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PIGMENTS OF ORGANIC ORIGIN.

THE CONVERSION OF METRIC INTO ENGLISH WEIGHTS AND MEASURES --CENTIGRADE AND FAHRENHEIT THERMOMETER SCALES 469

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.

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.

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 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 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:--

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 . The manufacture of these colours is a separate branch of chemical industry.

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 , is mixed with a solution of bichromate of potash, whereupon a precipitate of lead chromate 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.

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 ; 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 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 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.

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 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.

ASSISTANT MATERIALS.

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 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 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.

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