indies; I. cærulea, the “black indigo” of India; I. glauca, in Egypt and Arabia; I. pseudo-tinctoria, cultivated in some parts of the E. Indies, and said to yield the best dye; I. cinerea, I. erecta, I. hirsuta, and I. glabra, in Guinea. Considerable local supplies are obtained from the following plants:—Isatis tinctoria, in Europe and China (see Woad); I. indigotica, cultivated in some parts of China; Amorpha fruticosa, in Carolina; Baptisia tinctoria, wild, in the United States; Gymnemia (Asclepias) tingens, in Burmah; Polygala tinctoria, in Arabia; Polygonum Chinense, P. tinctorium, P. perfoliatum, P. barbatum, P. aviculare, in China and Japan, and introduced into Belgium; Ruellia indigotica, largely cultivated in Assam, as well as in India, and at Che-king, in China; Tephrosia tinctoria, and T. apollinea, in India and Egypt; Wrightia tinctoria (Nerium tinctorium), the Palas indigo of the Carnatic.

The cultivation of indigo (chiefly Indigofera tinctoria) is very extensively carried on in India, especially in the district included between 20° and 30° N. lat. The soil best suited for the culture is a rich loam, with a subsoil which is neither too sandy nor too stiff; alluvial soils give the best returns, but good crops are sometimes raised on higher grounds. The land is ploughed in October-November, after the rains; the seed, about 12 lb. to the acre, is sown in February-April. Too rapid growth diminishes the yield of dye. In July-September, the plants are in full blossom, and the harvest takes place. The preparation of the dyestuff may be performed in either of two ways, which are distinguished as the “dry-leaf,” and the “green-leaf” process. The latter is considered the better, and is the more general; it is conducted as follows:—The flowering plants are cut down at about 6 in. from the ground, and immediately taken to the steeping vats, within which they are spread out and pressed down by beams fitted to the side posts of the tanks. Enough water is then admitted to cover the plants; if this be delayed, fermentation may set in and spoil the product. The duration of the steeping is liable to considerable modification, and needs much judgment and experience; with a temperature of 96° F. in the shade, 11-12 hours may suffice; in cooler weather, 15-16 hours may be necessary. Moreover, very ripe plants require less time than young and unripe ones. The following general conditions indicate the time for suspending the maceration:—(1) The sinking of the water in the vat; (2) the immediate bursting of the bubbles that arise; (3) an orange tint mingled with the green, when the surface water is disturbed; (4) the emission of a sweetish, pungent odour, quite distinct from the raw odour of the unripe liquor. At this point, men enter the vat, and stir up its contents either by hand or by a wooden paddle. The agitation is at first gentle, but increases as the fecula begins to separate; this is known by the disappearance of the froth, and by the colour of the liquor changing from green to blue. The “beating,” as it is called, is continued for 1¾-3 hours, the following conditions being a guide as to its sufficiency:—(1) The ready precipitation of the fecula from a sample of the liquor, and the madeira-wine colour of the latter; (2) a brownish colour observed on dipping a cloth into the liquor, and wringing it out; (3) the appearance of a glassy surface on the liquor, and the subsidence of the froth with sparkling and effervescence.

Next a little pure cold water, or weak lime-water, is sprinkled over the surface of the liquor, to hasten the settlement of the fecula, which occupies 3-4 hours. After this, the water is drained away from the top, by means of plug-holes in the side of the vat. The precipitated fecula is then removed to a boiler. Here it is made to boil as promptly as possible, and is kept boiling for 5-6 hours; it is constantly stirred, and skimmed with a perforated ladle. After boiling, it is run off to a straining table, where it stays for 12-15 hours to drain; next it is pressed for about 12 hours, and then cut, stamped, and placed to dry. The ordinary dimensions of a steeping-vat are 16 ft. by 14 ft. by 4½ ft. deep; this will contain about 100 maunds (8200 lb.) of plants, which may yield from 40 lb. downwards of indigo. The beating-vat is less deep.

Such are the methods of cultivation and manufacture most generally in use throughout India. In limited districts, however, some modifications are in vogue. On land subject to inundation, the plants last only one year. South of the Ganges, the seed is sown at the beginning of the rains, and the plants remain on the ground for two years, thus giving a double crop, the second of which is the larger and better. In very strong land, a third crop is sometimes secured. Occasionally, sesame is sown on the same ground, and harvested before the indigo is cut. Small quantities of indigo are grown on poppy lands, and irrigated. The seed is sown in March-April, and the crop is gathered at the end of the rains, in time for an opium crop to be taken off the land. Indigo is sometimes manufactured by collecting the fecula, and dropping it in cakes to harden in the sun; this is “gaud” indigo, of very inferior quality. The fecula is improved by boiling it in coppers and pressing it into boxes. The production of the indigo blue is the result of the decomposition of the colouring principle of the plant, which exists as a glucoside. Plants grown on poor soils, and in dry climates, yield almost the whole of this glucoside to the ordinary process of steeping and beating described above; but plants raised on rich alluvial soil, and in damp heat, contain an amount of glucoside which cannot be utilised by the ordinary process. In order to prevent this waste, which causes the richest plants to give the least return, it is necessary either to prolong the fermentation, and raise the heat to 95°-100° F., or to add a solution of sugar or glucose to the vat-liquor. Olphert adopts the use of steam, to raise the temperature of the vat to 111° F., and thus obtains 25 per cent. more colouring matter.

Japan possesses several large factories for preparing indigo from the native Polygonum tinctorium. The plants, 2-3 ft. high, are cut into three parts, the uppermost being the most valuable. The best dye is made from the leaves alone, which, after a few hours’ exposure to air and sun, are placed in straw bags. They are afterwards removed from the bags and moistened with water, which must be proportioned with the greatest exactitude. They are then spread upon, and covered by, mats, for a few days, after which the sprinkling is repeated. The process continues for about 80 days, the moistening being renewed about 25 times for the best leaves, and 9 for the inferior. After this fermentation, the leaves are pounded in wooden mortars for two consecutive days, by which they are reduced to a pulp; this is then formed into balls of dark-blue colour.

The central provinces of Java yield large quantities of indigo, which are exported to Holland, and thence widely distributed. The indigo prepared by the natives is of an indifferent quality, in a semi-fluid state, and contains much quicklime; but that prepared by Europeans is of a very superior quality. An inferior variety, having smaller seeds, and being of quicker growth, is usually planted as a second crop on land where one rice crop has been raised. In these situations, the plant rises to a height of about 3½ ft. It is then cut, and the cuttings are repeated three, or even four times, till the ground is again required for the annual rice crop. But the superior plant, when cultivated on a naturally rich soil, not impoverished by a previous heavy crop, attains a height of 5 ft., and grows with the greatest luxuriance. The plants intended for seed are raised in favourite spots, on the ridges of rice-fields in the neighbourhood of the villages, and the seed of one district is frequently exchanged for that of another. That of the rich mountainous districts, being esteemed of best quality, is occasionally introduced into the lowlands, and is thought necessary to prevent that degeneration which would be the consequence of cultivating for a long time the same plant upon the same soil. The climate, soil, and state of society of Java seem to offer peculiar advantages for the extensive cultivation of this plant. The periodical droughts and inundations of the Bengal provinces are unknown in Java, where the plant, in favoured situations, may be cultivated nearly throughout the whole year, and where it would be secure of a prolonged period of that kind of weather necessary for the cutting. The dye is prepared in a liquid state by the natives, by infusing the leaves with a quantity of lime; in this state, it forms by far the principal dye-stuff of the country. The indigos prepared in Java by Sayers’s process are of unusually high and constant quality. They contain an average of 70½ per cent. of indigotine, and a minimum of 65-66 per cent.; and an average of 2·77 per cent. of ash. Ordinary commercial indigos seldom attain 65-66 per cent. of indigotine; and their ash averages about 16½ per cent.

The Philippines produce considerable quantities of indigo, the best coming from Luzon. The plants suffer from locusts and storms, but the cultivation is very profitable. The yield of indigotine is large, but the preparation is conducted in such a primitive manner that the value of the product is much deteriorated.

In many parts of Africa, as Sierra Leone, Liberia, Abeokuta, the Niger valley, Natal, Cape Colony, Tunis, and the Soudan, species of indigo plants are found in a wild state, and from them the natives prepare an inferior dye-stuff.

In some of the S. States of America, notably S. Carolina, indigo culture has been attended with more or less success. The method of preparation pursued here varies but very slightly from the ordinary Indian process, almost the only important modification being the addition of a little oil to the liquor in the beating-vat, when the fermentation becomes too violent. The precipitated fecula is placed in coarse linen bags, and hung up to drain. The drying is finished by turning it out of the bags upon a floor of porous timber, and working it up. It is frequently exposed to the sun for short periods at morning and evening, and is then placed in boxes or frames, to cure till it is fit for the market.

Several of the Central American States have figured conspicuously as indigo producers. The dye is precipitated in the beating-vat by the sap contained in the bark of Tihuilate (Yonidium), Platanillo (Myrosma Indica), or Cuaja tinta. The fecula is left during the night; and, on the following day, is boiled, filtered, pressed, and sun-dried. In most districts, the cultivation is declining, partly owing to the carelessness exhibited in the preparation of the dye.

Indigo is judged commercially by its lightness, by a copper gloss on the surface, and by exhibiting no foreign ingredients when broken. There are several ways of testing it chemically, to ascertain the exact proportion of indigotine present; one method is as follows:—Finely pulverised indigo, 1 part; green copperas, 2 parts; and water containing 10 per cent. of caustic soda, 200 parts; are well boiled in a flask, and left to cool. The clear liquor is exposed in shallow vessels to the air, when the soluble indigo is oxidised, and precipitated as pure indigotine. The residue in the flask is thus treated three times; the whole indigotine is then collected on a filter, dried, and weighed. The consumption of indigo is still very large. Artificial indigo has not, as yet, been manufactured on a commercial scale, nor at a commercial price; but it has been produced, in the laboratory, from coal-tar derivatives, and further experiment may reveal a process for preparing the article at a sufficiently low price to compete with the natural colour.

Several preparations of indigo are in use:—(1) Sulphopurpuric acid, phenicine, or indigo-purple, is made by mixing 1 part of indigo with 4 parts of sulphuric acid (sp. gr. 1·845), and heating for ½-1 hour; the acid mass is thrown into 40-50 parts of water, when the purple falls down; it is collected on a filter, and washed with dilute hydrochloric acid. (2) Sulphindigotic acid is prepared by mixing indigotine, 1 part, with sulphuric acid (sp. gr. 1·845) 6 parts; the operation must be performed in a leaden vessel, cooled outside, and the indigo must be added by degrees, to avoid heating; the mixture is then left for 8 days, when the conversion will be complete. Fuming or anhydrous acid may be used, in less proportion, but the reaction is more difficult to manage. Weaker acid will require a longer period, say a month for “brown acid” (145° Tw.). (3) Sulphindigotic acids are transformed into neutral paste, or “carmine,” by neutralising with carbonate of soda, and washing the paste, on a woollen filter, with a solution of chloride of sodium (common salt).

Manganese Blue.—(1) Kuhlmann found a blue mass of manganate of lime in furnaces used for making calcium chloride by calcining a mixture of chalk and residues from chlorine making. The formation of this beautiful coloured manganate he attributes to the decomposition of the calcium chloride by steam, and to a certain solubility of the lime in undecomposed calcium chloride. Unsuccessful attempts to reproduce this result were apparently due to the lime not being under such favourable conditions for acting upon the manganese oxide as when it is in solution in the calcium chloride. As accidentally produced in reverberatory furnaces, the manganate of lime is of an ultramarine tint, and is insoluble in water though not permanent under its influence; it is acted upon by the weakest acids.

(2) Bong has proposed several formulæ for making manganese blues, the ingredients in each case being heated to redness in an oxidising atmosphere, taking special care to avoid iron. The following are his recipes:—(a) 6 parts soda ash, 5 of calcium carbonate, 3 of silica, and 3 of manganese oxide; (b) 8 of barium nitrate, 2 of kaolin, and 3 of manganese oxide; (c) 8 of barium nitrate, 3 of silica, and 3 of manganese oxide. The tint can be varied from violet to green by altering the proportions.

Prussian Blue.—This blue owes its colour to a combination of iron and ferrocyanogen. The commercial products vary very much in tint, depth of colour, covering power, and solubility. They are used for a variety of purposes, nearly all of which require the blue to have some property different from what it should have for other uses. For some purposes a green shade blue is wanted, for others a violet shade blue; some users want the blue to be soluble in water, others for it to be soluble in oxalic acid, others require it to be insoluble. Ordinary Prussian blue is insoluble in water, acids, and alkaline salts; bleaching powder has no action on it, and therefore it is largely used for tinting paper. It is capable of resisting acids; but alkalies, such as caustic soda, caustic potash, the carbonates of the same metals, lime and ammonia, decompose it, oxide of iron and a solution of a ferrocyanide of the alkali being formed, the decomposition being shown by a change of colour from blue to a reddish brown. On this account Prussian blues cannot be used for colouring soaps and alkaline products, or used as a pigment in distemper painting along with lime. The change of colour, from blue to brown, by the action of alkalies, distinguishes this blue from other blues.

Prussian blues require to be tested for their solubility in oxalic acid, by taking about 20 gr. of the blue, mixing with 1 oz. of water and 20 gr. of oxalic acid, in which a good blue ought to dissolve completely. Some brands are soluble in strong hydrochloric acid while others are not. It is decomposed by boiling sulphuric acid, and turned green by boiling nitric acid. For all the ordinary uses of a pigment Prussian blue is quite durable, and possesses a depth of colour and a definite tint which is proof against the destructive agencies of light and air; and though its covering powers are not great, it is one of the most important blue pigments in use.

When a salt of the higher oxide of iron is added to a solution of yellow prussiate of potash (or ferrocyanide of potassium) a blue compound is formed which is called Prussian blue. But this is not the method adopted for its commercial manufacture. In that case a ferrous salt, the proto-sulphate of iron, is added to a potassium ferrocyanide solution, the result of which is that a dirty bluish white precipitate is thrown down. On adding to this a little solution of bichromate of potash and sulphuric acid, the full deep blue is obtained. This is the industrial method of manufacturing Prussian blue.

Yellow Prussiate.—The first step is the preparation of the yellow prussiate of potash. The manufacture of this substance, although an industry of considerable importance, is comparatively little understood, either from a scientific or a practical point of view. At all events, many prussiate makers seem completely at sea with regard to the most favourable conditions for carrying on the manufacture, and there can be no doubt that in many cases great waste occurs, through ignorance of the various reactions which take place during the process. The raw materials usually consist of carbonate of potash, iron filings or turnings, and organic matters containing carbon and nitrogen—such as dried blood, woollen rags, horn, hair, leather scraps, &c. The most suitable substances for use are, of course, those containing the largest proportion of nitrogen. The following are the percentages of nitrogen in various kinds of animal matter:—

Animal matters always contain more carbon than is necessary for the formation of cyanogen by combining with the nitrogen also present. Consequently, when such substances are heated with pearlash, the excess of carbon reduces a portion of the carbonate to the metallic state, and this potassium combines with the cyanogen to produce potassium cyanide. The manufacture of yellow prussiate of potash may be conveniently divided into three stages: (1) The production of the molten mass technically known as “metal”; (2) the lixiviation; and (3) the crystallisation.

(1) The “metal” is made by fusing animal matters with pearlash, almost invariably with the addition of iron scrap. The animal substances are sometimes used in their original condition, whilst sometimes they are previously charred. Generally speaking, however, a judicious mixture of the fresh and charred materials has been found to give the best results. The charcoal which is left on carbonising animal matters contains a certain amount of nitrogen, decreasing in proportion as the temperature rises; but a smaller quantity of charcoal is also thereby produced. For example: 100 parts of rags carbonised at a certain temperature left 75 parts charcoal containing 12 per cent. of nitrogen, while the same rag carbonised at a higher temperature yielded 25 parts of charcoal, which contained only 2 per cent. of nitrogen. The animal matters employed should not leave much ash on ignition, as this would both thicken the mass and decompose a portion of the potash. In this respect sand is specially objectionable, for on ignition 1 part will decompose 2 of pearlash, owing to formation of silicate of potash. It is not necessary that the pearlash should be quite pure; in fact, a certain proportion of sulphate is stated to be useful, as it is changed into sulphide by ignition with the carbonaceous materials.

The theory of the formation of yellow prussiate of potash may be briefly stated as follows: The carbonate and sulphate of potash react with the carbon, nitrogen, and iron, forming in the first instance sulphide of potassium, which afterwards converts the iron into sulphide, whilst potassium cyanide is simultaneously produced. It should be here explained that ferrocyanide of potassium (yellow prussiate) is not formed during the ignition of the above mentioned materials, but results from the lixiviation of the fused mass with water, when the cyanide of potassium and iron sulphide decompose each other, producing ferrocyanide and sulphide of potassium. It is quite obvious that even if any ferrocyanide were produced during the process of fusion, it would almost immediately be decomposed, at the intense heat to which the mass is subjected, into potassium cyanide, iron carbide, and nitrogen gas. If any doubt were felt on this point, the experiments of Liebig conclusively prove that the formation of ferrocyanide takes place on dissolving the ignited mass in water, but not previously. Liebig found that if the fused mixture be allowed to cool, and then treated with moderately strong alcohol, potassium cyanide alone is extracted, and the residue when dissolved in water no longer yields ferrocyanide. As ferrocyanide is not formed during the process of fusion, the presence of iron in the preliminary stages may appear superfluous; but such is not the case. The presence of iron is necessary for two reasons, firstly, because the sulphate of potash which is generally present is converted into sulphide and bisulphide, and these, in the absence of iron, would decompose some of the cyanide of potash into sulphocyanate, thereby causing a loss of cyanogen so far as yellow prussiate is concerned; and secondly, because potassium bisulphide has a very corrosive action on the iron pot in which the fusion takes place. When iron is present it readily decomposes any alkaline sulphides, thereby preventing formation of sulphocyanate, and being itself converted into iron sulphide, which is again changed into prussiate by the action of the aqueous cyanide.

Pear-shaped iron pots were formerly used for fusing the raw materials. The arrangement now generally adopted in large English works consists of a series of iron pots almost hemispherical in shape, set in brickwork, and each heated by a separate fire and circular flue. These vessels are closed by iron lids, with apertures for the admittance of animal matters, the aperture being at once closed by a slide after each addition. Through every lid there passes a vertical spindle, carrying a set of blades for mixing the materials, and set in motion by a suitable shaft worked by steam power. Instead of the ordinary iron pots, reverberatory furnaces are often employed, especially in Germany. The reason for this preference is, that ordinary iron vessels are worn out in a comparatively short time, the destructive action being greatest on the under surface of the muffle. A much larger quantity of raw material can also be operated upon at one time if a reverberatory furnace be used. The mode of procedure depends to some extent upon the condition of the organic materials employed. If fresh, the muffle or furnace must be left open, so as to permit the mixture to be well and frequently stirred, and additions to be made at intervals until eventually ammonia ceases to be evolved. The furnaces are arranged in such a manner that when the carbonate of potash has once become fused the doors of the fire-place may be shut, and no fresh firing is required during the introduction of the animal matters. The molten mass is kept well stirred by means of a thick iron bar, suspended by a chain, and fixed in an aperture in the side of the furnace. By the use of this arrangement the stirring is much more easily and thoroughly effected than is the case with the old fashioned pots. Ordinary reverberatory furnaces cannot be used for the fusion, because the silica in the hearth would combine with the potash to form silicate of potash. Gas generators with air blast are now sometimes employed instead of ordinary fuel in the manufacture of yellow prussiate of potash. Several advantages are gained by operating in this manner, especially that of permitting the regulation of temperature and the admission of oxygen, so as to obtain an ordinary, a neutral, or a reducing flame, according to requirements. In the preparation of the “metal,” for every 100 parts of pearlash from 100 to 125 parts of fresh animal substances are required, together with 6 or 8 parts of iron in some form or other. The pearlash, or a mixture of 1 part of pearlash with 2 to 4 parts “blue salt” or “blue potash” (this substance will be referred to later on), is melted in the furnace and heated to bright redness, so that the temperature of the mass may not be reduced too much by the addition of the animal matters. These, in their original condition, or an equivalent quantity of carbonised materials, together with the proper proportion of iron, are then introduced—first pretty frequently, afterwards at longer intervals. Each addition of animal matter causes a somewhat violent frothing and escape of combustible gases, along with water and carbonic acid, and the whole becomes thick—not so much owing to the introduction of solid substances as by the fall of temperature, resulting from the production of such large quantities of gas. In order to hasten the decomposition, vigorous stirring must be applied. When the reaction is at an end, the semi-fluid mass is transferred to cast-iron dishes, and the furnace is again filled with carbonate of potash and heated. In this way four or five charges may be accomplished every day, and the process carried on continuously. The most favourable conditions for effecting the melting part of the process are attained when the heat approaches whiteness, and a bright, clear flame is produced as soon as the raw materials are introduced. According to one authority, woollen rags and good pearlash, with a small proportion of waste iron, have produced the largest yield of yellow prussiate, although even in this case two-thirds of the total nitrogen present was lost in the form of ammonia.

(2) Lixiviation.—The fused mass, if properly prepared, should yield about 16 per cent. of prussiate on dissolving in water. In this part of the process, the “metal” when cold is broken into lumps and placed in cold water mixed with the weak lyes from former operations. Heat is then applied until the temperature rises to about 180°-190° F., and the liquid is stirred vigorously so as to promote rapid solution, because some of the potassium cyanide is apt to be decomposed during lixiviation. When the solution attains a density of 30°-40° Tw. it is left to clarify, the heat being withdrawn. The clear solution is decanted, and evaporated in pans, which are generally heated by the waste heat of the furnaces. When it has a density of 54° Tw. it is run off into the crystallisers, where it deposits the crude salt.

(3) Crystallisation.—This is a very important stage of the manufacture, as it is the final process by which the crude prussiate is rendered sufficiently pure to be placed on the market. The impure substance is dissolved in warm water until the solution stands at 54° Tw.; after all insoluble matter has deposited, the clear liquor is placed in the crystallising vessels. These are occasionally made of wood; but when such vessels are used, the crystallised salt generally possesses a green colour, which is believed to be due to the tannin present in the wood. On this account cast-iron crystallisers are more frequently employed. The crystallisation proceeds slowly—often going on for several weeks in large vessels. The mother liquor is then drawn off, and if not too impure is used for dissolving fresh quantities of the crude prussiate. The ferrocyanide is deposited in crusts in the crystallisers; but by hanging lumps of the solid salt in the solution, long clusters of crystals may be obtained, and by suspending these in fresh prussiate lyes immense crystals are produced. From 100 parts crude prussiate about 90 parts pure potassium ferrocyanide are obtained, or sometimes in the case of purer materials 97 parts.

Sulphate of potash is often present in commercial yellow prussiate. The separation of this impurity is best effected on the large scale by evaporating the prussiate solution to a density of 62° Tw., at which point most of the sulphate will crystallise out. If the clear liquor be then drawn off, diluted to 52° Tw., and allowed to cool, almost pure potassium ferrocyanide will gradually deposit. This may be rendered absolutely pure by gently fusing the crystals, dissolving in water, and treating with a small quantity of acetic acid, which will decompose any carbonates and cyanides. On adding sufficient strong alcohol, the ferrocyanide is precipitated, and when crystallised once or twice more from water it may be regarded as chemically pure.

Blue salt.—This substance, to which we have previously referred, is a residue obtained in the manufacture of prussiate of potash. The last mother-liquor contains a large quantity of carbonate of potash, along with smaller amounts of hydrate, silicate, chloride, and sulphocyanate. It is concentrated until the liquid has a density of 90° Tw., when most of the chloride, silicate, &c., separates out, and the strong liquor containing the greater proportion of the carbonate is evaporated to dryness, and calcined in a reverberatory furnace. The dry residue constitutes the “blue salt” or “blue potash,” and contains from 70 to 80 per cent. carbonate of potash. It may be employed instead of pearlash, or mixed with it, for the next batch of yellow prussiate. The composition and amount of the insoluble residue left on lixiviation of the “metal” vary according to the proportions and character of the raw materials used. Other conditions being equal, horn gives the lowest percentage of insoluble matter on lixiviation.

The large proportions of potash and phosphates contained in the insoluble residues render them well suited for use in the manufacture of artificial manures. As already mentioned, when regarded from a scientific or economical point of view, the yellow prussiate industry is carried on under very imperfect conditions. In addition to the amount of potash, there is a very considerable waste of nitrogen, firstly, because the larger proportion of that element present in the animal substances is not converted into cyanogen at all, but passes off chiefly in the form of ammonia salts; and, secondly, because part of the potassium cyanide which is actually produced is lost by decomposition, and another portion is left in the mother liquor. It has been calculated that out of every 100 parts of ferrocyanide which should theoretically be obtained, 4 parts are lost when fairly pure materials have been employed, and 14 in the case of impure ingredients.

The following analyses indicate the percentage composition of two samples of insoluble residue:—

According to Karmrodt, the following proportions of the nitrogen contained in various animal substances are actually converted into cyanogen during the manufacture of yellow prussiate of potash:—

As is well known, human excreta contain a considerable proportion of nitrogen, and there seems no reason why this should not be employed in the manufacture of yellow prussiate. It is quite possible that municipal bodies might find this a convenient and profitable plan of disposing of a portion of the sewage with which they have to deal. It is obvious to all persons who have given this subject much consideration, that the nitrogen required in the manufacture of yellow prussiate of potash might be obtained with comparative ease from the surrounding atmosphere. Indeed, from a theoretical point of view this seems a charming process. About fifty years ago the Society of Arts awarded Lewis Thompson a medal in connection with this very process. Thompson ignited a mixture of 2 parts pearlash, 2 parts coke, and 1 part iron turnings in an open crucible for a considerable time at a full red heat. The resulting black mass was found to contain a large quantity of ferrocyanide, together with excess of carbonate of potash, &c. This process, or a similar one, in which a current of air was passed over a mixture of charcoal and iron saturated with carbonate of potash, was tried on a large scale for two years at Bramwell’s works at Newcastle. About 1 ton of yellow prussiate was made daily by this process; but it was not found to work profitably, and was eventually abandoned, chiefly, it is said, owing to the large amount of fuel required, and because the cylinders, whether of iron or fireclay, were not able to stand for any length of time the intense heat to which they were subjected.

The annexed illustrations, [Figs. 14 to 17], show the arrangement of a prussiate of potash furnace at Sir E. Buckley’s works, at Clayton, Manchester, which are well designed to prevent nuisance: A, iron pot; B, fire-place; a, cover of pot; b, stirrer; c, hinged pipe conveying vapours to the flues; d, flues surrounding the pot, and leading to the chimney-shaft; e, chain to lift up cast-iron vapour hood.

Brunquell, a German manufacturer, has criticised the present method of conducting operations, and proposes that it is necessary as far as practicable to aid the secondary formation of cyanogen by ammonia and incandescent charcoal, and to avoid loss of potash by using pure animal substances, and preventing contact with the solid products of combustion from the furnace. With this view he adopts a horizontal reverberatory furnace, the hearth of which is a cast-iron tray about 4½ ft. long, 4 ft. wide, and 3½ in. deep. The crown of the furnace is built as flat as possible, the working space is limited, and the charge is kept from contamination by the fire. Such a furnace, despite certain drawbacks, presents important advantages. Fuel is economised; the process is hastened so that seven or eight charges can be dealt with in a day, instead of only four; and the furnaces cost less and endure longer. The charge consists of 220 lb. potash, of which two-thirds is from evaporated mother-liquors, and one-third fresh; 44 lb. animal charcoal from the carbonisation of substances poor in nitrogen; 140-150 lb. of pure animal matters as dry as practicable; and 17½ lb. iron. The firing is urged and the charge is stirred till all the potash is fused, when the ash-pit is closed, and the damper turned on for charging half the animal charcoal. The firing and stirring are again pushed on till the proper consistency is attained, and potassium vapour begins to burn off. In this state the mass is ready to receive the animal

Figs. 14, 15, 16, 17.—Yellow Prussiate Furnace.

substances, those rich in nitrogen being first added in small portions at a time. Their effect is to render the mass hard, dry, and difficult of fusion, whereupon the remainder of the animal charcoal should be promptly introduced. After thorough agitation, the working door is closed for a short time, and the contents of the furnace are rapidly discharged into a covered iron pan.

The character of the animal matters employed varies so much that it is impossible to lay down hard and fast rules for the proportions of the several ingredients, or the duration of the roasting. Nor is the value of a raw material always in proportion to its richness in nitrogen, because the poorer material may waste less potash, consume less fuel, and require less labour. The addition of iron filings or turnings is useful only in prolonging the life of the cast-iron crucibles.

Combination of the Cyanide and Iron Solutions.—A great number of recipes are in vogue for combining the two solutions of ferrocyanide and an iron salt, both with reference to their proportions, and to the addition of foreign matters of various kinds. These variations in the formulæ give rise to distinct names for some kinds of Prussian blue, which will be referred to below. The ordinary common Prussian blue has a greenish tendency, and is chiefly made according to one or the other of the following directions:—

(1) Mix a solution of 100 lb. yellow prussiate with a solution of 100 lb. green copperas (ferrous sulphate) and 18 lb. alum, to which 9 lb. sulphuric acid has been added, and let the mixture stand for 2-3 hours, or until the solid portion has completely settled out. Decant the clear supernatant liquor, and well wash the precipitate with clean waters. Finally throw it on a filter and subject it to repeated disturbance, so as to ensure the admission of air to every particle, in order that the requisite oxidation may take place. The proportion of alum used is subject to very great variation according to individual fancy; it renders the subsequent grinding of the pigment a very much easier matter, but it causes the shade of blue to be paler than it otherwise would be.

(2) The simple solutions of green copperas and yellow prussiate in equal proportions are mixed together without any other ingredient being added, and the precipitate produced is washed, filtered, and aërated as in (1). It is, however, inferior by reason of the oxide of iron formed in the pigment spoiling the purity of the colour, and necessitating the treatment of the wet mass with hydrochloric acid, at some expense, for removal of the iron oxide.

Antwerp Blue.—This pale variety of Prussian blue has but little importance now. It is prepared by adding a solution of 4 lb. yellow prussiate in 5-6 gallons of water to one of 2 lb. sulphate of iron, and 1 lb. each of alum and sulphate of zinc in an equal quantity of water. The resulting pigment consists of a mixture of the ferrocyanides of iron, alumina, and zinc; it is washed, filtered, aërated, and dried as other forms of Prussian blue.

Bong’s Blue.—When cyanide of potassium is added to an acid solution of a copper salt, a red colour is produced, which has already been mentioned by different observers. The substance formed is very changeable, at least in the liquid where it is formed. It is decomposed by acids, alkalies, cyanide of potassium, and even decomposes spontaneously, the colour changing to yellow. It is precipitated by insoluble cyanides; hence when a dilute acid is added to the red solution, the dye is at once thrown down along with the cyanide of copper. If the precipitate thus obtained is treated with sulphuretted hydrogen, it is decomposed and the substance is set free. This substance can combine with iron, like cyanogen, so as to conceal the properties of the iron. This compound is very permanent, and has lately been studied by Bong, who gives the following directions for its preparation:—

Cyanide of potassium is added in excess to an acid solution of a copper salt until the red colour at first formed has disappeared, when a ferric salt is at once added. On the addition of the iron salt, of course, a copious precipitation of Prussian blue takes place, and the liquid again turns to a dark purple-red. To separate the colouring substance from the alkaline salts in the liquid, a dilute acid is added, which precipitates it and the cyanide of copper. This precipitate is combined with the Prussian blue, which also contains a considerable quantity of the colouring substance, and then treated with a boiling solution of carbonate of ammonia, in which it dissolves. As the cyanide of copper also goes into solution, it is separated by again precipitating it with an acid, and treating the precipitate with sulphuretted hydrogen. The colouring substance thus liberated now contains a certain amount of hydroferrocyanic acid, which is removed after neutralisation by acetate of lead. It is now filtered, and the purification is completed by precipitating with a silver salt and treating the precipitate with sulphuretted hydrogen.

This purple-coloured compound crystallises very indistinctly. To determine its composition, Bong precipitated it with acetate of copper. When dried at 212° F., the rose-coloured precipitate had the following composition: Carbon 24·31, nitrogen 28·04, hydrogen 1·88, iron 13·66, copper 17·67, oxygen 14·44. Total 100·00. These numbers correspond to the formula Cu, Fe Cy₄ (HO)₄.

This substance is likewise precipitated by salts of zinc, mercury, and silver. All these precipitates are pink or purple, very beautiful, and of remarkable brilliancy. They are soluble in alkalies. Iron salts yield no precipitate, nor do lead salts, except in the presence of ammonia, when a blue-violet precipitate is formed. When treated with sulphuretted hydrogen, these precipitates yield purple-red and acid liquids, which undergo change in the air, especially if warm, forming Prussian blue. When these liquids are neutralised with alkali, purple compounds are formed, which are permanent in the air, soluble in water, slightly so in alcohol, and insoluble in ether. Their colouring power is exceptionally great. These pigments will unite with ferrocyanides, and in its preparation such a compound is produced in considerable quantity; it is likewise of a purple colour, and gives a rose-coloured precipitate with acetate of lead. Both alone and in this compound it is very permanent; it resists the action of sulphurous acid, concentrated and boiling alkalies, and dilute acids, but is rapidly destroyed by chlorine and nitric acid. If this pigment could be prepared cheaply enough, it would probably be used with advantage in the arts, on account of its resistance to chemical reagents and light, the variety of its shades, and its brilliancy. It does not colour fibres directly, but can readily be fixed on them from slightly acid solutions, if they are previously mordanted with metallic oxides.

Brunswick Blue.—This pigment is made in pale, medium, and deep shades, and is an extremely useful colour, being very fine, requiring no grinding, thoroughly permanent in light and air, hardly acted upon by acids, but turned brown by alkalies, and liable on standing to separate into two portions—a white and a blue—the latter coming to the surface while the former sinks, and necessitating a thorough stirring of the paint before use.

It generally consists simply of barytes, or gypsum, or china clay, coloured by a small percentage of Prussian blue, with or without the addition of a lesser proportion of ultramarine. The barytes or other base is very thoroughly agitated in water, while a solution of green copperas and a solution of yellow prussiate are gradually added without ceasing the agitation. When the incorporation of the ingredients has been completely accomplished, the precipitate is settled, washed, filtered, and dried. Following are a few recipes:—

Pale. (1) 1 cwt. barytes, 1 lb. green copperas, 1 lb. yellow prussiate.

Pale. (2) 1 cwt. china clay, 2 lb. green copperas, 2 lb. yellow prussiate.

Pale. (3) 1 cwt. gypsum, 1½ lb. green copperas, 1½ lb. yellow prussiate.

Medium. (1) 1 cwt. barytes, 3 lb. green copperas, 3 lb. yellow prussiate.

Medium. (2) 1 cwt. china clay, 6 lb. green copperas, 6 lb. yellow prussiate.

Medium. (3) 1 cwt. gypsum, 4½ lb. green copperas, 4½ lb. yellow prussiate.

Deep. (1) 1 cwt. barytes, 5 lb. green copperas, 5 lb. yellow prussiate.

Deep. (2) 1 cwt. china clay, 10 lb. green copperas, 10 lb. yellow prussiate.

Deep. (3) 1 cwt. gypsum, 7½ lb. green copperas, 7½ lb. yellow prussiate.

In each case about 50-60 gallons of water are required.

To determine the amount of barytes present in a sample, boil about 50 gr. with caustic soda, filter, wash the residue free from soda, treat with sulphuric acid, well wash the insoluble residue, dry, and weigh.

Chinese Blue.—This well-known and favourite form of Prussian blue is prepared with great care, and is usually sold in fine powder or little cubes. Its composition is virtually identical with that of ordinary Prussian blue, but it is more free from impurities, and shows a fine bronze bloom or lustre on newly fractured surfaces. Being pure, it is entirely dissolved by oxalic acid; and its composition is about 52 per cent. oxide of iron, 43½ cyanogen, and 4½ water. In dyeing and calico-printing it is extensively employed. Its tint varies from greenish to violet, according to modifications in the method of manufacture, the chief difference being that yellow prussiate gives a greenish tone and red prussiate a violet.

The process of preparation is mainly as follows. In about 40 gallons of cold water dissolve 1 cwt. of green copperas selecting it carefully for freedom from insoluble oxide; add about 5 pints of sulphuric acid. This liquor very rapidly undergoes oxidation, by which oxide of iron is thrown down, and the solution is rendered unfit for making the best quality pigment. Therefore it should be prepared only immediately before it is used. In another vessel containing about 40 gallons of cold water, dissolve 1 cwt. of yellow prussiate (if a green shade is desired), or of red prussiate (if a violet tint is wished for). Even larger quantities of water may be used for the solutions, as the more dilute they are the finer is the colour precipitated and the greater the lustre on the surface of the finished pigment.

When the two solutions of yellow or red prussiate and acidified green copperas are brought together, a bluish-white precipitate is thrown down. This is allowed to completely separate itself, and then the clear supernatant liquid is drawn off.

The next step is to thoroughly oxidise the precipitate. This cannot be satisfactorily accomplished by utilising the oxygen of the atmosphere, as is done in other cases, because that method entails the production of a certain amount of oxide of iron, which prejudicially affects the purity of colour of the finished article. Of the chemical oxidising agents which are available, the most satisfactory in point of cost and efficiency is chloride of lime (bleaching powder). For each cwt. of green copperas, mix about 20 lb. of bleaching powder into a thin cream with water, and add it, in small quantities at a time, to the precipitate, constantly stirring so as to ensure the absorption of the whole of the chlorine by the blue. Without the addition be made gradually and under agitation, the chlorine will be generated more quickly than it can be absorbed, entailing a waste of gas and a noxious vapour to be breathed by the workmen. Sometimes the bleaching powder is added at an earlier stage, viz. to the green copperas solution, and in that case the blue assumes a violet tone.

After the addition of the bleaching powder solution to the bluish-white precipitate, it is acidified with hydrochloric acid, which develops the blue. When the whole has settled, the supernatant liquor is drawn off, and the blue powder is well washed and strained on a filter, then placed in pans and dried very gradually indeed in the dark, at a temperature never exceeding 130° F. The slower the drying the better is the gloss of the pigment. It is most essential that iron be excluded during the final grinding operation, or it may cause ignition of the mass, and its conversion into oxide of iron would speedily follow.

It has been proposed to treat the white precipitate (obtained in the usual manner from green copperas and yellow prussiate) by the chlorine contained in aqua regia (nitro-hydrochloric acid). The copperas, however, must be as free as possible from basic sulphate (oxide), which is ensured by keeping a little metallic iron in the acid solution of copperas. It is also desirable to effect the precipitation with crude prussiate, so as to avoid absorption of oxygen and premature development of the blue colour. Habich considers that the mistake is generally made of using too little copperas, and he has found that when 90 lb. of copperas have been added to 100 lb. of yellow prussiate, a drop of iron solution in the filtered liquor produces no precipitate, while the white precipitate has carried with it a certain proportion of prussiate, which can be washed out. He therefore proposes to avoid this waste by pouring the copperas solution into the prussiate solution, with constant agitation, till no further precipitate goes down, then adding one volume of the copperas solution equal to one-ninth of that already used. After fifteen minutes stirring, it is certain that all the prussiate carried down is decomposed.

The drained precipitate is blued (peroxidised) by adding aqua regia prepared several days previously, and in proportions depending on the strengths of the two acids. Generally, the aqua regia mixture will be 100 lb. of commercial nitric acid at 30° B. (containing 35·4 lb. of anhydrous acid) and 62·2 lb. commercial hydrochloric acid at 23° B. (containing 23·9 lb. of the anhydrous acid); and 40 lb. of this mixture will suffice for bluing the precipitate resulting from 100 lb. yellow prussiate. The addition of the aqua regia should take place in a wooden vessel with constant agitation.

According to another modification, the white precipitate obtained in the usual way is blued by adding a solution of perchloride of iron, which may be made from a hematite ore free from clay and carbonate of lime, or from rouge. The iron oxide, from whatever source, is ground to a very fine state, and treated with crude hydrochloric acid in a lead-lined tank, where the mixture remains for several days, and is constantly stirred. When saturated with iron the clear liquid is withdrawn for use. To receive it, the white precipitate is rapidly heated to boiling in a copper vessel, and is then transferred to a wooden vat, and the iron perchloride solution is stirred in till the desired tint is produced. The pigment is washed and dried in the ordinary way, while the supernatant liquor (essentially protochloride of iron) is poured over old scrap iron and used instead of copperas for a fresh batch of yellow prussiate.

A solution of perchloride of manganese may be used instead of perchloride of iron. Inferior qualities of manganese ore can be employed, and the residues left after treatment with hydrochloric acid may be washed and dried for sale as purified or peroxidised manganese.

Paris Blue.—(1) A synonym for the violet-tinted kind of Prussian blue.

(2) A series of compounds described below. [a] A thorough mixture of 2 parts sulphur and 1 part dry carbonate of soda is gradually heated in a covered crucible to redness or till fused; a mixture of silicate of soda and aluminate of soda is then sprinkled in, and the heat is continued for an hour; the little free sulphur present may be washed out by water. [b] An intimate mixture of 37 parts china-clay, 15 parts sulphate of soda, 22 parts carbonate of soda, 18 parts sulphur, and 8 parts charcoal, is heated in large crucibles for 24-30 hours; the mass is re-heated in cast-iron boxes at a moderate temperature till the desired tint appears, and is finally pulverised, washed, and dried. [c] Gently fuse 1075 oz. crystallised carbonate of soda in its water of crystallisation; shake in 5 oz. finely-pulverised orpiment, and, when partly decomposed, as much gelatinous alumina hydrate as contains 7 oz. anhydrous alumina; add 100 oz. finely-sifted clay, and 221 oz. flowers of sulphur; place the whole in a covered crucible, and heat gently till the water is driven off, then to redness, so that the ingredients sinter together without fusing; the mass is then cooled, finely pulverised, suspended in river-water, and filtered. The product is heated in a covered dish to dull redness for 1-2 hours, with occasional stirring. Colourless or brownish patches may occur, and must be removed.

Saxon Blue.—Following is a recipe for the preparation of this pigment, which possesses limited importance.

Dissolve 8 lb. alum and 1 lb. green copperas in 16 gallons of water. Add separate solutions of pearlash and yellow prussiate till precipitate ceases to go down. Collect the precipitate when it has completely settled; wash thoroughly, and dry.

Soluble Blue.—This term is applied to a variety of Prussian blue which, while possessing no difference in the matter of chemical composition, yet has the distinctive feature of being soluble in water, which the other varieties are not. It no longer enjoys the popularity it once had as a dye, on account of the severe competition of the coal-tar colours. Below are some of the most satisfactory formulæ for its preparation.

(1) Mix 10 lb. of Prussian blue thoroughly in about 10 gallons of cold water. Then add 5 lb. of yellow prussiate and let the whole mass boil steadily for several hours. Strain off the liquor and well wash the precipitate on a filter. Finally dry for use.

(2) Dissolve about 1 cwt. of red prussiate in water and make the solution hot. Prepare another solution of about 73 lb. of green copperas in hot water. Mix the two solutions together and boil them for about a couple of hours. Allow the solid matters to settle out, then put them on a filter and wash with clean water until a blue coloration manifests itself in the drainings. The blue residue is then dried as usual.

(3) Make one solution of 10 lb. of yellow prussiate, and another of 8 lb. of green copperas, water being the solvent in both cases. Mix these two solutions together and give them an hour’s boiling.

Add 3 lb. of a mixture of nitro-sulphuric acid, containing 2 parts of the former to 1 of the latter. Boil for another hour. Let the solid pigment precipitate itself thoroughly, and then filter, wash, and dry as in the other cases.

(4) Dissolve about 1 cwt. of perchloride of iron and 10 lb. of sulphate of soda in water. Also dissolve in another vessel 2 cwt. of yellow prussiate and 10 lb. of sulphate of soda. Pour the first solution into the second (never the contrary) and take care that the prussiate solution is always preponderant. The Glauber’s salt is useful in rendering the precipitation of the blue pigment more complete by reason of the insolubility of the latter in saline fluids. When the blue sediment is all thrown down it is drained off on a filter, and repeatedly washed till a blue tint appears in the wash-waters, when it is dried for use.

Turnbull’s Blue.—This is an old-fashioned name often applied, like the term Paris blue, to the violet shades of Prussian blue which have been prepared with red prussiate.

Ultramarine.—According to Rowland Williams, F.C.S., natural ultramarine is, perhaps, the most beautiful blue pigment known. It was formerly, and is now to a small extent, manufactured (chiefly for artists’ use) from lapis lazuli, a blue mineral which occurs, intermixed with limestone and iron pyrites, in Siberia, Thibet, and China. In order to obtain ultramarine from lapis lazuli, the roughly pulverised mineral is ignited, dipped into vinegar to remove carbonate of lime, and then reduced to the finest possible state of division. The powder is next mixed with a cement composed of rosin, linseed oil, white wax, and Burgundy pitch, and the resultant paste is worked under water until all the ultramarine is separated. The ultramarine is washed several times with water, and afterwards with alcohol, which removes any of the resinous compound which may have adhered. When treated in this manner, lapis lazuli yields from 2 to 3 per cent. of ultramarine. According to Clement and Desormes, lapis lazuli has the following composition:—

It will be seen, therefore, that ultramarine essentially consists of alumina, silica, soda, and sulphur, and may be regarded as a sodium aluminium sulphate, in combination either with polysulphide of sodium alone, or with a polysulphide and a polythionate of sodium. Clement and Desormes believe that the iron in lazulite (lapis lazuli) is an accidental impurity, and is neither essential to the mineral itself nor to the ultramarine derived from it. There is still some doubt on this point, however, many eminent chemists holding the opinion that iron is a necessary constituent of ultramarine blue.

Natural ultramarine has been almost entirely replaced by the artificial product, since methods have been devised for the manufacture of the latter on a large scale. The possibility of preparing artificial ultramarine suggested itself in a curious manner. About seventy years ago a French alkali maker noticed the occasional appearance of a blue coloured substance in his soda furnace. On analysis, Vauquelin found the substance to have the same chemical composition as lapis lazuli, and this incident led him to believe that ultramarine might be built up from its elements. Several years passed away before Guimet succeeded in manufacturing artificial ultramarine on anything like a large scale, but Gmelin is said to have prepared it in small quantity half a dozen years previously. There are four varieties of artificial ultramarine: (1) the pure deep blue, equal in colour to average native ultramarine; (2) pale blue; (3) violet or pink ultramarine; (4) green ultramarine. The latter is obtained in the first stage of the ultramarine manufacture, being the result of incomplete ignition of the materials employed. Ultramarine is generally manufactured by one of the following processes:—(a) “Sulphate”; (b) “Soda”; (c) “Silica.”

(a) “Sulphate” Ultramarine.—This may be prepared from sulphate of soda (Glauber’s salt), charcoal, and kaolin (china clay). The materials should be as free as possible from iron, and it has been found that clay having approximately the formula Al₂O₂ (SiO₂)₂ gives the best results. The clay and sulphate of soda must be thoroughly calcined. They are then intimately mixed with charcoal in the following proportions:—

Sometimes a portion of the sulphate of soda is omitted, and some carbonate of soda and sulphur added instead. The composition of the mixture then becomes:—

Caustic soda is also sometimes used instead of carbonate. These mixtures (whether sulphate alone or sulphate and carbonate) are made with a view to have the soda present in sufficient amount to combine with one-half the silica contained in the clay, and to leave sufficient soda to form polysulphide of sodium with a portion of the sulphur. There should then remain enough soda and sulphur to produce ordinary sulphide of sodium (Na₂S). If either of the two mixtures be ignited out of contact with air, a white compound is formed, which is sometimes termed white ultramarine. On leaving this exposed to the atmosphere for some time it becomes green, and on further ignition, with free access of air, it is converted into ultramarine blue. In actual working the carefully prepared mixture of the above mentioned materials is heated for several hours to a high temperature in fire-clay crucibles, only a limited supply of air being allowed to enter, and the temperature being eventually raised to a white heat. The product of this operation, when cool, has a grey or yellowish-green appearance. It is washed several times with water, dried, reduced to a fine powder, and then represents the green ultramarine of commerce. Stölzel found that green ultramarine had the following composition:—

Green ultramarine is transformed into blue by heating with about 4 per cent. of sulphur at a low temperature, with free access of air. Sulphur is afterwards added, if necessary, in small quantities at a time, and the heating is continued until the desired shade of blue is obtained. The mass is then powdered, the soluble matter (sulphide of soda, &c.) is removed by washing with water, and the blue is dried and assorted according to quality.

(b) “Soda” Ultramarine is sometimes made with soda alone (either carbonate or caustic), and at others with a mixture of soda and sulphate of soda. Rowland Williams found the following proportions of the respective ingredients to answer satisfactorily:—

The proportions for soda and sulphate of soda ultramarine have been previously given under “sulphate ultramarine.” The ignition is carried on in a manner similar to that already described. The resultant green product, owing to its avidity for oxygen, is partially changed into ultramarine blue by simple contact with the air. It is entirely converted into the blue variety by roasting with an additional quantity of sulphur. With care, ultramarine blue may be manufactured in one operation, by increasing the proportions of soda and sulphur.

(c) “Silica” Ultramarine is manufactured in the same way as soda ultramarine, except that, in addition to the other materials, silica to the extent of 5 or 10 per cent. of the weight of clay is employed. By this process, ultramarine blue of a slightly reddish tint is obtained in one operation. The method has, however, one decided drawback, viz. that the materials employed are rather liable to fuse during ignition. The faintly reddish hue of “silica” ultramarine becomes more intense according to the proportion of silica present. “Silica” ultramarine is said by some to be less readily attacked by acids and by strong alum solutions than ultramarine prepared by the “sulphate” and “soda” processes; but Rowland Williams’ experience does not confirm this statement. He mentions that good artificial ultramarine withstands the action of weak acids much better than is generally imagined. He had occasion to test many samples which resisted the action of dilute acids to a remarkable degree. Most strong acids, of course, decompose both artificial and native ultramarine, with evolution of sulphuretted hydrogen. Native ultramarine is, however, less susceptible to the influence of acids (both strong and dilute) than the artificial compound. This difference of behaviour is probably due to the fact that the former contains considerably less sulphur than the latter, and it is also possible that the constituents of natural ultramarine may be combined in a somewhat different manner from those of the artificial product.

Notwithstanding the large amount of research with reference to the chemical composition of ultramarine, the origin of its blue colour still remains in doubt. According to Wilkens (Ann. Ch. Pharm., xcix. 21), ultramarine consists of two portions, one of which is easily attacked by hydrochloric acid, and is regarded by him as the essential constituent, whilst the other portion is insoluble in hydrochloric acid, and contains variable proportions of clay, sand, oxide of iron, and sulphuric acid. From his analyses of the pure blue, Wilkens deduces the formula (2Al₂O₃ 3SiO₂) (Al₂O₃ 4SiO₂) Na₂S₂O₃ 3Na₂S:—

Wilkens regards the blue colouring principle of ultramarine as a compound of hyposulphite and sulphide of sodium. He considers the presence of iron is not necessary for the production of the blue; whilst Dr. Elsner, in a paper published in 1841, states that about 1 per cent. of iron (which he presumes to be in the state of sulphide) is essential. Rowland Williams asks whether it is not conceivable that the blue colour of ultramarine may be due to the presence of a small quantity of black sulphide of iron, most intimately combined with a colourless or comparatively colourless compound (such as white ultramarine), the whole mass (owing to the dilution of the black sulphide) showing a blue reflection.

Ultramarine is insoluble without decomposition in any known menstruum. According to P. Ebell (Ber. 16), ultramarine, when in the most finely divided state, will remain suspended in pure water for months. The liquid may be filtered unchanged through several layers of Swedish filter paper, and appears perfectly clear when examined in a ¾ in. layer, and on evaporation deposits the ultramarine as a lustrous coating on the sides of the vessel. Rowland Williams repeated the above experiment, and can confirm Ebell’s statement. This result shows the necessity of due precautions being taken during the washing of the ultramarine in the process of manufacture, otherwise a considerable amount of the finely divided blue may be lost. Ultramarine is largely used in calico printing for pigment styles, being fixed on the fibre by means of albumen. It is also employed for blueing linen and cotton, wax candles, lump sugar, &c. Ultramarine is not adulterated to a large extent, the chief sophistication being barium sulphate (barytes), and occasionally chalk and china clay.—(Rowland Williams, in Industries.)

Another writer in Industries says that the manufacture of ultramarine has perhaps hardly received the attention it deserves in England. The importance of the industry has been recognised in Germany, however, and though the palmy days of the trade, when the whole production was in the hands of a few firms, and the price was a matter of private friendly arrangement, are gone for ever, yet the business is in a flourishing state, and should prove lucrative if properly managed. It is a characteristically English failing to overlook branches of business not dealing with large quantities of staple commodities, and thus many of the smaller but remunerative industries have passed out of our hands. When one observes that almost every sheet of ordinary blue official paper is decolorised when accidentally brought into contact with an acid, betraying the fact at once that its colouring matter is ultramarine, one realises that a very considerable consumption for this and similar purposes must take place. Like most trades based upon chemical principles, the manufacture of ultramarine has recently made rapid strides, and some of the latest developments are recorded in a paper by J. Wunder, appearing in a recent number of the Chemiker Zeitung, which is worthy of some attention.

With most people not directly interested in it, the term ultramarine is taken to mean the blue pigment known under that name, the words being reckoned almost synonymous. Others, more erudite, recognise the existence of a green variety, but that the production of such colours as red and violet is possible is scarcely suspected. Of course the blue is the most important, but even that does not correspond to one specific substance, products of different shade being prepared by modifying the process of manufacture. As usually made, ultramarine is formed by heating together carbonate of soda, kaolin, sulphur, and charcoal, with limited access of air, the resulting pigment being green; this, on roasting with sulphur, becomes blue. If the operation be conducted with complete exclusion of air, so-called ultramarine white (in reality grey) is produced, which becomes green on further heating. Ultramarine blue capable of resisting the action of alum is sometimes required, and may be obtained by the use of a highly silicious charge and much sulphur, the burning being conducted in crucibles or in mass according to the purpose for which the pigment is required. The former process is costly, while the latter gives a product containing a good deal of free sulphur, which is objectionable for such purposes as calico printing. Removal of the excess of sulphur by heat or caustic soda is not feasible, as the colour suffers in either case, but a certain amount of success has attended experiments with sodium sulphide, the colour often brightening noticeably.

It is curious that chemically pure sodium carbonate, or such as is made by the ammonia-soda process, is not well fitted for the manufacture of ultramarine; Leblanc soda, containing a little caustic, is distinctly preferable. Sprinkling the soda with a strong solution of sodium sulphide before use is a good plan, and one easy to adopt. The more silicious the mixture the more difficulties are encountered, but the product is a deeper, richer colour, and withstands the action of alum and weak acids better. Excess of oxygen must be guarded against; many a manufacturer has had a batch turn out a hard cold blue, instead of a soft rich colour, solely on account of a too-excellent draught, an accident especially liable to happen in winter time. So much dreaded is this catastrophe that some makers habitually limit the air supply—smothering the neighbourhood with smoke, and wasting coal. The need for exact control here indicated points to a probable advantage from the use of gaseous fuel. Considerable economy has resulted from the use of the waste gases from one furnace serving for the preliminary heating of another; a better plan would probably be the introduction of regenerative heating.

The crude ultramarine as it comes from the furnace contains a large proportion of soluble salts, notably 20 to 24 per cent. of sodium sulphate, which have to be removed before it is merchantable. Usually, after grinding, it is simply stirred up repeatedly with hot water and the aqueous extract is siphoned off. That such a crude method should be in vogue at the present time is very significant of the ample margin of profit that must exist. By systematic extraction and filtration under pressure the washing may be effected with so little water that the solution is sufficiently concentrated to pay for evaporation by the heat of waste furnace gases, the recovered sodium sulphate serving to replace part of the raw material.

The quality of ultramarine largely depending upon its fineness, it is graded by levigation, the coarser portions being filter pressed, and the finest “floating” quality, which remains in suspension for an inconveniently long time, precipitated by the addition of a trace of an ammonium salt, gypsum, or even hard water, and filtered by the aid of a suction tube on the principle of an ordinary Bunsen pump.

The first successful attempt to produce ultramarine violet was made by Professor Leykauf in 1859. By heating ordinary ultramarine with calcium chloride in the presence of air and moisture, he obtained a violet-toned pigment, but it was not a full colour. The active substance in this change was probably hydrochloric acid, produced by the decomposition of the calcium chloride. Later experiments with other reagents, such as chlorine and gaseous hydrochloric acid, led to the following methods being devised. In the first, ultramarine blue is spread out on stoneware shelves in iron chambers and treated with a mixture of chlorine and steam at a temperature of 300° F. to 480° F. for about three hours. In the second, the plant is very similar, but at the bottom of the chambers are stoneware dishes, into which hydrochloric acid is poured from time to time. As the temperature is raised, copious vapours arise from these, evaporation being aided by a strong draught, and the ultramarine blue, after being kept at 428°-446° F. for some seven hours, becomes converted into a dull violet, which brightens on continuing the process with a temperature gradually falling to 320° F. The ultramarine violet produced by either of the above methods resists the action of lime, and is of general applicability.

The pigment produced by a third and simpler process, consisting merely in heating ultramarine blue mixed with salammoniac and a little sodium nitrate, is unfortunately not so stable. Another shade of considerable interest is a pure bright light blue, formed by heating the violet variety in hydrogen to about 536°-554° F. It has not yet been prepared on a commercial scale, but certainly merits the attention of manufacturers. An ultramarine red has been made by acting on the violet produced by either of the first two methods with the vapour of either nitric or hydrochloric acid at 275°-293° F., the sole essential determining condition being the temperature. Iron vessels could be used in the case of nitric acid at this temperature, but if hydrochloric acid were employed stoneware would have to be substituted. In the manufacture of the violet the temperature is above the limit at which hydrochloric acid acts on iron.

It is now only necessary for some successful experimenter to put on the market yellow and orange shades of ultramarine for almost the whole of the spectrum to be represented. The problem of the cause of the colour of ultramarine, attempts to solve which have been repeatedly made, seems increasingly difficult when its protean character is considered; but this from the industrial point of view is of secondary importance, provided all required shades can be produced with ease and economy. Nevertheless, it is certain that here, as in other cases, substantial technical progress would follow from adequate scientific investigation.—(Industries.)

Ultramarine was also made the subject of a very interesting paper, by Herbert J. L. Rawlins, read before the Society of Chemical Industry, in December 1887.

After referring to the native form, lapis lazuli, Rawlins goes on to observe that “analysis could give no clue as to the cause of the blue colour. To prepare it artificially became a great object, and the efforts in this direction were stimulated by the offer of prizes, amongst which was one of 6000 francs, offered by the ‘Société d’Encouragement’ of France, to be awarded to the discoverer of a method of making ultramarine, provided it did not cost more than 90s. per lb. How strange it seems to think of this in these days when the value has fallen to less than half that price per cwt.!

“As early as 1814, two German chemists, Tessärt and Kuhlmann, had observed the formation of a blue product in soda kilns and calcination kilns, but Guimet, in 1828, first discovered how it was produced, and gained the 6000 francs prize. He did not, however, publish his method, and grew immensely rich, although the price sank to about 16s. per lb. In 1828 he was producing at the rate of 120,000 lb. annually.

“About the same time, or, as is positively asserted by some, even prior to Guimet, Gmelin made the same discovery and published his researches in full, thus perhaps laying the foundation stone of the present supremacy of Germany in this manufacture.

“In spite of the valuable discoveries of Hoffmann, Unger, and others, our knowledge of the chemical constitution of ultramarine is very limited and uncertain, many different theories having been advanced regarding the cause of the blue colour.

“According to Wilkins, ultramarine is composed of two portions, one of which consists of two silicates of alumina with sulphite and sulphide of sodium, and is constant in its composition; the other being a mixture of variable quantities of sand, clay and oxide of iron, with sulphuric acid. The blue colouring principle he considers to be a compound of sodium sulphite and sulphide. Another ingenious theorist, Stein, in two papers published in the Jahresberichte in 1871 and 1872, concludes that blue ultramarine contains sulphurous, and not thiosulphuric acid, that neither sulphites nor thiosulphates are necessary to its composition, and that it owes its colour to the presence of black sulphide of sodium, which is formed at high temperatures by the action of sulphide of sodium on alumina—admitting, therefore, that it is not a chemical compound, but merely a mechanical mixture, the blue colour of which is due to the bodies composing it.

“Brunner considers ultramarine to be a compound of aluminium silicate, with sodium sulphate and sulphide; while Brünlin regards it as a double silicate of aluminium and sodium, in combination with pentasulphide of sodium. Green ultramarine he considers to be the same double silicate in combination with bisulphide of sodium.

“Again, according to Ritter, ultramarine contains a double silicate, not only associated with polysulphide, but also with thiosulphate of soda; and Schülzenberger, on the other hand, considers that it is a mixture of a double silicate with sulphite and monosulphide of sodium.

“Endemann considers that the blue colour is due to a ‘colour nucleus,’ consisting of unchanging proportions of aluminium, sodium, oxygen and sulphur, in each variety of ultramarine the proportion being different, while the rest of the sodium and aluminium and the whole of the silica merely act as a vehicle necessary to the preparation and existence of the colour. He considers that this ‘colour nucleus,’ in the case of white ultramarine, which he calls the ‘mother-substance in the manufacture of blue ultramarine,’ has the formula AlNa₄O₂S₂. By the action on two molecules of this of sulphurous acid gas, Na₂O is removed, and green ultramarine Al₂Na₆O₃S₄ is formed, which then, by the action of oxygen, which forms sodium sulphate, passes into the pure green compound, having the formula Al₂Na₄O₃S₃. In the ‘indirect process’ of manufacture, green ultramarine is converted into blue by being burned with sulphur. By this means Endemann considers that more sodium and sulphur are removed, and blue ultramarine Al₂Na₂O₃S₃ is formed. He considers that the other portion, not included in the ‘colour nucleus,’ differs in different samples. In one which he mentions it has about the composition 3Al₂O₃.5Na₂O.16SiO₂.

“But of all chemists who have worked on this subject, none has done more to increase our knowledge of ‘the blue marvel of inorganic chemistry,’ as he himself has called it, than Reinhold Hoffmann. His position of manager of the Marienberg Ultramarine Works, near Benscheim, in the Grand Duchy of Hesse, renders his acquaintance with the manufacture perfect, and his untiring researches on the subject have been well rewarded by results both interesting and valuable. He considers ultramarine to be a double silicate of sodium and aluminium, together with bisulphide of sodium, the variety poor in silica, characterised by its paleness and purity of tint, and easy decomposition by acids, having the formula 4(Al₂Na₂Si₂O₈) + Na₂S₄; while that rich in silica, characterised by its dark and somewhat reddish tint, and more difficult decomposition by acids, has the formula 2(Al₂Na₂Si₃O₁₀) + Na₂S₄. He also considers it very doubtful whether green ultramarine is really a chemical compound, and indeed it is now generally considered that the colour is only due to small traces of sodium salts in very intimate mechanical mixture with the blue variety, for by heating the green body for some time at 160° with water in closed tubes, it is converted into the blue product, and small traces of sodium compounds are found in solution in the water; and further, on heating blue ultramarine strongly with sodium sulphate and charcoal—that is, acting upon it with sodium sulphide—the green variety is formed.

“In a paper by Knapp, an abstract of which appeared in the Journal of the Chemical Society for March 1880, there are some curious facts recorded with regard to the colouring agent. It was noticed that when silicic acid was replaced by boracic acid, a blue, nearly as stable in its properties as that of ordinary ultramarine, was produced. It was found that a blue could be obtained without alumina being introduced. Hence silica without alumina, and alumina without silica, can be employed with a certain amount of success. The blue, however, formed without silica, is not so strong or stable as that formed with it.

“One very curious property which ultramarine possesses is its power of giving up its sodium in exchange for other metals. Thus, by heating blue ultramarine with a concentrated solution of silver nitrate in sealed tubes to 120° for fifteen hours, a dark yellow silver ultramarine is produced, containing about 46·5 per cent. of silver. This corresponds to about 15·5 per cent. of sodium, which is just about the amount that the original body contained.

“When this body is heated with an aqueous solution of sodium chloride to 120° in sealed tubes, about three-quarters of the silver is replaced by sodium, but the other quarter cannot be so replaced; in fact, blue ultramarine, when heated with silver chloride, takes up silver, and becomes green. But by heating silver ultramarine with sodium chloride in the dry way, at rather a higher temperature, the whole of the silver is replaced by sodium, but the ultramarine thus regenerated does not equal the original body in colour. The change is probably due to the loss of sulphur in the formation of the silver ultramarine.

“If in the above experiment potassium chloride be substituted for the sodium salt, and the temperature not allowed to exceed 400°, a bluish-green potassium ultramarine is formed. Barium ultramarine is a yellowish-brown product, zinc ultramarine is violet, and magnesium ultramarine is grey. These may all be obtained by acting on the yellow silver ultramarine with the corresponding metallic chloride.

“From the experiments of Dollfus and Goppelsröder some very striking differences have been brought to light between the three types of colour which they examined—namely, the blue, green and violet—in their behaviour with various reagents. Thus, an aqueous solution of caustic soda or potash does not act on the blue or green, but turns the violet to blue, and when heated with carbonic oxide the same result ensues. Many other reagents have the same effect on the violet variety, but when acted upon with sodium sulphide, the green turns grey, and when heated with potassium chlorate becomes darker and loses its brightness of colour. Dollfus and Goppelsröder attempt no explanation of these facts, but simply state them as results of their observations, and profess their inability to give any chemical formulæ for the three ultramarines, though they consider that there is sufficient proof that each has its distinct constitution. They give as their opinion, however, that they are double silicates of aluminium and sodium, in which a part of the oxygen is replaced by sulphur.

“Violet and red ultramarines are more bodies of scientific interest than of any practical use, as their colouring power is not sufficiently great. The violet variety may be prepared by exposing the underground blue product to chlorine gas under a high temperature, while the red may be obtained from the violet by acting on it, under a low temperature, by dilute nitric acid fumes.

“The first artificial method of producing ultramarine was that known as the ‘indirect process’—that is, first the manufacture of green ultramarine; and secondly, its conversion into blue. It was carried out as follows:—

“An intimate mixture of Glauber’s salts, china clay, and coal or rosin, finely ground together, was placed in crucibles and baked or burned in an oven for about six hours. It was then transferred to iron trays, and heated with flowers of sulphur to the point where the sulphur took fire, when it was allowed to burn itself out. By this second process the green was converted into blue. It was then washed, ground with water, and settled out, the first deposit being of a darker shade than the second, and the colour becoming lighter as the powder settled was finer in grind. This is essentially the method employed now at many German works—those at Marienberg, for instance—and produces what is known as “sulphate ultramarine,” distinguished by its pale shade and almost greenish blue tint.

“There are, however, some objections to the indirect process, and it was considered advisable to find a plan by which ultramarine could be made in bulk in a muffle furnace. The following is a method which is employed at the present time in some of the German works:—

“A mixture of china clay, carbonate of soda, sulphate of soda, sulphur, sand and charcoal or rosin, finely ground together, are placed upon the floor of a muffle furnace, being pressed down so as to present an even surface. The mixture is then entirely enclosed with fire-clay tiles, the spaces between which are filled in with thin mortar. When the oven is so charged, the front is built up, a small hole being left for watching the temperature of the flue between the tiles and the top of the furnace, and for drawing samples during the process, which is done through a corresponding hole in the front of the fire-clay tiles, temporarily closed with a fire-clay stopper. The oven is now heated, slowly at first, and afterwards more strongly, so that at the end of eight or nine hours it is at a dull red heat. It is kept at this temperature for about 24 hours, when the heat is raised so that a clear red glow is obtained, which is kept up to the end of the operation.