Josef Bersch

“ It is known that a substance quite devoid of colour will furnish a perfectly white powder, because the colourless particles reflect the light in all directions without breaking it up into its constituent yellow, red and blue rays. Chalk, too, is in reality a colourless substance, and reflects light with greater uniformity in proportion as the fineness of the particles increases. Consequently, when one has a chalk that is not perfectly white, it can, nevertheless, be made to furnish a very handsome product by bestowing great care on grinding and levigation. ”

“ A certain temperature is necessary for the combustion of every substance; if a cold body is placed in the flame of a candle or an oil lamp it becomes covered with soot, because it cools the flame to such an extent that the carbon is no longer heated to the temperature necessary for its combustion, and is, therefore, separated in a finely divided condition. Use is made of this phenomenon in a method of making soot black which has many advantages, of which the most important is that expensive buildings are not required, and the manufacture can be conducted in restricted space. ”

“ When the clay has been derived from the weathering of such rocks, it is easy to understand that it may contain admixtures of quartz and mica, which are frequently visible to the naked eye, or at any rate under the microscope. Whereas clay forms a white, amorphous mass, the grains of quartz sand are decidedly crystalline, transparent and of vitreous lustre; the scales of mica, on the other hand, appearing as thin tabular crystals, mostly of a green or brown colour and exhibiting, when viewed at certain angles, a brilliant metallic sheen. ”

“ 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. ”

“ Most frequently, earth colours are named after the localities where they are either discovered or prepared, in combination with the word indicating the colour of the product—for example: Cologne white, Vienna white—or the term “earth” (Verona earth, Veronese green, etc.) . Whilst these names give, to some extent, an indication of the nature of the pigment, others have no reference to it at all; such as colcothar, bole, umber, etc. ”

“ 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. ”

“ The method of formation of terra di Siena was probably on the same lines as that already described in the case of ochre, namely by the breaking down of minerals—in this case brown ironstone—and natural levigation, the powder being deposited in places where the water containing the ferric hydroxide in suspension came to rest and allowed the solid particles to settle down.
The best lumps of terra di Siena in point of purity and colour can be used as pigments without any preparation; but in most cases the earth is lightly calcined, in order to improve the colour. ”

“ In whatever way and from whatever materials carbon is made for use as a pigment, the manufacturer must always endeavour to obtain it as pure as possible, and in a condition of the finest division; the depth of colour and the covering power depend on these two conditions. Carbon only shows a pure black colour when it is pure; if it contains relatively small quantities of impurities, it has a more or less brown shade. ”

“ The somewhat expensive pigment, Indian red, is—when pure—really nothing but a very pure ferric oxide of Indian origin. Ferric oxide, however, often contains impurities which considerably influence the colour of the product.30 Owing to the fact that large quantities of ferric oxide are formed as by-products in certain chemical processes which are carried out on a very extensive scale, this oxide, which is very pure, can be advantageously used for making iron pigments, especially as its application for other purposes is very restricted, and it can therefore be had at a very low price. ”

“ If a colour manufacturer is in the fortunate position of placing his works on a river, he has not only an unrestricted water supply, but may also be able to use water power, the cheapest of all powers. When water power is available, it will be used to raise the large volumes of water required and to move the machinery for grinding the raw materials, rasping dye-woods, etc. In a colour works of any size a boiler is required; if water power is not used, it must be of sufficient power to give steam for driving the engine, for boiling liquids and for heating the drying stoves. ”

“ The employment of barytes in the colour industry is often regarded as adulteration, which, however, it is not when the case is considered from the right point of view. For instance, the only preparation which can properly be termed white lead consists of basic lead carbonate. This, when pure, is a rather expensive pigment, whereas, for certain purposes, the consumer requires a product that can be obtained at a low price. ”

“ It cannot be denied that certain manipulations impart a greater brilliancy to the product, although it is impossible to assign a physical or chemical reason; but the manufacturer will quickly be able to differentiate the valuable from the worthless in these processes. Two conditions have the greatest influence on the beauty of the pigment—the temperature at which the black sulphide is sublimed, and the complete freedom of the vermilion from excess of sulphur. ”

“ On exposure to the air, green vitriol turns an ugly brown colour, and is no longer completely soluble in water, passing gradually into the condition of basic ferrous sulphate. This is because ferrous oxide is a highly unstable substance, which attracts oxidation and changes into ferric oxide. This latter, however, requires for the production of soluble salts a larger quantity of acids than does ferrous oxide, and therefore the oxidation of ferrous sulphate in the air leads only to the formation of salts that are imperfectly saturated with acid, namely basic salts. ”

“ 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 treatment of the crude earth is best carried on in the same vessels as are to be used in the subsequent levigation process. After the acid liquor has been drawn off, the earth is brought into contact with water, stirred up well, and the water run off, by opening tapholes in the side of the vessel, into settling-tanks, where it is left until all the suspended matter has completely subsided.
The colour of green earth can also be toned by the addition of yellow ochre, thus producing a range of greens with a yellowish tinge. ”

“ If a white body reflects the majority of the rays of light falling on it, but decomposes a small number, we perceive a white which has a yellow, blue or red tinge. [Pg 73]
The most valuable white for the colour maker is evidently that which reflects, unaltered, all the rays of light; it is the most brilliant, and free from every tinge of colour. The physical condition of the substance is most important. Solid substances are either crystalline, that is, possess definite shapes formed according to a regular law, or they are amorphous, that is, are composed of irregularly formed particles. ”

“ When precipitation is ended, the deposit is allowed to settle down, and the clear supernatant liquid is carefully drawn off so as not to disturb the delicate sediment, which is then stirred up thoroughly with clean water, left to subside, washed again, and then spread out to dry on cloths which are suspended by the four sides. The surplus water drains away and the residue gradually assumes the consistency of paste, in which condition it can easily be moulded to any desired shape. If left long enough to dry completely, it forms a very delicate powder, furnishing a pigment of excellent quality. ”

“ The more oxygen an oil requires to burn with a non-smoky flame, the more suited it is for the manufacture of blacks. Generally speaking, the difficulty of completely burning these oils is in proportion to their specific gravity and the height of their boiling point.
By the distillation of rosin, oils are obtained which also consist of carbon and hydrogen and form a useful material for the manufacture of soot blacks. The mineral known as earth-wax or ozokerite, a substance intermediate between asphaltum and petroleum, can also be used, but since it is a solid it must be burnt in troughs. ”

“ As a pigment, chalk possesses many valuable properties. The organic structure of chalk gives it high covering power as a wash, a thin layer applied to a surface sufficing to mask the colour of the underlying ground completely. The lime in chalk being combined with carbonic acid, its basic properties are so extensively weakened that chalk can be mixed with even the most delicate colours without fear of their shade being affected. A coating of pure chalk paint on any surface will never change colour in the air ”

“ When the finest charcoal powder, made by carbonising any organic substance, is examined under a powerful microscope the structure of the particles is at once recognised, so that it is almost always possible to decide whether the charcoal is of plant or animal origin. A very careful examination may show what particular part of the plant has been carbonised. If a manufacturer succeeded in producing by direct carbonisation a black with the covering power of a soot black, that black would soon be the only one found in commerce. ”

“ The testing of a mineral for its suitability as pigment is a very simple matter, all that is required being to subject a small quantity to the same treatment that is applied to the earth colours on a large scale. For this purpose a few pounds of the mineral are levigated, and the residue is dried. To ascertain the tones obtainable by calcination, small samples—of about 100 grms.—are placed in crucibles, and gradually heated in a furnace. When the masses have attained a sufficient temperature, the samples are taken out of the furnace, at intervals of ten minutes, and left to cool. ”

“ To prevent loss of copper by oxidation the molten metal should not come in contact with air; it should be covered by a layer of red·hot coal, which prevents oxygen from reaching it. When the copper is completely melted, which is ascertained by stirring with a piece of wood, the whole of the zinc is added. Some skill is required in this operation, otherwise a large proportion of the zinc will be volatilised, and the vapours will burn when they come in contact with air, in which case dazzling bluish white flames are seen over the crucible. ”

“ 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. ”

“ Red lead is made in two ways—directly from metallic lead or by heating easily decomposed lead salts. When it is made from the metal, the following is the process: the lead, which must be very pure, is melted in a reverberatory or calcining furnace, oxidised to massicot by the air passing over it, and the massicot then, by careful heating, changed into red lead, a process in which particular care must be taken that the mass is not melted. By continued heating the massicot absorbs about 2 per cent. of oxygen, and changes in colour to a bright red. ”

“ If Chinese green is really made by this process, it is not a lake but a vegetable colouring matter, to which clay has been added to increase its weight or to make it plastic. Very fine lakes can be obtained from Chinese green by dissolving it in a solution of alum, and adding soda solution. When Chinese green is dissolved in acetic acid and ammonium chloride and a zinc salt added, a blue lake is produced on the further addition of sodium acetate. A bluish violet lake is obtained by treating Chinese green with a strong reducing agent and adding calcium acetate. ”

“ Unfortunately there is no clear sign to indicate the temperature most favourable for the production of the greatest quantity of soot, the appearance of the flame alone can be taken as a guide. If the flame appears pure white and very luminous and shows at its end no thick black smoke the combustion is very complete, and but a very small yield of soot is to be expected. On the other hand, if the flame continually threatens to go out the air supply is too small and a liberal amount of soot will be formed, but mixed with a large quantity of the products of dry distillation. ”

“ Barium sulphate, known as permanent white, enamel white, blanc fixe, barytes white, is the only white pigment which is absolutely unaltered by exposure to the atmosphere. Lead pigments are discoloured in the course of time, and in the end turn black; bismuth white behaves in the same manner; zinc white is much more lasting, but not quite permanent.
Enamel white is really permanent; it deserves the greatest attention from the colour manufacturer, especially as it can be made by a very simple and cheap method. ”

“ 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. ”

“ The treatment may be stopped when the solution acquires a slight brownish colour. When the soot has been purified so far it has lost its brown shade and now appears as a velvety black powder, very soft and very light, and distinguished by extraordinary covering power.
It is not so easy as it appears at first sight to judge of the quality of a black pigment by its appearance. To the unaccustomed eye a pigment may appear to be of an unexceptionable black, which to the expert appears decidedly brown; only long practice can give the eye the requisite keenness. ”

“ Red Mercuric Sulphide, or vermilion, exhibits, for a sulphur compound, considerable resistance to the action of chemical reagents; dilute mineral acids do not decompose it. Unfortunately, vermilion has another property which makes it quite unsuitable for the artist’s use: in the course of time it gradually turns dull and at last is completely discoloured. This alteration of colour can only be ascribed to a return of the red crystalline modification into the black non-crystalline. When a white pigment is tinted by vermilion it should not be a lead pigment, or in a brief time it will turn black. ”