THE MANUFACTURE OF MINERAL
AND LAKE PIGMENTS

CONTAINING DIRECTIONS FOR THE MANUFACTURE
OF ALL ARTIFICIAL ARTISTS’ AND PAINTERS’ COLOURS,
ENAMEL COLOURS, SOOT AND METALLIC PIGMENTS

A Text-Book For
Manufacturers, Merchants, Artists and Painters

BY
Dr. JOSEF BERSCH

TRANSLATED FROM THE SECOND, REVISED EDITION

BY
ARTHUR C. WRIGHT, M.A. (Oxon.), B.Sc. (Lond.)

FORMERLY ASSISTANT LECTURER AND DEMONSTRATOR
IN CHEMISTRY AT THE YORKSHIRE COLLEGE, LEEDS

WITH FORTY-THREE ILLUSTRATIONS IN THE TEXT

LONDON
SCOTT, GREENWOOD & CO.
Publishers of the “Oil and Colourman’s Journal”
19 LUDGATE HILL, E.C.
1901

[The sole right of publishing this work in English rests with the above firm.]

PREFACE.

We know hardly another branch of chemical technology which has made such remarkable advances of late as the manufacture of colours; a large number of pigments have been recently discovered, distinguished by beauty of shade and permanence. Chemists are continually endeavouring to replace handsome and poisonous colours by others equally handsome but non-poisonous.

In writing this work I have endeavoured to give it such a character that it may be a text-book for the practical man, only those methods have been given which certainly lead to a good result; in the case of new pigments I have only described methods of preparation which I have myself found to give good results.

Since it cannot be imagined that any one quite ignorant of chemistry could successfully manufacture colours (colours being always made by chemical processes which occasionally are rather complicated), I have, therefore, presupposed a knowledge of the principles of chemistry. In the short sketch of the chemical properties of the raw materials used in making ordinary pigments, the principal properties of the materials used by the colour maker are given.

In order to make this treatise useful to dealers in and consumers of colours, the chapters dealing with the examination of pigments have been so arranged that the nature or adulteration of a pigment can be determined quickly and with certainty by any one.

Recipes, which originated at a time when empiricism ruled in chemistry, have been omitted, since they would only detract from the clearness of the matter.

As far as it is possible I have avoided the “recipe fetish,” and have endeavoured to make clear to the reader the chemical processes to which regard must be had in the manufacture of the different pigments. Since the appearance of the first edition there have been many valuable innovations in the mineral colour industry, to which regard has been given in preparing this second edition in so far as they possess a really practical value.

A critical examination of proposals and formulæ, which are found in large numbers in the journals, has been avoided, since I wished to keep for my book that character of a reliable text-book and book of reference which was ascribed to it in the form of its first edition.

Dr. JOSEF BERSCH.

TRANSLATOR’S PREFACE.

If excuse be needed for presenting a translation of Dr. Bersch’s book at so long an interval after the publication of the original (1893), it must be sought in the paucity of the English literature on the subject. It is hoped that the practical nature of the work will make it acceptable to the English reader.

The subject-matter of the original has been preserved in the translation without alteration or addition, with the exception of an unimportant change in the order of arrangement.

The metric system of weights and measures has been used throughout; for the convenience of those who are not familiar with this system, directions are given in an appendix for converting into English weights and measures.

The section on paint grinding (Chapter LXIX.) is perhaps somewhat incomplete; for a more detailed and modern account of this branch of the subject the reader is referred to Practical Paint Grinding, by Mr. J. Cruickshank Smith, B.Sc., shortly to be issued by the same publishers.

A. C. WRIGHT.

Hull, January, 1901.

TABLE OF CONTENTS.

CHAPTER I.
INTRODUCTION.

It is doubtful whether another branch of applied chemistry is recorded of so great an age as the colour industry; at the present time there is hardly a race on the face of the earth which does not make use of colours in some form, either for the decoration of their persons or surroundings. The art of preparing colours is as ancient as their use. It is true that we find from the most remote historical records that the so-called earth colours were almost solely employed, and principally those which exist ready formed in nature. But these natural colours also require their particular process of preparation before they fulfil their object, even though this be merely a mechanical operation, such as powdering or levigating. That the oldest nations of whom we possess lasting records, either written or otherwise, really understood the preparation of colours by chemical processes is shown by the common occurrence in the Egyptian mural pictures of figures clad in brightly coloured garments, a proof that the Egyptians not only understood the science of colour manufacturing, but also the more advanced art of fastening colours upon fabrics—dyeing.

The writings of the ancient Greeks, and in part also the scanty remains of their buildings, prove to us completely that they understood the use of colours to such an advanced degree that they already employed them for pictures as works of art. That the Greeks were also acquainted with the preparation of colours and dyeing follows from various passages from the classical writers, in which magnificently decorated rooms and beautifully coloured garments are often described.

Among the Romans, who were the pupils of the Greeks in the arts and manufactures, the prodigal luxury which existed in Rome, especially under the emperors, caused a great demand for colours, which were used in the most profuse manner for the decoration of house and attire. The Roman colour makers had advanced so far in their art that they could colour the human hair rose-red.

A glance at East Indian fabrics and pictures, or at the ancient Chinese buildings, whose colouring is a matter of marvel to-day, shows that the Oriental were not behind the Western nations in the discovery of colours and the art of manufacturing them.

In so old an industry it is not remarkable that great changes have taken place in the course of time. The thousands and thousands of experiments made by the alchemists in the attempt to prepare gold failed in their main object, but the tremendous expenditure of time and trouble in this work was not fruitless; upon the great mass of chemical facts discovered by the alchemists were laid the foundations of scientific chemistry. We find on reading the writings of the alchemists that the colour industry is indebted to them for an immense number of its products; the reason being that the alchemists worked by preference on metals, earths and mineral compounds, and from these substances a large number of colours are obtainable, of which many are still in use to-day, and, on account of their cheapness, will continue in use.

The period in which the painters were also the colour makers lies not far behind us. The preparation of many a colour of particular beauty was treated by the fortunate owner of the recipe as a great secret. It was sold by him at a great price. What a difference between that time and the present! There is now no painter among civilised races to whom it would occur to prepare his own colours; the chemical works provide them for him at a low price and in such a condition that they can be immediately used for painting. The Italian painters prepared the highly prized blue pigment, ultramarine, by laborious toil from the costly lapis lazuli; to-day, this same colour, more beautiful and deeper in hue, is made by several works, and sold at a price which bears no comparison with that of the colour obtained from the mineral. The latter was worth many times its weight in gold: a pound of the finest ultramarine can now be bought for a shilling or two.

We find a similar comparison in the case of the fine scarlet pigment known as vermilion: formerly the natural vermilion, cinnabar, was sold at a very high price; at the present time the finest vermilion, prepared artificially, can be bought at a low rate. It is no longer necessary for any one to use natural Chinese vermilion as an artists’ colour.

Whilst formerly mineral colours were used in great preponderance, we now know a great number of vegetable and animal colouring matters. The discovery of the sea route to India and the discovery of America had an important influence in this development. From these countries, as from other tropical lands, come the majority of the plants which contain colouring matters. The attempt to change these colouring matters into insoluble compounds led to the discovery of the lake pigments.

With the advance of chemical knowledge the number of colours grew apace; e.g., the discovery of chromium was of great importance to the colour industry: it presented us with a large number of new colours. To a more limited extent, the discovery and study of uranium, molybdenum and other metals were the occasions for the invention of new colours.

In more recent times, efforts in the colour industry have been especially directed to making colours more permanent and, at the same time, harmless. In the first respect, the position at present leaves much to be desired; but, as regards the second property, great advances have been made. The colours in use in former days were almost all very poisonous compounds; the greater number were derived from lead, copper, mercury or arsenic. More recently these poisonous substances have been in many cases replaced by innocuous materials, so that among the colours now in use, though the list is much more comprehensive than of old, there are but few poisonous to a high degree.

In all civilised states the use of poisonous colours has been much restricted by law, and in those cases in which an article is to be manufactured for use as food the employment of such colouring matters has been absolutely forbidden. For example, in Germany by the law of 5th July, 1887, concerning the use of dangerous colours in the preparation of foods and condiments, the application of the permissible colours has been exactly defined.

During the last decades the colour industry and, still more, dyeing have undergone a complete change. The momentous discoveries which have been made in these departments leave far behind the advances which have been made in other branches of chemical technology, the manufacture of explosives, perhaps, excepted. We allude here to the beautiful colours which have been made from coal-tar, colours which far surpass in beauty all hitherto known, and which we can already prepare in every shade and hue. Unfortunately, we can only employ the coal-tar colours, as such, in a restricted measure among the pigments; they are of more importance in dyeing. We use the term pigments here in the narrow sense of such substances which, when spread out on certain materials, provoke a certain sensation of colour. Dyeing is, on the contrary, that branch of colour chemistry which generally has for its object the simultaneous production of the colour and its fixation upon a fabric. This definition was at least applicable to the majority of the colours which were in use before the discovery of the coal-tar colours and their introduction into the industry. Since, however, the latter have acquired so great a preponderance in dyeing, it is no longer applicable, for the dyers use at present a large number of substances which are included in the narrow definition of pigments. The greater part of the coal-tar colours are substances which, in solution, when brought in contact with a fabric, adhere to it and colour it permanently.

According to their use and preparation, pigments are divided into a number of classes, and one speaks of painters’, artists’, enamel, porcelain and glass colours, also of oil, honey, water and cake colours. Although this division is important for trade purposes, it is of little moment for the colour maker, for he can prepare the same colour for both purposes, either for oil or water colour. What is of the greatest interest for the colour maker is the preparation of the pigment itself. The conversion of the prepared pigment into (oil or water) paint is unaccompanied by difficulties.

When we look for a practical classification for pigments, we find that there are colours which exist ready formed in nature, and others which can only be obtained by certain chemical processes, at times very complicated.

As regards the first group of pigments—those which exist ready formed in nature—the processes which they undergo at the hands of the colour maker are almost entirely mechanical treatments—grinding, sieving, levigating and similar operations—in order to convert them into such a condition that they can be used for painting. Since a large number of these pigments belong to that class of minerals which mineralogists call earths, these pigments have also been designated earth pigments, a term which we shall retain on account of its general use, although it is incorrect, since many of the so-called earth pigments are not obtained from “earths” in the mineralogical sense.

Among the pigments which are prepared by human skill many divisions can be drawn. A large number of pigments are prepared from mineral sources; an equally important number are derived from the animal and vegetable kingdom, the latter consisting of combinations of organic materials with certain inorganic substances. Some few pigments (putting aside the coal-tar colours) are simply organic products, as, for example, the majority of the black pigments, which consist of carbon.

The following classification is drawn up on the lines indicated above:—

1. Natural Colours or Earth Pigments.—Found ready formed in nature and requiring only mechanical preparation to be usable. A large number of handsome and also cheap colours belong to this class.

2. Artificially Prepared Mineral Pigments.—Obtained by certain chemical processes, and, according to their composition, either compounds of metals with sulphur, oxygen, iodine, cyanogen, etc., or of oxides with acids, i.e., salts.

3. Lakes.—Compounds of colouring matters from the animal or vegetable kingdom with a mineral substance, such as lead oxide or alumina.

As a fourth group we might take those colours which do not fall into the previous classes, as, for example, the black pigments composed of carbon; but since this division is not made in practice we shall not regard this species of pigment as a particular group, but shall discuss them in the proper place.

As an entirely new group of colours are to be classed those which are generally called coal-tar colours. These colours, which, at present, are the most important in dyeing and calico printing, are prepared from so-called organic compounds (more properly, carbon compounds). The manufacture of these colours is a separate branch of chemical industry.

CHAPTER II.
THE PHYSICO-CHEMICAL BEHAVIOUR
OF PIGMENTS.

In a work which, as its title indicates, is devoted to a description of the manufacture of pigments, the properties of those substances which are necessary for the preparation of colours cannot be exhaustively considered; we must, therefore, presuppose a knowledge of the elements of chemistry. We have to consider in this book the chemistry of colours; the reader will, therefore, not expect an exposition of general chemical laws; we shall only state certain facts which are of value to the manufacturer. With the description of the manufacture of each pigment and of the materials required for that manufacture, we shall still discuss the chemical processes which must be conducted in the preparation of the colours, so far as it is necessary in order to understand them. In this chapter we shall say a few words about the physical and chemical behaviour of pigments in general.

The great majority of pigments are prepared by the process of precipitation, generally by mixing the solutions of two substances, upon which an interchange of the constituents occurs and the less soluble compound separates in pulverulent form from the solution as a precipitate. Most of these colours are obtained by the admixture of the solutions of two salts; the preparation of the so-called chrome yellow may be taken as an example. In the preparation of this pigment, a solution of a lead salt, sugar of lead (lead acetate), is mixed with a solution of bichromate of potash, whereupon a precipitate of lead chromate (chrome yellow) is formed, whilst potassium acetate remains dissolved. The lead chromate is formed because the acetic acid has a greater affinity for potash than for lead oxide, wherefore an interchange of acid and base takes place, but the lead chromate being insoluble in water consequently separates in the form of a precipitate.

Many mineral pigments are produced in the form of precipitates by passing sulphuretted hydrogen or carbonic acid gas into certain metal solutions. In these cases a similar exchange takes place between the reacting substances to that given in the case of chrome yellow; the metals have a greater affinity for the sulphur or for the carbonic acid than for the substances with which they are already united, they unite with the former, and the new compound separates as an insoluble substance. We have examples of such compounds in cadmium sulphide, which is obtained by passing sulphuretted hydrogen into the solution of cadmium in an acid, and in white lead, which is formed by the saturation of a solution of lead acetate by carbonic acid.

Many organic colouring matters, soluble in water, have the property of forming compounds with metallic oxides, soluble with great difficulty, when their solutions are mixed with a salt of lead, tin or aluminium, and the oxide is separated from the solution by an alkali. The precipitates obtained in this way are insoluble compounds of the colouring matter and the oxide of the metal; they are called lake pigments, or, briefly, lakes. A large number of pigments, often of great beauty, is obtained in this manner. The lakes are widely used in all branches of painting and dyeing.

Of great importance for the quality of the pigment is the physical condition of the precipitate; this is either crystalline or amorphous, that is, non-crystalline. When a crystalline precipitate is examined under the microscope, it is seen to consist of very small, coloured, transparent crystals. The amorphous precipitates are, however, in such a fine state of division that even with the highest magnification they transmit little or no light, and consequently appear opaque. These different characters of precipitates have the greatest influence on that property of pigments which we call covering power. In consequence of its transparency, a crystalline precipitate will allow the colour of the surface upon which it is spread to appear through, hence it must be laid on much more thickly than is necessary with an opaque pigment, of which a thin coating is sufficient to make the colour of the surface beneath invisible.

How extremely important is the crystalline or non-crystalline nature of a precipitate in practice is seen by a consideration of white lead. This pigment, lead carbonate, can be made by mixing solutions of a lead salt and a soluble carbonate (soda); but in this case a lead carbonate of crystalline nature is formed, which, being transparent, is of so small covering power that this process has no application in the manufacture of white lead; but a far more troublesome method is used by which a non-crystalline product, amorphous lead carbonate, is obtained.

Many pigments are formed by burning (oxidising) metals, as, for example, zinc white; others are prepared by melting salts together, as Naples yellow; others again are formed by very complicated processes still partially unexplained, as is the case with ultramarine. In the manufacture of colours we find all chemical processes in use.

It may be here remarked that it is quite possible to manufacture some colours, indeed a large number, according to fixed directions, without any particular chemical knowledge being necessary to carry on the processes. Indeed, in works we find most processes being carried out by ordinary labourers who are quite destitute of any knowledge of chemistry. We must, however, add that we are convinced that any colour maker who works simply in a purely empirical manner, according to a stereotyped recipe, will never be in the position to raise himself above the position of a workman; he will not be able, when a slight mishap occasions a change in the ordinary course of the process, to devise a means of overcoming the defect, but will be compelled to dispose of the faulty product in the condition in which it exists. Such a manufacturer is in a condition of blind dependence on the chemical works and dealers from whom he receives the raw materials requisite for the preparation of his colours. If he should receive materials which contain impurities not to be detected by empirical methods, the inevitable result will be that the colours produced from them will not be equal to the standard. If, in making a colour which is the outcome of several processes, a workman once makes a mistake, the product will not be of the required quality.

On the contrary, if the manufacturer possesses a certain amount of chemical knowledge, it will not be difficult for him to ascertain the causes of a failure in a process, and, at the same time, to devise means by which the defects may be removed. The manufacturer is more and more in the habit of buying the chemicals which he requires for his manufactures rather than of making them himself. He should, therefore, be in a position to form an opinion as to the usability and purity of these substances, which will only be possible when he has the knowledge requisite for subjecting them to a chemical examination.

Although we shall presuppose, as we have said, that those who intend to concern themselves with colour manufacturing possess an acquaintance with the principles of chemistry, yet this book has been so planned that it may be of use (we hope) to the practical man who is innocent of chemical knowledge. On this account, we have devoted care to the description of those raw materials which are bought in large quantity, and to the simple investigation of their purity.

When the manufacturer has the advantage of a chemical education, apart from his endeavours to produce colours lacking nothing in beauty or depth of shade, he will direct his endeavours in two directions, in respect of which great advances are yet to be made—the permanence and harmlessness of his colours.

Many pigments possess the undesirable property of losing their brightness under atmospheric influences; many, indeed, fade away completely in the course of time. We have only to examine a picture some centuries old; in spite of the care bestowed on its preservation, we can say with certainty that, in the course of time, it will be so completely altered that nothing will remain of the original colours. It is the endeavour of the sensible manufacturer of colours to make only such as remain unaltered by atmospheric action, and also undergo no change when they are mixed with other pigments. Although it may be highly desirable that the painter should possess a knowledge of the chemical properties of the colours he uses, still it should be the first object of the maker to take care that he places on the market only colours which will remain as much as possible unaltered when used alone, and will remain undecomposed when mixed. This is, unfortunately, not the case with many colours now in use. We shall return later to this point, of such extraordinary importance to the artist.

The second point to be observed, is to produce only harmless colours. The advances of chemistry have made known to us a series of colours which have the advantage over others known for a longer period that they are non-poisonous. Unluckily, these harmless colours frequently fall behind the poisonous colours in brilliance, and generally they are more expensive. Here, too, is opened to the manufacturer a wide field of activity. The more completely poisonous substances disappear from the colours in use, the more widespread will be the use of colours. We should remark that the expression “poisonous colours” is to be used with a certain reserve. Many pigments which contain lead, copper, antimony, mercury, etc., are poisonous, because they contain poisonous metals; but poisoning with them will not readily take place on account of their insolubility. It is different with the very poisonous arsenic compounds, which should be removed from the list of colours in common use; many a misfortune caused by them would then be avoided.

Endeavours to produce innocuous colours have been more successful than the efforts after permanence. There are now very few commonly used colours which can be accounted very poisonous compounds, and which cannot be replaced by other colours of equal beauty. On the whole, we are now in the position to prepare harmless colours suited to most purposes. Special endeavours should be made to sell these, so that such cases of poisoning should not occur as, for example, caused by gingerbread which had been wrapped in paper coloured by emerald green.

CHAPTER III.
RAW MATERIALS EMPLOYED IN THE
MANUFACTURE OF PIGMENTS.

As we have mentioned before, the manufacturer of colours now generally uses materials supplied to him by chemical works. The purer these are, the easier it will be to work with them, and the finer will the colours turn out. We have indicated that it is important for the manufacturer to know accurately the properties of his materials in order to be able to estimate their value. Many substances required in certain cases must be made by the colour manufacturer, since, on account of their condition, they cannot form articles of commerce—chlorine and sulphuretted hydrogen, for example.

In addition to the substances which are not to be bought, there are others which do occur in commerce, but are sold at so high a price that the manufacturer is compelled to make them himself. This is the case with the cobalt compounds, from which many beautiful colours are made. The producers of these demand such prices that it is to the interest of the colour maker to prepare them for his own use.

In the following chapters, we shall deal with the more important raw materials which are employed in colour manufacturing, and shall restrict our remarks to what is of particular importance thereto. For more detailed accounts of these raw materials the reader is referred to the text books of chemistry, in which he will find them minutely described, in so far as they are chemical products.

The materials employed may be divided into assistants in the processes and components of the manufactured pigment. The assisting substances are those which are used in the manufacture of a colour without entering into its composition; from the component materials the colours are directly derived. For example, in the manufacture of Prussian blue, yellow prussiate of potash, an iron salt, water (in which the salts are dissolved) and nitric acid are used. In the blue obtained are contained portions of the iron salt and of the yellow prussiate, these are, therefore, component materials, whilst water and nitric acid are simply assistants, since they do not enter into the composition of the pigment.

In colour making a large number of assisting materials are employed, which comprise a considerable number of elements and compounds. Since these are of great importance for our purpose, we shall describe their properties, and, when necessary, briefly the method of preparation.

Among the component materials are to be reckoned a large number of salts of the alkaline earth and earth metals and of all the heavy metals. In addition, there are also the substances of animal or vegetable origin used in lake making.

In the description of the raw materials, if we were to overstep the line drawn here, we could include a great variety of compounds, those, for example, used in the manufacture of the so-called aniline dyes. These substances form, however, as we have stated, the object of a particular branch of manufacture, which forms a separate division of colour chemistry, but with which is not to be confounded what has been hitherto designated the manufacture of colours.

CHAPTER IV.
ASSISTANT MATERIALS.

Water, H₂O = 18.[1]—This substance plays a tremendous part in colour making; almost all the substances which are used in solution are dissolved in water; the removal from precipitates of admixed foreign bodies, the so-called washing, is always accomplished with water. The chemist does not understand by water quite that liquid which in general speech is so designated. We must consider the water which is at the disposal of the colour maker.

[1] We append the chemical formula and the molecular weight to the description of each compound.

Water, in the chemical meaning of the word, is a liquid composed only of hydrogen and oxygen, and leaving no residue when evaporated. Such water is not found in nature; it can only be obtained by distillation of well or river water. The water which falls in long continued rain, or is obtained by melting snow, is most nearly like distilled water; it contains only small quantities of dissolved substances, and generally such as would be without influence in colour making. Water of this description is available for but a limited use; the large quantities of water required in a colour works must be taken from springs or streams. These waters contain, however, more or less large amounts of dissolved salts, which act in a marked manner upon the substances dissolved in them.

In almost all spring and well waters is found carbonate of lime; such waters are called “hard”. River water contains generally little carbonate of lime; it is then called “soft water”. The influence of the carbonate of lime is especially evident when salts of lead, copper, iron and other heavy metals are dissolved in water; the carbonate of the particular metal gradually separates from solution, and the liquid becomes very turbid.

When only hard waters containing much lime are at the service of the manufacturer, turbid solutions are often obtained, which must be filtered before use. In many cases this can be avoided by adding milk of lime to the water in a large vessel; the free carbonic acid unites with the lime, and thus the carbonate of lime, which is only soluble in water containing free carbonic acid, separates as a fine precipitate. Water which has been treated in this way becomes clear after some time, through the deposition of the carbonate of lime; it is then soft water. In order to separate the carbonate of lime in this way, no more than the requisite quantity of milk of lime should be added, so that no lime remains in excess, since this would cause precipitates when salts of lead, copper, iron, etc., were dissolved. In many cases—for example, when lead or barium salts are dissolved—the lime contained in the water can be made harmless by slightly acidifying with acetic or nitric acid. Water which contains sulphate of lime (gypsum) is equally useless for many purposes, as, for example, the solution of lead and barium salts. These metals form insoluble compounds with the sulphuric acid, which render the solution turbid, and can be removed only with difficulty by filtering, on account of their great fineness. They are more easily removed by allowing to settle.

Water containing gypsum often contains in addition small quantities of sulphuretted hydrogen. However small the quantities of this gas may be, they still make the water absolutely useless for certain purposes in the manufacture of colours; for example, for the preparation of all pigments containing lead which are obtained by precipitation. The sulphuretted hydrogen forms black compounds with lead, copper, bismuth, mercury and other metals, which impair the brilliance of the colour. A colour made under these conditions is never clean, its hue is injured by the admixture of the black substance.

Water which contains much common salt (sodium chloride) is unsuitable for the solution of lead, mercury and silver salts. In consequence of the great affinity of these metals for chlorine, turbid solutions are obtained when their salts are dissolved in water containing common salt.

Some waters contain a considerable quantity of iron. Such waters deposit on evaporation, and often on standing exposed to the air, a brown powder of ferric hydrate, which would have considerable influence on the shade of a pigment. White pigments, in the preparation of which such a water is used, have always a brownish tinge; yellow and red pigments are also unfavourably affected.

Carbonate of lime and common salt occur in small quantities in every well water. The colour maker must do the best he can with such a water; its use will not particularly harm the shade of the colours prepared with it if the amount of the impurities is not very large. Water containing much iron is practically useless; the oxide of iron would injure the colours so much that it would not be possible to obtain brilliant shades. Water from wells in the neighbourhood of deposits of turf or cemeteries often contains considerable quantities of organic substances which act injuriously on the shade of pigments; such water should not be used in colour making.

The impurities in a water are more or less harmful according to the purpose for which it is to be used. Sulphate of lime is generally more injurious than carbonate of lime, since the precipitates which the latter causes in solutions of the salts of certain metals can be prevented by the addition of acids. This is not the case with sulphate of lime; when lead or barium salts are dissolved in water containing this substance, a precipitate of lead or barium sulphate is obtained, which is insoluble.

In dealing with the salts of costly metals, such as mercury or silver, it is better to dissolve them in distilled water, or, at least, very pure rain water. The rain water which runs from zinc or well tiled roofs is generally very pure; for practical purposes it may be regarded as free from carbonate and sulphate of lime, sulphuretted hydrogen and common salt. The colour maker should take care to obtain as much of this pure water as possible by erecting large rain-water tanks.

The less impurity a water contains the more useful it is for our purpose. After rain water soft river water is the best, and after this the softer well waters. All mineral waters distinguished by a high content of salts or gases are quite useless for colour making; for this reason sea water is disqualified.

An accurate analysis of a water is much too complicated for the manufacturer; it is sufficient for him to convince himself of the absence of certain substances. Water which, some time after the addition of a little tannic acid solution, acquires a clear green or a bluish to black shade contains much iron, and is useless. Water which coagulates a large quantity of a solution of soap in alcohol is very rich in carbonate or sulphate of lime. In order to decide approximately in what relative proportion these salts are present, a solution of barium chloride is added to the water so long as a precipitate forms. If this disappears completely on the addition of nitric acid, the water contains only carbonate of lime; if it only partly dissolves, sulphate of lime is also present. The presence of chlorine is shown by a considerable turbidity on acidifying the water with nitric acid, boiling and adding silver nitrate. If the precipitate obtained on the addition of a lead salt is not pure white, but discoloured, the water contains sulphuretted hydrogen, which has formed black lead sulphide. In order to test the water for organic substances, about a litre is evaporated to dryness in a porcelain dish, and the residue heated to redness; if it turns brown and black, and possibly gives off a smell of burnt feathers, the water contains much organic matter.

Pure water is coloured permanently red by a solution of potassium permanganate; but if it contains organic matter, the solution is decolourised after some time and a brown precipitate is deposited at the bottom. From the amount of this precipitate an idea of the quantity of organic matter present may be obtained.

It is only necessary to be very scrupulous concerning the quality of the water when it is to be used for the solution of salts or the extraction of dye-woods. For washing precipitates, which requires a large volume of water, there can generally be used, without detriment, water containing much lime, but it must be free from iron and sulphuretted hydrogen. The latter is particularly harmful to most of the lead colours, which would lose in beauty by washing with water containing this substance.

It is hardly necessary to say that the water used in colour making must be quite clear. Muddy river water must in every case be completely freed from the solid particles contained in it, either by settling or by filtering. Filters filled with well washed sand give good results for this purpose.

Chlorine, Cl = 35·5.—For some operations in colour making it is necessary to employ chlorine. This is a greenish yellow gas at ordinary temperatures, which is characterised by a suffocating smell and the energy with which it unites with most elements. On account of its injurious effects on man certain precautions have to be observed in preparing chlorine, and it is advisable to erect the apparatus necessary for its production in a separate room, so that the workmen are not injured by the gas.

Fig. 1.

Formerly chlorine was exclusively made in lead apparatus, because this metal is one of the least readily attacked. When such an apparatus is used for the first time a layer of lead chloride is formed, which, like a varnish, protects the metal beneath from further attack. [Fig. 1] represents an apparatus formerly employed for the preparation of chlorine in chemical works. In the upper part of the pear-shaped vessel, K, there are four openings, two of which, D and C, are provided with water lutes. This means that the opening is surrounded by a moat containing water, into which the rim of the cover dips, thus making a joint. Through the middle opening goes the axle of the stirring apparatus, R; in the fourth is a lead safety funnel, J. Solid materials are introduced through D, liquid through J; the tube C carries away the chlorine formed; the tube A, furnished with a stop cock, can draw off the fluid contents of the apparatus.

Since lead melts at low temperature, the apparatus cannot be heated over the fire without danger, therefore it is surrounded by an iron jacket, W, which is filled with water, or else the apparatus is heated by steam introduced into W. Larger quantities of chlorine are more conveniently prepared in an apparatus, of similar structure, made of stone or earthenware, which have the advantage over lead that they are not at all attacked by chlorine.

Fig. 2.

[Fig. 2] exhibits the construction of such an apparatus of medium size. It is constructed of sandstone or earthenware; the lid and some of the smaller parts can be made of either earthenware or lead. The pyrolusite is introduced at G in large pieces; H is the funnel for pouring in the acid; E K D, the steam pipe; C, the perforated false bottom upon which the pyrolusite lies; F, the delivery tube for the chlorine; J, the opening for running off the manganese chloride; a, the leaden cover.

To prepare chlorine, 1 part (by weight) of common salt, 1 part of powdered pyrolusite, 2½ parts of vitriol and 1¼ part of water are used. The salt and the pyrolusite are introduced through D into the apparatus ([Fig. 1]); the acid, diluted with the water, is poured in through the funnel, the materials are mixed by the stirrer and gently warmed until chlorine appears, when the application of heat must be considerably diminished or the chlorine will be violently evolved.

Pyrolusite and hydrochloric acid are now generally used for the preparation of chlorine, because the solution of manganese chloride, left at the end of the operation, is valuable.

If all the chlorine made in one operation is not at once required for the manufacture of a colour, it can be utilised by sending it into a box filled with slaked lime, which is converted into chloride of lime or bleaching powder. The liquid run away from the apparatus at the conclusion of the operation contains manganese and sodium sulphates, or manganese chloride as the case may be, and can be used for the preparation of manganese pigments.

Ammonia, NH₃ = 17.—Ammonia is obtained from chemical works in the form of a strong solution of ammonia gas in water, which is generally very pure. The density of an aqueous solution of ammonia is the smaller the more ammonia it contains, and thus the strength of a solution of ammonia can easily be formed by means of the hydrometer. The following table shows the percentage of ammonia, NH₃, in a liquid of known specific gravity at the temperature of 14° C.:—

The Hydrometer.—In the above table the percentage content of the ammonia solution is given according to its specific gravity, that is, according to the ratio between the weight of any volume of the liquid and the weight of an equal volume of water. According to scientific principles, only those hydrometers should be used which are graduated in specific gravities. In spite of all exertions in this direction, manufacturers have not yet been induced to use such instruments in every case. Hydrometers, with quite arbitrary scales, such as those of Baumé and Twaddell, are frequently found in works. These hydrometers generally only show that a liquid is of so many degrees on the particular scale, and the manufacturer in using them is restricted to the following out of a certain recipe which requires the use of a liquid of a certain strength which is expressed in degrees Baumé, etc. He does not learn by this how many per cent. of the particular substance are dissolved in the water when the liquid has a certain hydrometric strength.

For the sake of uniformity, it is urgently to be desired that all manufacturers who use the hydrometer to estimate the content of a liquid in ammonia, potash, soda, hydrochloric, sulphuric, nitric acids, etc., should employ simple specific gravities. This is desirable, because the percentage strength of a solution, corresponding to the specific gravity, can be at once accurately found from tables. On these grounds, in the present work, we have restricted ourselves to tables showing simply the specific gravities of solutions and the corresponding composition.

Sal Ammoniac or Ammonium Chloride, NH₄Cl = 53·5.—This substance comes into commerce in the form of a white crystalline meal, more rarely in the form of sugar loaves (crystallised sal ammoniac) or of flat cakes (sublimed sal ammoniac). It is usually very pure, since impure forms, generally containing much iron, are difficult of sale. At a particular temperature sal ammoniac is volatile; it is used in certain mixtures in order to prevent the temperature, on heating, from rising beyond a certain point. Like ammonia, it is more used in dyeing.

Ammonium Sulphide, NH₄HS.—This compound is obtained by leading sulphuretted hydrogen into ammonia solution so long as it is dissolved, and a test portion of the liquid still gives a white precipitate with a solution of magnesium sulphate. Ammonium sulphide decomposes by long standing in the air, sulphur being separated. It gives precipitates with the salts of certain metals, for example, iron, cobalt, manganese, zinc, nickel. These precipitates, which consist of the sulphides of the metals, are not formed by sulphuretted hydrogen in acid solutions.

Acids.

In colour making many acids are used for the solution of metals, the production of precipitates, for oxidations and so forth. Commercial acids, especially inorganic acids, generally contain not inconsiderable quantities of impurities which are injurious in the manufacture of many colours.

Hydrochloric Acid, HCl = 36·5.—The commercial acid (muriatic acid, spirits of salt) generally contains large quantities of iron, which colour it yellow—fortunately, in many cases, this is not a disadvantage, and also at times the iron can be removed from solutions made in the acid. Another impurity is sulphuric acid. This can be detected by diluting and adding barium chloride; if sulphuric acid be present, a white precipitate, or, at least, a cloudiness, appears.

Ordinary hydrochloric acid is a solution of hydrochloric acid gas in water. The strongest acid contains 42·85 per cent. of the gas, and has the specific gravity 1·21. The following table gives the strengths of acids of various specific gravities:—

Sulphuretted Hydrogen, H₂S = 34.—This is a gas of acid properties smelling like rotten eggs; it precipitates the sulphur compounds of many metals when led into the acid solution of the corresponding salt. This substance is seldom required in colour works, so that it is convenient to have an apparatus which permits of the preparation of any required quantity. [Fig. 3] represents an apparatus devised by the author, which is well adapted for the preparation of sulphuretted hydrogen. It consists of a small, wooden tub, on whose upper edge lies a thick paper ring, so that the lid may be pressed down air-tight by the screws B. Through the lid pass a tap-funnel, T, a movable screw, S, and a tube, R, to carry away the gas. On the screw S hangs a basket, K, by a handle; this is filled with pieces of iron sulphide as large as nuts. The tub is filled to about one-third of its height with a mixture of 9 parts of water and 1 part of sulphuric acid.

Fig. 3.

When sulphuretted hydrogen is required the basket is lowered by the screw, S, until it dips in the liquid; according as the basket dips more or less into the liquid a fast or slow current of the gas is obtained. When the gas is no longer required the basket is raised out of the liquid, and the evolution of gas at once ceases. The funnel, T, serves for the introduction of the liquid, the tap, H, for drawing off the iron sulphate solution, which can be used with advantage for the preparation of fine iron colours. The apparatus should not be opened so long as sulphide of iron remains in the basket.

Sulphuric Acid comes into the market in two different forms: oil acid; both are used in colour making. Oil of Vitriol, H₂SO₄ = 98, is a colourless, oily liquid of high specific gravity; it is generally tolerably pure, and contains, as a rule, only a small quantity of lead, the presence of which is indicated by a turbidity on largely diluting the acid. The amount of pure sulphuric acid in the liquid is practically determined by taking the specific gravity. The table indicates the relation between the specific gravity and the content of sulphuric acid.

Nordhausen Sulphuric Acid, H₂S₂O₇ = H₂SO₄ + SO₃, is generally a yellowish brown liquid, which gives off white fumes in the air. It contains varying quantities of sulphur trioxide dissolved in sulphuric acid. It often contains selenium, which separates as a red powder when the acid is diluted. The presence of this impurity does not interfere with the use of the acid for dissolving indigo, the only purpose for which it is required in the colour factory.

Nitric Acid, HNO₃ = 63.—This acid, which is used in the preparation of many colours, is distinguished by the readiness with which it gives up part of its oxygen, and thus converts metals like antimony and bismuth into oxides, and transforms other compounds into a higher state of oxidation. There are two kinds of nitric acid: ordinary nitric acid, a colourless liquid which is more or less pure; and fuming nitric acid, a yellow or orange-coloured liquid, fuming strongly in the air, which consists of a solution of nitrogen peroxide, NO₂, and nitric oxide, NO, in nitric acid.

Since the action of nitric acid chiefly depends on its oxidising properties, which are possessed by both kinds, it generally does not matter which is used. The usual impurities are chlorine and sulphuric acid; the presence of the first is shown by silver nitrate solution, of the latter by barium chloride, in each case added after diluting. When the acid is used for oxidations these impurities do not interfere, but nitric acid containing chlorine cannot be used to dissolve silver, because the chlorine would form insoluble silver chloride.

The strength of nitric acid is gauged by its specific gravity as given in the table.

Aqua Regia.—A mixture of 2 parts of hydrochloric acid and 1 part of nitric acid gradually turns orange or yellow and evolves chlorine. This liquid, which can dissolve gold in consequence of the free chlorine it contains (hence its alchemistic name, from gold, the “king of metals”), is used as a very powerful oxidising agent in colour making.

Carbon, C = 12, is the only one of the non-metallic elements to be mentioned here; by itself it forms a group of very important pigments, which we shall describe in detail at a later stage.

Carbonic Acid Gas, CO₂ = 44, is used in the manufacture of white lead, which it precipitates from lead acetate. This is, however, a particular branch of colour making carried on in special works. In describing this manufacture we shall return to the preparation of carbonic acid on a large scale.

Organic Acids.

The organic acids which are important in colour making are acetic, oxalic and tartaric acids.

Acetic Acid, C₂H₄O₂ = 58.—The very dilute form of this substance is known commonly as vinegar, the stronger as pyroligneous acid, and the purest as glacial acetic acid; the latter is, however, scarcely used. Formerly in colour making ordinary vinegar was used, but now pyroligneous acid is almost exclusively employed. This is distinguished by its strong empyreumatic smell, which, however, is without importance in colour making.

The strength of a solution of acetic acid cannot be found by a simple estimation of specific gravity, since the density does not increase with the percentage of acetic acid. If an accurate estimation of the strength of acetic acid is required, it must be obtained by neutralising the acid with an alkali by a process of volumetric analysis.

For practical purposes, where it is generally known whether a very strong or a more dilute acetic acid is under consideration, the following table, showing the connection between specific gravity and percentage strength, is sufficient.

Oxalic Acid, C₂H₂O₄.2H₂O = 126, has but a limited use in colour making. It comes into commerce in the form of more or less pure white crystals which readily dissolve in water, and are almost pure oxalic acid, containing only small quantities of oxalate of lime, the presence of which is without importance for the purposes to which the acid is put in colour making. Frequently, instead of oxalic acid, the acid potassium oxalate (salt of sorrel) is used.

Tartaric Acid, C₄H₆O₆ = 150, occurs as white or yellowish crystals, with a slightly burnt smell, which dissolve readily in water, and have a strong acid taste. The pure acid, which is white and without smell, is considerably dearer than the yellow variety. The impurities of the latter, which are small in quantity, are without influence on the colours prepared by its help, so that this form is generally used.

CHAPTER V.
METALLIC COMPOUNDS.

Alkalis.

The compounds of the alkali metals, potassium and sodium, play a considerable part in colour making. Formerly the potassium compounds were in general use, but the sodium compounds are at present obtainable at a much lower price, and in most cases they can be used equally well. Thus, in colour making, sodium compounds are chiefly employed. The cyanogen compounds are an exception; their potassium compounds are used exclusively.

Potassium Compounds.—The potassium compounds which are chiefly used in colour making are potassium carbonate (potashes, pearl-ash), potassium hydroxide (caustic potash), potassium nitrate (saltpetre), potassium tartrate (tartar), and potassium ferrocyanide and ferricyanide (yellow and red prussiate of potash). The cyanogen compounds have peculiar properties. We shall describe them separately after the potassium and sodium compounds.

Potassium Carbonate (carbonate of potash), K₂CO₃ = 138, is known commercially as potashes, a name derived from its former method of preparation by heating the ashes of plants in pots. At present potashes are prepared in large quantities from other sources.

Pure potash forms crumbling lumps with a slight yellow or bluish grey tinge, rapidly absorbing moisture from the air, and in time completely liquefying. The yellowish tinge is caused by oxide of iron, the bluish by manganese compounds. The so-called calcined potash has been strongly heated, and thus all organic substances contained in it have been destroyed.

Potashes are in no way pure potassium carbonate; they contain a mixture of all those salts which are found in plants—potassium sulphate and chloride, small quantities of silicic acid, etc. These impurities are rarely harmful, still it is generally necessary to know the percentage of pure potassium carbonate contained in potashes.

Although at present in commerce the strength of potashes is frequently guaranteed, it is still desirable to estimate the strength. It is sufficient for practice to allow a small quantity, say 100 grammes, to stand with an equal quantity of very cold water for some hours, then to filter and pour a similar quantity of water over the residue on the filter. The weight of undissolved substance subtracted from 100 gives with sufficient accuracy the weight of pure potassium carbonate contained in 100 parts of potashes. This method is founded on the fact that potassium carbonate dissolves readily even in cold water, but the other salts with difficulty. This procedure can also be used to obtain pure potassium carbonate from crude potashes; it is only necessary to filter and evaporate to dryness the solution obtained by pouring very cold water on crude potashes.

Potassium Hydroxide (Potassium Hydrate, Caustic Potash), KOH = 56.—The commercial variety consists of very deliquescent white lumps, generally containing a large quantity of impurities. On this account caustic potash, or rather a solution of it, is prepared in the colour works.

With this object 11 parts of potash, contained in a tub with an opening at the bottom, are mixed with 100 parts of cold water. Two hours later, the clear solution is run off into a clean iron pan, in which it is heated to boiling. To the boiling solution is added milk of lime prepared from water and 3·5 parts of quicklime. After the liquid has boiled a few minutes, a small portion is filtered and hydrochloric acid added to the clear filtrate; if no effervescence occurs, then all the potassium carbonate is converted into caustic potash. Should effervescence occur, milk of lime is added until a new portion no longer effervesces on the addition of hydrochloric acid. Then the pan is covered with a well-fitting lid, and the cooled liquid, if not required for immediate use, preserved in well-corked glass bottles.

The strength of a caustic potash solution can be found by means of a hydrometer. The following table shows the relation between the specific gravity of a solution and the percentage of caustic potash it contains:—

Potassium Nitrate (Saltpetre), KNO₃ = 101, consists of large crystals, which quickly dissolve in water. On heating it readily gives up oxygen, and thus finds use as an oxidising agent. In former times, when the colour manufacturer was compelled to make his own materials, saltpetre was of great importance in colour making; at present, when such materials are to be bought at low prices and no colour maker prepares his own, saltpetre is little used.

Potassium Bitartrate, C₄H₅KO₆ = 188.—This salt, known as tartar in large crystals, and as cream of tartar in the form of meal, is occasionally used in colour making. It is little soluble in cold water, but more easily in hot. The hot solution is generally used.

Potassium Bichromate (Bichromate of Potash), K₂Cr₂O⁷ = 295.—This salt is made in special works, by melting chrome iron ore with saltpetre and extracting the mass with water, when a yellow solution of potassium chromate is obtained; to this sulphuric acid is added, which unites with half the potassium, thus leaving potassium bichromate, which is obtained by evaporation of the solution in fine red crystals. These are purified by recrystallisation. At present, in place of the above method, calcium chromate is formed by roasting chrome iron ore with lime; the calcium chromate is then decomposed by a soluble potassium salt, thus forming potassium chromate.

Potassium bichromate is unaltered in air; it dissolves easily in water, and is of great importance in the preparation of many colours, in particular chromium oxide and the lead pigments. The commercial salt generally contains potassium sulphate, with which at times it is intentionally adulterated. The adulteration is detected by dissolving in water, adding half the volume of pure hydrochloric acid, and cautiously and carefully dropping in spirits of wine. A rapid action takes place, which is only assisted by warming when necessary. The red liquid changes to emerald green. If barium chloride be now added, a white precipitate is obtained in the presence of potassium sulphate.

Potassium Sodium Chromate, KNaCrO₄ = 279, is also used in colour making. Its solution is made by adding soda to a solution of potassium bichromate so long as an effervescence of carbonic acid occurs, and until the liquid turns red litmus paper blue; the solution of the double salt is yellow.

Chrome Alum, KCr(SO₄)₂.12H₂O = 499.—This salt occurs in commerce as beautiful violet crystals. It is obtained as a by-product in the manufacture of aniline and anthracene dyes, and may often be bought at lower prices than other chromium salts; 100 parts of water dissolve approximately 20 parts of chrome alum.

Potassium Ferrocyanide, K₄Fe(CN)₆.3H₂O = 422.—The potassium iron cyanogen compounds are made in special works, particularly in the neighbourhood of large towns, by melting potashes with nitrogenous organic substances and iron, washing out the mass and purifying the salt so obtained by recrystallisation. Potassium ferrocyanide (yellow prussiate of potash) forms large transparent crystals of a peculiar soft nature, which dissolve readily in water. It often contains considerable quantities of potassium sulphate, up to 5 per cent., and it is to be noted that the impurity is much the cheaper of the two salts. When barium chloride is added to a solution of the salt, a white precipitate forms if sulphate be present.

The behaviour of yellow prussiate towards iron salts is noteworthy. With ferrous salts, for example green vitriol (copperas), it gives a white precipitate which gradually turns blue in the air; with ferric salts, for example ferric chloride (“nitrate of iron”), it at once gives a blue precipitate.

Potassium Ferricyanide (Red Prussiate of Potash), K₃Fe(CN)₆ = 329, is obtained by passing chlorine through a solution of yellow prussiate until the liquid smells strongly of chlorine and no longer gives a precipitate with a solution of a ferric salt. The solution then contains potassium ferricyanide and chloride. The former is obtained by evaporating and allowing to crystallise.

Pure potassium ferricyanide forms beautiful dark red crystals, which readily dissolve in water. The solution gives a blue precipitate with ferrous salts, but only a brown colouration and no precipitate with ferric salts. Both yellow and red prussiate are used in the preparation of several much used colours, for Prussian and Chinese blues, and several others. All cyanogen compounds, with the exception of yellow prussiate, are extremely poisonous. The following table gives the solubility of potassium ferricyanide at different temperatures:—

Sodium Salts.—In chemical properties the sodium salts are very similar to the potassium salts, and, being cheaper, they are generally used in place of the latter.

Sodium Carbonate (Soda Crystals), Na₂CO₃.10H₂O = 286, is made in enormous quantities in great works and in a very pure state. It forms large transparent crystals, which effloresce in the air, losing a large quantity of water, and so falling to a white powder. Although this property does not interfere with the use of soda, since it is generally used in solution, yet efflorescence should be as far as possible avoided by keeping the salt in well-closed packages, because effloresced soda dissolves more slowly than crystallised, since it has to combine with water before it can enter into solution.

In the retail trade a form of soda is found which is adulterated with very large quantities of Glauber’s salt. This is recognised by the different form of the crystals. Manufacturers sell soda stating its strength. The colour maker should only buy with this guarantee.

Sodium Hydroxide (Sodium Hydrate, Caustic Soda), NaOH = 40, comes into commerce in the form of hard masses, the use of which would be very convenient for the colour maker if it were not often very impure. Thus it is better to prepare a solution oneself, which is accomplished in the manner given above for caustic potash.

Caustic soda and caustic potash have similar properties; they have a corrosive action on the skin, readily unite with carbonic acid from the air, and separate the heavy metals from their solutions in the form of hydrated oxides.

The strength of caustic soda solutions is given in the following table:—

Besides soda and caustic soda, few soda salts are used in colour making. However, sodium nitrate (Chili saltpetre), NaNO₃, is frequently used instead of ordinary saltpetre, from which it differs in being deliquescent.

Sodium Thiosulphate (Hyposulphite), Na₂S₂O₃.5H₂O = 248, is a common article of commerce, being much used by photographers; it is used in a few cases in colour making. It forms large crystals with a somewhat bitter taste, permanent in air and readily soluble in water.

Sodium Chloride (Common Salt), NaCl = 58·5, which has a little application in colour making, is sufficiently pure in the form in which it is generally used for household purposes.

Salts of the Alkaline Earth Metals.

The metals which are known as the alkali earth metals have much similarity with the alkali metals in their compounds, with the difference that their alkalinity is much less, and that their salts are much less soluble in water. For colour making the compounds of three of these metals—calcium, barium and magnesium—are used.

Calcium Compounds.

The most important calcium compounds are lime and carbonate and phosphate of lime. Carbonate of lime is used as a pigment, and will be dealt with in detail among the mineral colours; it will be but briefly described here.

Calcium Oxide (Quicklime), CaO = 56.—When chalk and limestone, which consist of calcium carbonate, are heated, carbonic acid is evolved, and calcium oxide, commonly called quicklime, is left. For our purposes only very pure quicklime is to be used. Its ordinary impurities are iron oxide and magnesia: the former is found in lime made from red or brown limestone; the latter in lime made from dolomitic limestone. The presence of oxide of iron is recognised by the reddish tinge of the quicklime; if magnesia be present, a small quantity of the quicklime, when mixed with a very large quantity of water, leaves an insoluble residue which consists of magnesia.

When quicklime and water are brought together they unite very energetically and form calcium hydroxide or slaked lime. According to the quantity of water used for slaking, either dry slaked lime, lime paste, or milk of lime is produced, all of which find a use in colour making.

Calcium Hydroxide (Slaked Lime), Ca(OH)₂ = 74.—In order to prepare slaked lime, which contains lime united with just the necessary quantity of water, the pieces of quicklime are sprinkled with water from a watering can. The water is rapidly taken up and the sprinkling is repeated until the lumps begin to fall to a fine powder; in the process the lime becomes very hot. The slaked lime is then passed through a sieve in order to separate the larger pieces of quicklime which have not been slaked; the powder must be kept in well closed packages, since it energetically absorbs carbonic acid out of the air.

If so much water is added to the lime that a homogeneous wet mass is formed which can be readily moved with a shovel, one has then lime paste, which can be conveniently kept in pits as the masons do; it may be stored in this way for many months without appreciable alteration, still it is better to keep it covered. To prepare milk of lime, so much water is used in slaking that a milky liquid is formed, or the lime paste is mixed up in the proper quantity of water. Slaked lime dissolves in 700 to 800 parts of water; on standing, the undissolved slaked lime settles to the bottom of the milk of lime: thus it is better to prepare milk of lime immediately before use, and to stir it well to prevent the settling of the solid particles.

Slaked lime in one of its forms is often used instead of the more costly caustic soda in order to precipitate metallic oxides from their salts.

At times one finds a too strongly burnt lime, so-called “dead-burnt” lime, which is very slowly slaked by water. Such quicklime is slaked by allowing it to lie in water for days, or by means of hot water, which accomplishes the slaking more quickly.

Calcium Carbonate, CaCO₃ = 100, is found naturally in large quantities as chalk, which consists of the skeletons of extremely small animals. By powdering and levigating, it is converted into a soft powder, which is used to lighten the shade of lakes and other colours.

Calcium Sulphate (Gypsum), CaSO₄.2H₂O = 172.—This mineral, when finely powdered, is added to some colours.

Calcium Phosphate (Bone Ash), Ca₃(PO₄)₂ = 310, is sometimes used to lighten the shade of certain colours which might be injured by calcium carbonate. It comes in large quantities as a fine, white powder from South America. Naturally, only quite white bone ash can be used; if not completely burnt it is grey, and will then impair the shade of colours with which it is mixed.

Magnesium Carbonate (Magnesia), MgCO₃ = 84, is also used as an addition to colours in order to obtain pale shades. It is most cheaply obtained by dissolving magnesium sulphate (Epsom salts) in water and adding soda solution so long as a precipitate forms, which is then washed and dried. The magnesium carbonate prepared in this way is a very fine, light powder, insoluble in water, which can be mixed with the most delicate colours without harming them. White magnesia is an extremely light powder, which may be used when it can be bought at as low a price as the above preparation.

Barium Compounds.—The raw material used for the preparation of barium pigments is either barium sulphate (barytes, heavy spar) or barium carbonate (witherite). The latter is much more rare than barytes, which is almost exclusively employed in the preparation of barium compounds. The barium compounds of particular importance for our purpose are barium chloride and nitrate.

Barium Chloride, BaCl₂.2H₂O = 244.—This salt is now a common article of trade, and can be bought at a low price. When pure it forms colourless crystals readily soluble in water. If the colour maker is able to get cheap barytes and fuel, it may be advantageous for him to prepare barium chloride himself.

To prepare barium chloride from barytes, the latter is very finely ground and levigated, intimately mixed with coal, and the mixture subjected to a very high temperature, when barium sulphide is formed, which is dissolved by washing out the mass with water and converted into barium chloride by adding hydrochloric acid, sulphuretted hydrogen being evolved.

The best method is to mix 4 parts of barytes with 1 part of bituminous coal and so much coal-tar that a plastic mass is formed, which is well kneaded and made into small cylinders 3 centimetres in diameter and 10 centimetres long. These cylinders are placed in layers in a cylindrical furnace with a good draught, which contains at the bottom a layer of coal 15 to 20 centimetres thick, then a layer of the cylinders, then again coal, and so on until the furnace is full. The lowest layer of coal is lighted, and the whole burnt at a bright red heat, when the barium sulphate is changed into sulphide. Hydrochloric acid is poured over the residue and the insoluble part, consisting chiefly of unaltered barytes, is used for the next operation.

Witherite (barium carbonate) can be converted into barium chloride in a very simple manner. Hydrochloric acid is added, in which it dissolves with the evolution of carbonic acid. The solution is allowed to stand twenty-four hours with excess of witherite; the whole of the dissolved iron is thus precipitated. The solution is then filtered, evaporated down and left, when pure barium chloride crystallises out.

Barium chloride and all soluble barium salts should only be dissolved in pure water (rain or distilled). Water which contains carbonates or sulphates always gives a turbid solution by precipitating barium carbonate or sulphate.

Aluminium Compounds.

The compounds of the earth metal aluminium play a very important part in colour making, since they form beautifully coloured compounds with many organic colouring matters. Formerly alum was the only material used in colour factories for the preparation of the alumina compounds; at present aluminium sulphate is used, and when it is sufficiently pure it is the most valuable material, because it contains the greatest quantity of alumina.

Under the designation of alum only one compound, the so-called potash alum, was at one time found in commerce, but now there are other alums, which contain soda or ammonia in place of potash. These salts are of equal use in colour making to potash alum. The preference is to be given to the compound which contains the largest proportion of alumina. The chief point to be observed in connection with alumina compounds for use in colour making is that they shall be free from iron, because iron oxide, which would be precipitated out of the solution along with the colours, in consequence of its red colour would spoil the shade of the pigment.

Aluminium Sulphate (Sulphate of Alumina), Al₂(SO₄)₃.18H₂O = 664.—Any manufacturer who can obtain cheap china clay (kaolin) and sulphuric acid can himself prepare this compound with advantage. The apparatus used for this purpose is an iron pan containing sand, in which is placed a large earthenware dish. In this dish are put very finely ground kaolin and strong sulphuric acid, and the mixture is heated so strongly that the acid boils, evolving heavy, white vapours. It is absolutely necessary to heat in this manner in order to avoid dangerous accidents. Sulphuric acid bumps so violently on boiling that it may even break a thick earthenware dish. The use of a sand bath makes the bumping harmless.

China clay, which consists of silicate of alumina, is decomposed by heating with sulphuric acid into silicic acid and sulphate of alumina. The original milky liquid becomes more transparent during boiling, and has at last the appearance of starch paste. Kaolin contains varying quantities of silica. The quantity of sulphuric acid necessary for its decomposition can only be found by trial. The quantities are chosen so that a small amount of kaolin remains undecomposed in order that the aluminium sulphate shall contain no free sulphuric acid.

When the decomposition is finished the pan is allowed to cool and the solid mass is brought into a vat filled with water, in which it is stirred until dissolved; then the liquid is left until the jelly-like mass of silicic acid has sunk to the bottom, when the clear solution of aluminium sulphate is drawn off and can at once be used.

If solid aluminium sulphate is required—and this is to be recommended when large quantities are to be prepared—the solution is evaporated in earthenware dishes until a portion solidifies when dropped on a cold plate. The molten aluminium sulphate is then cast in prismatic blocks, which are preserved in boxes. These blocks are of a pure white colour and very crystalline; they dissolve with difficulty in cold, but readily in hot water without residue. The solution has an acid taste, even when it contains no excess of sulphuric acid. When the blocks have a yellowish tinge, this denotes the presence of iron, and the solution must be freed from this impurity. Nowadays sulphate of alumina can be obtained so cheaply that it is hardly of advantage to make it.

The Alums.

These are double salts of aluminium sulphate and potassium, sodium or ammonium sulphate. There are also other double sulphates known as alums which, in place of aluminium, contain chromium, iron, etc., but they are not of interest here. It may still be observed that all alums, whatever their composition, possess the property of crystallising together from mixed solutions, so that crystals can be obtained in which every existing alum is contained.

The potassium, sodium and ammonium aluminium alums are used in colour making.

Potassium Aluminium Alum, KAl(SO₄)₂.12H₂O = 474.—This is the substance commonly called alum. Like all alums it crystallises in fine octahedral crystals, which at first are quite transparent, but slowly effloresce in the air and become covered by a white powder. It dissolves with difficulty in cold, but readily in hot water. The solution has at first a sweet taste, with an astringent after-taste.

Alum comes into the market in different forms, of which the following are the most important: as crystallised alum, in the form of large crystals united together; as alum meal, a coarse crystalline powder obtained by rapidly cooling and stirring a hot alum solution. On account of the larger surface this form dissolves more quickly than the large crystals. Roman alum is the name of a variety chiefly imported from Italy; it owes its reputation to its great purity—it contains a very small quantity of iron.

In order to prepare alum quite free from iron from the ordinary alum containing iron, it is recrystallised, that is, as much as possible is dissolved in boiling water and the solution quickly cooled with continual stirring; the small crystals so obtained are then washed with cold water. The residual saturated solution of alum, which contains the greater part of the iron, can be used for the preparation of those colours which are not injured by the presence of iron.

The solubility of alum in water varies greatly at different temperatures. The table gives the weight of alum dissolved by 100 parts of water at different temperatures.

When potash alum is heated it loses water, 75 per cent. of the total at 61° C.; at 92° C. it melts completely, and all the water is lost by continued heating at 100° C. The residue is known as burnt alum.

In alum the whole acidity of the sulphuric acid is not neutralised; the solution has always an acid reaction; if soda solution is added, the escaping carbonic acid causes the liquid to effervesce. If soda solution is added until a further addition would cause a precipitate, a solution of so-called neutral alum is formed which has no longer an acid reaction. Neutral alum is occasionally required in colour making. In preparing it the soda solution must be added with great care when the liquid is near its point of neutralisation. Any addition of soda solution after this point is reached will cause a separation of alumina. This is not desirable, since it is generally only wished to precipitate the alumina in combination with colouring matters.

Roman Alum.—Under this name, or that of “cubic alum,” a variety of alum is sold, generally at a rather higher price than ordinary alum, from which it is distinguished by its crystalline form. Ordinary alum forms octahedral crystals often the size of a child’s head, but cubic alum well formed cubes.

The property of crystallising in cubes may be imparted to any alum solution by the addition of a little potash. Much so-called Roman alum is made in German works in this way. When this alum contains very little iron it is quite equal in quality to the best Roman alum, for the higher value of the latter is entirely due to its small content of iron. The alum prepared in the province of Naples is still better than Roman alum; it contains less iron.

When alum is required for the preparation of lakes of bright and delicate shades, it is indispensable to use a preparation free from iron, because the brownish yellow oxide of iron would appreciably injure the shade. Alum, free from iron, is most simply prepared by dissolving alum in boiling water, running the boiling solution quickly through a cloth, and quickly cooling with constant stirring. The alum meal prepared in this way contains much less iron than the original alum, the iron salts remaining dissolved in the mother liquor. When this alum meal is collected and cold water poured over it to free it from mother liquor, it is generally sufficiently pure to be used for any purpose, but if not, it is again recrystallised. The alum liquors containing the iron are used for the preparation of colours which are not injured by the presence of iron.

To test alum for iron yellow prussiate of potash is used, which gives a blue precipitate with ferric salts. The test is carried out by dissolving 10 grammes of alum in 1 litre of water, placing the solution in a tall, narrow cylinder standing on white paper, adding 10 to 20 drops of a saturated solution of yellow prussiate, and well stirring. On looking down through the liquid, if a distinct colouration is at once evident, the alum contains much iron, and must be recrystallised; indeed, the crystals would generally be coloured yellow. On the contrary, if the solution does not show a blue tint until after standing several days, the alum contains but a small quantity of iron, and can be used for most purposes without further purification. Alum quite free from iron is a rare commercial article; the test will generally show a feeble blue colouration. If this is not intense and no blue precipitate is deposited at the bottom, the alum is tolerably pure, and can be used in colour works. The longer the time before the blue colouration appears the poorer is the alum in iron.

Soda Alum, NaAl(SO₄)₂.12H₂O = 458, is made in some alum works. It has the greatest similarity in properties with the potash salt, but is distinguished by a much greater solubility in water and more rapid efflorescence in air. Soda alum can be bought at varying prices; that containing iron is much cheaper than that free from iron. When the latter is to be bought at a fair price it is to be preferred to potash alum, since, as we shall show later, it contains a larger proportion of alumina.

Ammonia Alum, (NH₄)Al(SO₄)₂.12H₂O = 453.—This compound of aluminium sulphate and ammonium sulphate is now often met with; the more expensive potassium sulphate in ordinary alum is replaced by ammonium sulphate, which is cheaply obtained from the ammoniacal liquor of the gas works.

Ammonia alum is better for our purpose than potash alum since it contains more alumina, is generally cheaper and dissolves more easily in water. Unfortunately most commercial ammonia alum contains so much iron that it has to be recrystallised before it can be used in colour works.

One hundred parts of water dissolve at different temperatures the quantities of ammonia alum given in the table:—

Of the different alums, ammonia alum contains the largest and potash alum the smallest proportion of alumina. The composition of the three commonly occurring alums is given in the following table:—

Thus ammonia alum is to be preferred to soda alum and soda to potash alum, whilst the latter is used on account of its greater purity. In a colour works, in which large quantities of alum are used, it is advantageous to work with ammonia alum which is recrystallised on the works.

The alums and aluminium sulphate are the alumina compounds in ordinary use in colour works; aluminium acetate could also be used if it were to be had at a reasonable price.

Alumina, Al₂O₃ = 102, and Hydrate of Alumina.—Pure alumina, or rather hydrate of alumina, is required in the preparation of many colours. When the solution of an aluminium salt is precipitated by soda, the carbonic acid escapes with effervescence, and a gelatinous precipitate is formed which it is extremely difficult to wash clean. The precipitate, which consists of hydrate of alumina, shrinks very greatly in drying, and turns to a horny mass; when strongly heated it loses water, and becomes a white, insoluble powder of anhydrous alumina.

A variety of hydrate of alumina, heavy, and therefore easily washed, is obtained by boiling a solution of alum with a plate of zinc lying on a copper plate until all the alumina has separated. By collecting this on a filter and pouring hot water over it a number of times it is obtained quite pure.

Alumina plays a particularly important part in the manufacture of cobalt colours. When treated with a cobalt salt and heated it takes a fine blue shade.

In the foregoing those metals and their compounds have been treated which are extensively used in preparing colours without themselves forming coloured compounds. The chromates and prussiates are an exception to this. The ammonia compounds, the alkalis and alkaline earths, also the acids, are used in making many colours, although they do not contain colouring principles. The alumina compounds are in similar case. Themselves colourless, they form at the same time a carrier for the coloured compound and bring it into a suitable form for use as a pigment.

An example will explain what we mean by “carrier” of the colouring matter. Logwood contains a very handsome colouring matter which can be extracted by water. In order to be able to employ this colouring matter as a pigment it is combined with alumina, a compound insoluble in water being formed, which is called a lake. In this compound the alumina is to be regarded as the carrier of the colouring matter, which it has fixed in an insoluble form.

In dyeing, which in many respects is closely allied to colour making, the property of certain metallic compounds, themselves colourless, of fixing dyes is commonly utilised, the metallic compound being called the mordant. The fabric is first prepared with the metallic compound or mordant, and the colour then formed by bringing the mordanted material in contact with the colouring matter.

The “heavy metals” form, among their numerous compounds, a great number of coloured substances, and several of them are distinguished by a great wealth of coloured derivatives; for example, copper, chromium and cobalt form coloured compounds only. Although the use of pigments derived from the heavy metals has been considerably restricted in recent years by the discovery of a series of colouring matters which replace them, yet they are now, and always will be, of very great importance in the manufacture of colours. It is, therefore, necessary briefly to describe the various metals which are used in colour making, so that the manufacturer may know what metals produce harmless colours, permanent and unaltered by the atmosphere, and what do not.

The metals are divided into two great groups, designated, according to their specific gravities, the group of the light and of the heavy metals. The light metals comprise the alkali, alkaline earth and earth metals, whose important compounds we have just described. The specific gravity of each of these metals is less than five times that of water.

The heavy metals have a specific gravity exceeding 5; they are generally divided into groups, which are known by the name of the commonest metal in the group. These are as follows:—

To the zinc group belong the metals zinc, cadmium and indium, of which only the two first are of importance here. The iron group comprises iron, manganese, cobalt, nickel, chromium and uranium, all of which are used in the manufacture of colours. The antimony group contains antimony and bismuth, the latter of which is of little importance. To the tin group belong tin and the rare metals titanium, zirconium, thorium, niobium and tantalum; of these tin alone is important in colour making. In the lead group are lead and thallium; lead produces many important pigments. The silver group contains silver, copper and mercury, the two latter of which are important. Of the gold group gold alone is of interest, and its importance has been diminished by the discovery of far cheaper substances which replace it. In the platinum group, which contains platinum, iridium, rhodium, ruthenium, palladium and osmium, only platinum itself is used as a colour; it is employed in porcelain painting to produce the peculiar metallic shimmer known technically as “lustre.”

The behaviour of the compounds, of the metals mentioned above towards sulphuretted hydrogen is of the greatest importance to the colour maker, since on it depends the alterability of the pigments when exposed to the atmosphere. Many metallic compounds are unaltered by the sulphuretted hydrogen present in the air, whilst others are in a high degree affected by it and become gradually darker, so that their colour may in the end approximate to black.

The pigments containing lead, copper, mercury and bismuth are extremely susceptible to the action of sulphuretted hydrogen, by the action of which they form black compounds. Since colours which contain these metals are not permanent, but darken considerably, endeavours have been made for a long time to replace them by others not susceptible to the action of sulphuretted hydrogen. Thus it is desirable to manufacture only colours free from metals forming black compounds with sulphuretted hydrogen. For the same reason pigments which contain sulphur should not be mixed with those containing metals which form black sulphur compounds.

The rules laid down in the preceding paragraphs are of the greatest importance for the artist, for by following them he will succeed in composing a “permanent palette,” that is, containing only such colours as will not, by their composition, bring about the speedy ruin of the painting.

Although the majority of the compounds of the heavy metals are poisonous, some possess this property in an eminent degree. These are chiefly colours which contain arsenic, antimony, copper and lead. So far as it is possible, these colours should be dispensed with and harmless pigments sold in their place, though this is not always possible, since several poisonous colours cannot be replaced by innocuous ones.

Compounds of the Heavy Metals.

Zinc Compounds.—Zinc oxide, ZnO, and zinc sulphate, ZnSO₄.7H₂O, are the compounds of this metal used in the colour industry. Zinc oxide, which is used as a white pigment, is a powder which turns yellow when heated, and is not acted upon by sulphuretted hydrogen. Zinc sulphate (white vitriol) occurs as colourless crystals, or more frequently as greyish white crystalline masses. The freedom of zinc sulphate from iron is of particular importance; the commercial article is seldom satisfactory in this respect. In order to free commercial zinc sulphate from iron, the property of zinc hydroxide of precipitating iron oxide from neutral solutions may be employed. The zinc sulphate is dissolved in water, and ammonia added in small quantities until the precipitate of zinc hydroxide remains on stirring. When the liquid is left in contact with the precipitate and stirred up once or twice a day, if iron is present the precipitate will turn yellowish brown, owing to the separation of ferric hydroxide, and in the course of a few days all the iron will be removed from solution. The liquid should then give no blue colouration with yellow prussiate of potash.

Zinc oxide is used as a white pigment, zinc chromate as a yellow, and zinc cobalt compounds as green colours.

Cadmium Compounds.—Cadmium is a metal which possesses great similarity to zinc, with which it occurs in nature. In the preparation of cadmium compounds the metal is generally used. This is dissolved in dilute sulphuric acid, hydrogen is evolved, and a solution of cadmium sulphate obtained.

Cadmium is used in colour making only for the preparation of the beautiful cadmium yellows.

Iron Compounds.—These are of the greatest importance to the colour maker. Several, in which iron alone is the colour principle, are very valuable: ochre, rouge, Venetian red, sienna and umber, for example. Iron compounds are also used in the preparation of many colours. The most important is:—

Ferrous Sulphate (Green Vitriol, Copperas), FeSO₄.7H₂0.—This substance, which occurs commercially in a form of great purity at a very low price, is generally the starting point in the preparation of iron pigments. When pure, it forms fine sea-green crystals, with an astringent metallic taste, which are not poisonous and are easily soluble in water. After long exposure to the air, ferrous sulphate becomes covered with an ochre-coloured crust, consisting of basic ferric sulphate. The ferrous oxide contained in the green vitriol has united with oxygen and been converted into ferric oxide. The latter requires a larger quantity of acid than ferrous oxide for the formation of soluble salts, so that an insoluble basic salt is separated. The same thing occurs when a solution of ferrous sulphate is exposed to the air.

When green vitriol, or any other ferrous salt, is exposed to the action of oxidising agents, such as chlorine or nitric acid, the iron is rapidly changed into the ferric state. This transformation is of particular importance in the manufacture of certain blue pigments.

We give in a table the relation between the percentage of crystallised ferrous sulphate (FeSO₄.7H₂O) contained in a solution at 15° C. and its specific gravity:—

Yellow and red prussiate, which have been already mentioned, also belong to the iron compounds. They have been separately mentioned because the iron is contained in them in a peculiar form as a portion of an organic radical.

Ferrous Chloride, FeCl₂, may sometimes be used instead of green vitriol. When iron is dissolved in hydrochloric acid hydrogen is given off and a solution of ferrous chloride is obtained; but when rouge is dissolved in the same acid, ferric chloride is formed. When iron is dissolved in nitric acid, in consequence of the oxidising properties of this acid a ferric salt is obtained. Iron forms two series of salts: in the ferrous compounds the iron is in the same form as in green vitriol and the corresponding salts; in the ferric compounds the iron is contained in a higher state of oxidation. By powerful oxidising agents, as nitric acid or chlorine, ferrous compounds are converted into ferric.

Manganese Compounds.—Manganese (Mn) is a metal whose compounds show great similarity with those of iron, like which it forms two oxides (also others), manganous oxide (MnO) and manganic oxide (Mn₂O₃). The salts of manganous oxide are not oxidised in the air like those of ferrous oxide.

The raw material used in the preparation of manganese compounds is the mineral pyrolusite, which is manganese dioxide (MnO₂).

Manganese sulphate (MnSO₄) forms rose-red crystals containing varying quantities of water. The residues from the preparation of chlorine can be used as the material for the preparation of colours. According as pyrolusite and hydrochloric acid or pyrolusite, salt and sulphuric acid are used for this purpose a solution of manganous chloride or sulphate is obtained.

Manganese compounds have but a restricted use in colour making.

Nickel Compounds are generally coloured green, but they are not used as pigments.

Cobalt Compounds.—Among these are found many important pigments. All cobalt compounds are coloured; in beauty and variety of shade they can only be compared with those of chromium. In properties cobalt is very similar to iron and nickel.

The form in which cobalt is used in preparing colours is either cobalt nitrate, Co(NO₃)₂.6H₂O, or cobalt chloride, CoCl₂.6H₂O. Both salts are articles of commerce, but generally they are so dear that it is more profitable for the colour maker to prepare them direct from the cobalt minerals. A simple method for preparing cobalt compounds from the ores is therefore given. The most important cobalt ores are speiss cobalt, a compound of cobalt and arsenic, and cobalt glance, a compound of cobalt, arsenic and sulphur. The former mineral often contains only small quantities of cobalt, and it is advisable for our purposes to use cobalt glance, which contains from thirty to forty per cent. of cobalt. This mineral is first roasted, that is, is heated with a plentiful air supply, by which means the arsenic is driven off. On account of the poisonous nature of the arsenic vapours the roasting must be conducted in a furnace with a very good draught.

Under the name of zaffre, roasted cobalt ores come into commerce. These may be used in the preparation of cobalt compounds, by which means the operation of roasting is avoided. According to the quality of the ore which has been used to obtain zaffre, it contains a very varying proportion of cobalt. The varieties richer in cobalt must be used; they are technically known by the mark FS, or FFS (the best).

The roasted cobalt ores or zaffre are treated with fused acid potassium sulphate, when the salts of iron and manganese are decomposed, whilst cobalt and nickel sulphates remain unchanged. In a Hessian crucible are melted 300 parts of acid potassium sulphate, and 100 parts of the powdered zaffre are gradually added, mixed with one part of green vitriol and one part of saltpetre; the mixture is heated so long as sulphuric acid escapes. The mass is then boiled with water and the red solution treated with sulphuretted hydrogen so long as a precipitate is formed; this may contain copper, manganese, and bismuth. After filtering, soda is added to the boiling liquid; cobalt carbonate is precipitated, which can be converted into nitrate or chloride by solution in the corresponding acid. If cobalt sulphate is required, the solution, after treatment with sulphuretted hydrogen, need only be evaporated to crystallisation, when the sulphate separates in fine red crystals. Cobalt nitrate and chloride are very soluble in water; to obtain them their solutions must be strongly evaporated and quickly cooled whilst stirring. The crystals of cobalt nitrate and chloride absorb moisture from the air and deliquesce; they must be kept in glass vessels with well-ground stoppers.

The cobalt compounds which are to be used in colour making must be free from iron, nickel and arsenic, which would detract from the cleanness of the colours. If the precipitate produced by soda contains iron it is mixed with excess of solution of oxalic acid, and after a few hours the cobalt oxalate is filtered from the liquid, in which all the iron is dissolved. The cobalt oxalate can then be converted into nitrate or chloride by treatment with nitric or hydrochloric acids.

These salts form the material for the preparation of the cobalt compounds, a large number of which are used as extremely durable red, blue and green pigments; several of them, such as cobalt blue, cannot be exactly replaced by other pigments. On account of the industrial importance of the cobalt colours, these directions for the preparation of the soluble cobalt salts from the ores have been given with some detail. The preparation of the cobalt colours will be given in extenso later on.

Chromium Compounds.—As the name indicates, this metal yields numerous coloured compounds (χρῶμα, colour); in fact, only coloured chromium compounds are known, and the colours are most varied—yellow, green, red and violet. On this account the chromium compounds are among the most important used in colour making; a great number of colours are prepared by their aid. Chrome ironstone, as we have already stated, is the raw material for the preparation of chromium compounds. From it potassium bichromate is made on a large scale in special works, so that no colour maker is compelled to prepare chromium salts himself.

When the chromium pigments contain no metal blackened by sulphuretted hydrogen, they have the desirable property of being unaltered by the atmosphere. Like the cobalt compounds, they are distinguished by their great stability when heated; on this account, they have a large use in porcelain painting.

Molybdenum, Tungsten and Vanadium Compounds on account of their cost have a very limited use as pigments. Molybdenum compounds are obtained from molybdic acid; compounds of tungsten from the metal; and those of vanadium from ammonium vanadate.

Antimony Compounds can be used in the preparation of several pigments, but, on account of their behaviour towards sulphuretted hydrogen, the pigments cannot be regarded as really permanent, and their use is generally diminishing. The so-called antimony vermilion is the only antimony compound at all extensively employed.

Bismuth Compounds possess properties very similar to those of antimony. Only one bismuth preparation is used as a pigment, and this is very sensitive to the action of sulphuretted hydrogen, being changed into black bismuth sulphide.

Tin Compounds are employed in two ways: some are themselves colours, such as stannic sulphide (mosaic gold); others, themselves colourless, are used in making pigments, as stannous and stannic chlorides.

Stannous Chloride, SnCl₂.2 H₂O, is obtained when tin is dissolved in hydrochloric acid, hydrogen being evolved. Stannic Chloride, SnCl₄, is formed when tin is dissolved in a mixture of hydrochloric and nitric acids (aqua regia).

Tin compounds have, similarly to aluminium salts, the property of forming coloured insoluble compounds (lakes) with many organic colouring matters. Their use for this purpose is extensive.

Arsenic Compounds formerly played an important part in colour making. They were used in the manufacture of a large number of pigments, very beautiful but extremely poisonous. At the present time, we can, fortunately, entirely dispense with arsenic in colour making; the arsenic colours can be replaced by others equally handsome and less, or not at all poisonous. The most important of the arsenic compounds is the trioxide As₂O₃, or Arsenious Acid, commercially known as white arsenic. This substance is obtained in large quantities as a by-product in the extraction of several metals. It forms masses which are either glassy or have the appearance of porcelain. Freshly sublimed arsenic trioxide is glassy; this form gradually changes into the porcellaneous variety, it dissolves with difficulty in water. A strong solution can only be obtained by boiling for many hours.

The compounds of arsenic with sulphur, formerly extensively used as pigments, have now almost fallen into disuse.

Lead Compounds belong to the substances most largely used in making colours. Unfortunately all lead colours have two very important drawbacks. They are all very poisonous and at the same time extremely sensitive to sulphuretted hydrogen, so that they are very considerably altered by the action of the small quantities of that gas contained in the atmosphere of an ordinary dwelling. A striking example of this is seen in the lead paints used on the doors of water-closets. The paint, at first pure white, becomes gradually darker, and at last almost black, the lead compound having been changed into black lead sulphide.

On account of this great sensitiveness of lead compounds, it would be better if they could be excluded from the list of colours. Great care must be taken not to mix lead compounds with others which contain sulphur; a discolouration of the mixture would be the inevitable result in a very short time.

The oxides of lead and a number of its salts are themselves pigments, for example, litharge, PbO, red lead, Pb₃O₄, and white lead (basic lead carbonate). These pigments are prepared on a large scale in particular works. At this point only those lead compounds will be mentioned which are generally used for the preparation of other lead pigments; they are: lead sulphate, nitrate, acetate and chloride.

Lead Sulphate, PbSO₄, is formed when sulphuric acid or the solution of a sulphate is added to the solution of a lead salt; so obtained it is a white crystalline powder insoluble in water. This substance is generally not made in colour works, but is purchased from chemical works or dye houses, of which it is a by-product. In this form (lead bottoms) it is generally not sufficiently pure, but contains admixtures of sulphuric acid or aluminium salts, from which it is freed by washing. The lead sulphate is stirred up in water, the heavy precipitate allowed to settle, the wash water drawn off, and after repeating this process until the wash water no longer shows an acid reaction, the purified precipitate is dried. In this condition it is a heavy, white powder, and can alone be ground into paint. But on account of its crystalline nature, which reduces the covering power, such use is inadvisable.

Lead Nitrate, Pb(NO₃)₂.—This very important compound may be bought, but it is advisable to prepare it in the works. Water is placed in a wooden tub, then half the volume of nitric acid is added, and finely powdered litharge gradually stirred in, the liquid being kept in constant movement. When it is seen that the litharge is only slowly dissolved, the liquid is well stirred after each addition of litharge and then tested by litmus paper. When this is no longer reddened, the nitric acid is completely saturated, and the liquid contains only lead nitrate in solution. It is allowed to stand until the insoluble portions have settled, and then drawn off into another vessel where crystals of lead nitrate separate in a few days. If the salt be required in solid form, the solution may be evaporated in earthenware dishes; generally the solution is used as it is obtained.

Pure lead nitrate forms white crystals which are not particularly soluble in water, 1 part requiring 2 parts of water at the ordinary temperature. Lead nitrate is decomposed on heating, like all nitrates, and litharge remains. The solution of this salt is used in the preparation of those lead pigments which are obtained by precipitation, for example, chrome yellow.

Lead Acetate, Pb(C₂H₃O₂)₂.3H₂O.—The compounds of lead with acetic acid are of great importance. Two of these are to be considered: neutral lead acetate, commonly known as sugar of lead, and basic lead acetate. It may be advisable to manufacture both these compounds, the latter always. Neutral lead acetate comes into commerce in the form of colourless heavy crystals, which are often covered by a white powder of the basic acetate; they dissolve readily in water, and the solution has a sweetish taste, hence the name “sugar of lead”. Frequently the solution is very turbid; this is caused by the carbonates contained in the water. The turbidity may be removed by the addition of a little acetic acid. It is only economical for the colour maker to prepare sugar of lead when he can obtain cheap raw materials, lead or litharge and vinegar. Pyroligneous acid may also be used if it is colourless, its odour being without importance for this purpose.

The best method for preparing lead acetate from litharge is to place the vinegar in a tub and hang in it a strong linen bag filled with finely ground litharge. The tub is kept covered for a few days and its contents then tested with red litmus paper. When this is turned blue, the liquid is drawn off and vinegar gradually added whilst stirring, until blue litmus paper is just turned red. In this process, after neutral lead acetate has been formed, more lead oxide is dissolved, and the liquid thus acquires an alkaline reaction. The further addition of acetic acid reconverts the basic salt into neutral lead acetate.

Lead acetate solution may, with advantage, be prepared directly from metallic lead. For this purpose, lead is granulated by melting and pouring in a thin stream into cold water, where it solidifies in irregular pieces. This is done in order to give the lead as large a surface as possible. Three high narrow tubs placed one above the other so that liquid may flow from the highest to the middle, and from this into the lowest, are filled with granulated lead. Vinegar is placed in the uppermost vessel to cover the lead; after twenty-four hours it is allowed to flow into the middle, and after a further twenty-four hours into the lowest tub. In this way, a solution of basic lead acetate is formed, to which the necessary quantity of acetic acid is added to bring it into the neutral condition. If crystalline lead acetate is required, the liquid is evaporated down and quickly cooled with stirring, so that small crystals are formed. Generally, however, evaporation is unnecessary, since lead acetate is always used in solution in preparing colours.

If the lead acetate solution be not colourless, which is generally the case when coloured acetic acid is used, the defect may be removed by stirring a little bone black into the liquid and filtering after twenty-four hours, when a completely colourless solution is obtained.

It is always necessary to know exactly how much lead acetate is contained in the solutions prepared by these processes. The lead or the litharge is therefore weighed and the volume of the lead acetate solution measured. One hundred parts by weight of crystallised lead acetate are obtained from 62·54 parts of lead.

Lead acetate solutions must be kept in well-covered vessels. The carbonic acid of the air will turn the liquid turbid. The turbidity may be removed by the addition of acetic acid.

In the following table is given the percentage of crystallised lead acetate contained in solutions of different specific gravities:—

Basic Lead Acetate, Pb(C₂H₃O₂)₂.2PbO.—This salt may be regarded as a compound of neutral lead acetate with lead oxide. It is obtained by digesting vinegar with excess of litharge or with metallic lead, and also by treating lead acetate solution with litharge so long as the latter is dissolved. In the method last given, 100 parts of sugar of lead require about 118 parts of litharge to produce a saturated solution of basic acetate. The solution of this compound is alkaline; it turns red litmus paper blue. When exposed to the air, a turbidity is quickly produced owing to the separation of lead carbonate. In one white lead process basic lead acetate is the starting point of the manufacture.

Lead Chloride, PbCl₂, is seldom used in making colours. It may be prepared by stirring powdered litharge in common salt solution until the powder appears white. This, when washed, constitutes basic lead chloride. On adding hydrochloric acid to the washed mass until the liquid remains acid, lead chloride is obtained in the form of crystalline needles, which are very little soluble in cold, but more easily in hot, water.

Like any other soluble lead salt, lead chloride may be used in the precipitation of colours, but is seldom employed on account of its small solubility. Basic lead chloride was, at one time, used as a white pigment, and after melting, by which it is turned yellow, as a yellow pigment; it is no longer in use for these purposes.

Copper Compounds.—These are generally green or blue, and have an extended use in the production of colours. The metallic copper which is used in the preparation of colours is of the ordinary commercial quality. The impurities which it contains are generally so small in quantity that they are without importance for our purpose.

Copper Sulphate (Bluestone, Blue Vitriol), CuSO₄.5H₂O.—This is the commonest of the commercial copper salts, and on that account deserves our especial attention. It forms large sky-blue crystals, which effloresce slightly in the air, possess an unpleasant metallic taste, and are poisonous, like all soluble copper compounds.

Copper sulphate comes into commerce in a very pure form, but some qualities contain zinc sulphate or ferrous sulphate. The presence of zinc may be detected most easily by boiling the solution with excess of caustic soda, when copper oxide separates as a black powder, whilst zinc oxide remains dissolved. When sulphuretted hydrogen is passed through the liquid, a white precipitate of zinc sulphide is formed.

Iron is detected by passing sulphuretted hydrogen through the solution so long as a precipitate is formed, allowing the liquid to stand in a covered vessel, pouring it off from the precipitate, adding nitric acid, boiling and adding a solution of potassium ferrocyanide; a blue precipitate denotes the presence of iron.

Copper sulphate is rarely found which is quite free from iron and zinc. If these impurities are present in but small quantity, the zinc not exceeding 1 per cent. and the iron at most 0·5 per cent., the copper sulphate may be regarded as sufficiently pure for our purposes.

It may be here remarked that copper sulphate obtained from mints is generally of great purity, and hence particularly adapted for colour making.

When copper sulphate and other copper salts are dissolved in water, pale blue flocks of copper carbonate generally separate. This is due to the carbonate of lime contained in the water. An addition of a few drops of sulphuric, nitric or hydrochloric acid suffices to prevent this separation.

Copper Nitrate, Cu(NO₃)₂.6H₂O.—This salt may occasionally be obtained in colour works as a by-product. When nitric acid is poured over copper, there follows a copious evolution of nitric oxide, which produces brown fumes of nitrogen peroxide in air. Nitric oxide may be used to convert ferrous into ferric salts, a transformation required in making Prussian blue. In working in this way, nitric acid is poured over copper contained in a vessel provided with a delivery tube for the gas. The blue solution is at once used. Pure copper nitrate forms fine blue crystals, which very readily deliquesce in the air. The solution is therefore generally used as it is prepared.

Copper Acetate, Cu(C₂H₃O₂)₂.H₂O.—Copper is readily attacked by acetic acid. A number of salts are formed, of which some are used as pigments. For our purpose it will be sufficient to describe the manufacture of verdigris; few colour makers prepare any other copper acetate. A solution of this salt is most simply prepared in the following manner: Slaked lime is stirred with strong vinegar, and the solution left in contact with the excess of lime so long as it has a weak acid reaction. The solution, which contains acetate of lime, is poured into a solution of copper sulphate so long as a precipitate of sulphate of lime is formed. When this has been separated from the liquid, the latter is ready for further treatment. It contains only a very small quantity of dissolved sulphate of lime, which is not harmful in the preparation of colours.

In addition to the copper compounds mentioned here, several others were formerly used as pigments, or in the preparation of pigments which are no longer employed, because copper compounds of good colour can be obtained in a cheaper manner.

The same precautions should be taken in the use of copper colours which were mentioned for lead pigments; copper compounds are equally sensitive towards sulphuretted hydrogen, by which they are gradually discoloured.

Mercury Compounds.—Mercury forms compounds which, vermilion in particular, are used as pigments, and others which are used in the preparation of pigments. In many cases metallic mercury is the starting point in the preparation of the mercury compounds. The compounds commonly known as calomel and corrosive sublimate are also used.

Mercurous Nitrate, HgNO₃.—Nitric acid acts upon mercury in a manner differing according to its strength, and according to whether the mercury or the nitric acid is used in excess. In order to prepare mercurous nitrate, acid free from chlorine must be diluted at least with four times its volume of water, and the mercury must be in excess. On warming, the mercury is gradually dissolved, and, on cooling, the solution deposits colourless crystalline needles of the salt. A further crop of crystals is obtained after evaporating the solution.

When the action of nitric acid is over, the solution must be at once separated from the excess of mercury to prevent the formation of basic salts. If the salt has been properly made, it is completely soluble in water, but if a lemon yellow precipitate is formed on dissolving, the nitrate contains a basic salt, which can only be dissolved by warming and adding more nitric acid.

Mercuric Nitrate, Hg(NO₃)₂, is most simply obtained by warming mercury with very strong nitric acid. The heating must be continued until a test portion of the solution no longer gives a precipitate with hydrochloric acid. When this solution is evaporated, nitric acid is given off, and a salt crystallising in white needles is obtained, which dissolves in water with the separation of a yellow basic salt. It is therefore better to use the hot solution, which contains a little free acid, without evaporating.

Instead of mercurous and mercuric nitrates, the corresponding sulphates may be used, but the chlorides are more frequently employed since they can be readily obtained from the makers.

Mercurous Chloride (Calomel), HgCl, is obtained pure by adding common salt solution to a solution of mercurous nitrate and washing the precipitate, which is insoluble in water.