The manufacture of mineral and lake pigments

by 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.

PAGE
Chapter I.	Introduction	[ 1]
Chapter II.	The Physico-chemical Behaviour of Pigments	[ 8]
Chapter III.	Raw Materials Employed in the Manufacture of Pigments	[14]
Chapter IV.	Assistant Materials	[16]
	Water—Chlorine—Ammonia—The Hydrometer —Sal Ammoniac  or Ammonium Chloride—Ammonium Sulphide.
	Acids.—Hydrochloric Acid—Sulphuretted Hydrogen  —Sulphuric Acid: Oil of Vitriol, Nordhausen Sulphuric  Acid—Nitric Acid—Aqua Regia—Carbon—Carbonic Acid Gas.
	Organic Acids.—Acetic Acid—Oxalic Acid—Tartaric Acid.
Chapter V.	Metallic Compounds	[32]
	Alkalis.—Potassium Compounds—Potassium Carbonate  —Potassium Hydroxide—Potassium Nitrate—Potassium  Bitartrate—Potassium Bichromate—Potassium Sodium  Chromate—Chrome Alum—Potassium Ferrocyanide  —Potassium Ferricyanide—Sodium Salts—Sodium Carbonate  —Sodium Hydroxide—Sodium Thiosulphate—Sodium  Chloride—Salts of the Alkaline Earth Metals.
	Calcium Compounds.—Calcium Oxide—Calcium Hydroxide  —Calcium Carbonate—Calcium Sulphate—Calcium Phosphate  —Magnesium Carbonate—Barium Compounds—Barium Chloride.
	Aluminium Compounds—Aluminium Sulphate.
	The Alums.—Potassium Aluminium Alum—Roman Alum—Soda  Alum—Ammonia Alum—Alumina and Hydrate of Alumina.
	Compounds of the Heavy Metals.—Zinc Compounds—Cadmium  Compounds—Iron Compounds—Ferrous Sulphate—Ferrous  Chloride—Manganese Compounds—Nickel Compounds—Cobalt  Compounds—Chromium Compounds—Molybdenum, Tungsten and  Vanadium Compounds—Antimony Compounds—Bismuth  Compounds—Tin Compounds—Arsenic Compounds—Lead  Compounds—Lead Sulphate—Lead Nitrate—Lead Acetate  —Basic Lead Acetate—Lead Chloride—Copper Compounds  —Copper Sulphate—Copper Nitrate—Copper Acetate  —Mercury Compounds—Mercurous Nitrate—Mercuric Nitrate  —Mercurous Chloride—Mercuric Chloride—Silver Compounds  —Gold Compounds.
Chapter VI.	The Manufacture of Mineral Pigments	[71]
	White Mineral Pigments—White Lead.
Chapter VII.	The Manufacture of White Lead	[77]
	Manufacture of White Lead from Metallic Lead.—Dutch  Process: Casting the Lead into Sheets—Building  up the Stacks—Removal and Grinding of the White  Lead—White Lead Mills—Hard White Lead—Soft White  Lead.—German Process.—French Process: Preparation  of the Solution of Basic Lead Acetate—Preparation  of the Carbonic Acid and Precipitation of the White  Lead—Manufacture of White Lead by Means of Natural  Carbonic Acid.—English Process.—Other Methods.  —Oxychloride White Lead: Lead Sulphite—Lewis  & Bartlett’s White Lead Pigment.—White Lead-Antimony  Pigments: Lead Antimonite—Lead Antimonate.
Chapter VIII.	Enamel White	[116]
	Lithopone.
Chapter IX.	Washing Apparatus	[120]
	Filter Presses.
Chapter X.	Zinc White	[126]
	Griffith’s Zinc White—Tungsten White—White Antimony  Pigments: Antimony Trioxide—Antimony Oxychloride  —Bismuth White—Tin White—Manganese White  —Magnesia White or Mineral White—Annaline.
Chapter XI.	Yellow Mineral Pigments	[133]
Chapter XII.	Chrome Yellows	[134]
	Lead Chrome Yellow—Preparation of the Lead Solution  —Precipitation of the Chrome Yellow—The Pale  Chrome Yellows.
Chapter XIII.	Lead Oxide Pigments	[143]
	Red Lead, Minium.
Chapter XIV.	Other Yellow Pigments	[148]
	Cassel Yellow—Montpellier Yellow—Turner’s Yellow  or English Yellow—Naples Yellow—Antimony Yellow  —Calcium Chrome Yellow—Barium Yellow, Yellow  Ultramarine or Permanent Yellow—Zinc Chrome Yellow  —Cadmium Chrome Yellow—Cadmium Yellow—Lead  Iodide—Mars Yellow—Siderin Yellow—Aureolin  —Tungsten Yellow—Nickel Yellow—Mercury Yellow  or Turpeth Mineral—Yellow Arsenic Pigments—Lead  Arsenite—Thallium Pigments.
Chapter XV.	Mosaic Gold	[160]
	Chrysean.
Chapter XVI.	Red Mineral Pigments	[163]
	Vermilion—Black Mercuric Sulphide—Red Mercuric Sulphide.
Chapter XVII.	The Manufacture of Vermilion	[166]
	Dry Method: Chinese Vermilion.—Wet Method: Firmenich’s  Method—Liebig’s Process—Infusible White Precipitate  —Electrolytic Process—Mercuric Iodide.
Chapter XVIII.	Antimony Vermilion	[178]
	Appendix—Antimony Blue.
Chapter XIX.	Ferric Oxide Pigments	[180]
	Vogel’s Iron Red—Macay’s English Red—Indian Red.
Chapter XX.	Other Red Mineral Pigments	[186]
	Chrome Red or Chrome Vermilion—Cobalt Red—Cobalt  Magnesia Red—Cobalt Arsenate—Chromium Stannate  —Silver Chromate.
Chapter XXI.	Purple of Cassius	[190]
	Magnesia Gold Purple—Alumina Gold Purple.
Chapter XXII.	Blue Mineral Pigments—Iron-Cyanogen Pigments	[194]
	Chinese Blue—Prussian Blue—Mineral Blue—Soluble  Prussian Blue—Special Processes for the Manufacture  of Chinese Blue—Turnbull’s Blue—Antwerp Blue.
Chapter XXIII.	Ultramarine	[204]
Chapter XXIV.	The Manufacture of Ultramarine	[211]
	Preparation of Mixtures for Ultramarine—Ultramarine  Violet—Chlorine and Steam Process—Hydrochloric  Acid and Air Process—Ammonium Chloride Process  —Pale Blue Ultramarine—Ultramarine Red.
Chapter XXV.	Blue Copper Pigments	[226]
	Bremen Blue and Green—Neuberg Blue—Lime Blue  —Payen’s Mountain Blue—Oil Blue—Copper Hydroxide.
Chapter XXVI.	Blue Cobalt Pigments	[230]
	Cobalt Blue, Thénard’s Blue, Cobalt Ultramarine,  King’s Blue, Leyden Blue—Cæruleum—Cobalt  Zinc Phosphate.
Chapter XXVII.	Smalts	[233]
	Preparation of the Charge—Fusion of the Charge  —Grinding the Fused Mass—Tungsten Blue  —Tessié du Motay’s Blue—Molybdenum Blue.
Chapter XXVIII.	Green Mineral Pigments—Green Copper Pigments	[240]
	Green Copper Pigments: Copper Carbonate—Copper Arsenite  —Scheele’s Green—Swedish Green—Brunswick Green  —Green Verditer—Neuwied Green—Copper Oxychloride.
Chapter XXIX.	Emerald Green	[244]
	Manufacture of Emerald Green from Verdigris—Manufacture  of Emerald Green from Copper Sulphate—Mitis  Green or Vienna Green—Copper Stannate—Kuhlmann’s  Green—Elsner’s Green—Casselmann’s Green—Lime  Green—Patent Green—Copper Borate—Copper Silicate  (Egyptian Blue).
Chapter XXX.	Verdigris	[252]
	Blue Verdigris—Distilled or Crystallised Verdigris  —German Verdigris.
Chapter XXXI.	Chromium Oxide	[260]
Chapter XXXII.	Other Green Chromium Pigments	[264]
	Guignet’s Green—Emerald Green—Chrome Green Lake  —Turkish Green—Leaf Green.—Chromium Phosphate  Pigments: Arnaudan’s Green—Plessy’s Green—Schnitzer’s  Green—Chromaventurine—Chrome Blue (Garnier).
Chapter XXXIII.	Green Cobalt Pigments	[268]
	Cobalt Green.
Chapter XXXIV.	Green Manganese Pigments	[270]
	Manganese Green—Rosenstiehl’s Green—Böttger’s Barium  Green—Manganous Oxide—Manganese Blue.
Chapter XXXV.	Compounded Green Pigments	[273]
	Chrome Green—Elsner’s Chrome Green—Silk Green  —Natural Green—Non-arsenical Green.
Chapter XXXVI.	Violet Mineral Pigments	[276]
	Chromic Chloride—Manganese Violet—Tin Violet,  Mineral Lake—Copper Violet, Guyard’s Violet.
Chapter XXXVII.	Brown Mineral Pigments	[279]
	Lead Brown—Manganese Brown—Pyrolusite Brown  —Prussian Brown—Iron Brown—Copper Brown  —Chrome Brown—Cobalt Brown.
Chapter XXXVIII.	Brown Decomposition Products	[283]
	Humins—Bistre.
Chapter XXXIX.	Black Pigments	[285]
	Charcoal Blacks: True Charcoal Black—Vine Black  —Vine Black from Wine Lees—Vine Black from  Pressed Grapes—Bone Black or Ivory Black.
Chapter XL.	Manufacture of Soot Pigments	[294]
Chapter XLI.	Manufacture of Lamp Black	[307]
	Calcination of the Soot—Pine Black.
Chapter XLII.	The Manufacture of Soot Black without Chambers	[313]
Chapter XLIII.	Indian Ink	[316]
	Neutral Tint Black—Appendix: Black Mineral Pigments  —Chrome Copper Black—Chrome Black.
Chapter XLIV.	Enamel Colours	[319]
	White Enamels—Coloured Enamels: Yellow Enamels—Red  Enamel—Blue Enamels—Green Enamel—Violet Enamel  —Black Enamel.
Chapter XLV.	Metallic Pigments	[326]
	Shell-Gold—Shell-Silver—Imitation Silver.
Chapter XLVI.	Bronze Pigments	[329]
	Electrolytic Copper Bronze—Tungsten Bronze Pigments.
Chapter XLVII.	Vegetable Bronze Pigments	[339]
	Appendix: The Brocade Pigments.
PIGMENTS OF ORGANIC ORIGIN.
Chapter XLVIII.	Lakes	[343]
Chapter XLIX.	Yellow Lakes	[348]
	Dutch Pink—Weld Lake—Gamboge Lake—Prepared  Gamboge—Fustic Lake—Quercitron Lake—Purree  or Indian Yellow—The Colouring Matter of Saffron  —Colouring Matter of Gardinia Grandiflora.
Chapter L.	Red Lakes	[354]
	Cochineal and Carmine
Chapter LI.	The Manufacture of Carmine	[357]
	Cenette’s Method—Munich, Vienna, Paris or Florentine  Lake—Ammonia-Cochineal.
Chapter LII.	The Colouring Matter of Lac	[363]
	Lac Dye.
Chapter LIII.	Safflower or Carthamine Red	[366]
	Safflower Carmine—Alkanet.
Chapter LIV.	Madder and its Colouring Matters	[370]
	Garancin—Garanceux—Madder Extract—The Constituents  of Madder.
Chapter LV.	Madder Lakes	[375]
	Madder Carmine.
Chapter LVI.	Manjit (Indian Madder)	[378]
	Chica Red, Curucuru, Carajuru—Bigonia Chica.
Chapter LVII.	Lichen Colouring Matters	[380]
	Archil—French Purple—Cudbear—Litmus.
Chapter LVIII.	Red Wood Lakes	[384]
Chapter LIX.	The Colouring Matters of Sandalwood	[388]
	and Other Dye-woods
Chapter LX.	Blue Lakes	[390]
	Indigo—The Constituents of Indigo.
Chapter LXI.	Indigo Carmine	[394]
	Indigo Mills—Blue Lake.
Chapter LXII.	The Colouring Matter of Logwood	[398]
	Logwood Extract—Kohlrausch’s Process for Obtaining  Concentrated Extracts of Colouring Matters and  Tannins.
Chapter LXIII.	Green Lakes	[409]
	Chlorophyll—Sap Green—Chinese Green, Lokao  —Charvin’s Green.
Chapter LXIV.	Brown Organic Pigments	[414]
	Asphaltum—Sepia.
Chapter LXV.	Sap Colours	[416]
Chapter LXVI.	Water Colours	[419]
	Moist Water Colours.
Chapter LXVII.	Crayons	[423]
	Crayons for Earthenware.
Chapter LXVIII.	Confectionery Colours	[427]
Chapter LXIX.	The Preparation of Pigments for Painting	[429]
	Paint Mills.
Chapter LXX.	The Examination op Pigments	[434]
	Mineral Pigments—Examination with the Blowpipe  —Reactions of the White Pigments—Reactions  of the Yellow Pigments—Reactions of the Red  Pigments—Reactions of the Blue Pigments  —Reactions of the Green Pigments—Reactions  of the Brown Pigments—Reactions of the Black Pigments.
Chapter LXXI.	Examination of Lakes	[445]
	Reactions of the Organic Colouring Matters.
Chapter LXXII.	The Testing of Dye-Woods	[449]
	The Colorimeter.
Chapter LXXIII.	The Design of a Colour Works	[457]
Chapter LXXIV.	Commercial Names of Pigments	[460]
APPENDIX.
	The Conversion of Metric into English    Weights and Measures—Centigrade    and Fahrenheit Thermometer Scales	[469]
[INDEX.]

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.:—

Specific Gravity.	Ammonia per cent.	Specific Gravity.	Ammonia per cent.
0·885	36·00	0·953	11·50
0·886	35·00	0·955	11·00
0·889	34·00	0·957	10·50
0·891	33·00	0·959	10·00
0·893	32·50	0·961	9·50
0·895	32·00	0·963	9·00
0·897	31·30	0·965	8·50
0·905	25·39	0·970	7·50
0·925	19·54	0·972	7·00
0·932	17·52	0·974	6·50
0·947	13·46	0·976	6·00
0·951	12·00	0·978	5·50

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.