The pigment can be obtained at a far lower temperature when ferric chloride is used in place of ferrous sulphate. The mixture is made by grinding together 5 parts of cobalt hydroxide and 25 parts of ammonia alum, then adding a solution of ferric chloride, rapidly drying the whole to a powder, and whilst still hot filling it into the crucible in which the ignition is performed. When small quantities of ferric chloride are used chocolate brown shades inclining to violet are produced; the larger the quantity of the iron compound the more pure is the brown.
It has been stated that it is important, in the preparation of cobalt pigments, to prevent the entry of fire gases to the heated mixture; the reducing action of the gases would materially damage the beauty of the colour. It has been proposed to place a small quantity of mercuric oxide at the bottom of the crucible. This would decompose on heating, and create an atmosphere of pure oxygen in the crucible. Apart from the cost of the mercuric oxide, which is considerable, it would only be effective at the commencement of the operation, since it is completely decomposed at a low red heat. The author has found an addition of pyrolusite considerably more effective. This may be applied by spreading a small quantity, about 5 per cent. of the mixture to be heated, on the bottom of the crucible and covering it with powdered glass, upon which the colour mixture is placed. An alternative course is to place the crucible in a second, filling the space between the crucibles with powdered pyrolusite. At a strong red heat oxygen is slowly evolved from the pyrolusite; the entry of fire gases into the crucible is thus prevented. In using the first method the crucible is frequently spoilt, whilst in the second method the surrounding pyrolusite protects it, so that it can be used again.
CHAPTER XXXVIII.
BROWN DECOMPOSITION PRODUCTS.
Humins.—Wood decomposes, like many other substances of organic origin, to form compounds of a deep brown colour, which are known as humins, on account of their occurrence in the humus of tilled soil. The numerous compounds found in humus have all a deep brown colour, which, together with their great stability, makes them very suitable for use as pigments.
By various methods substances can be made so rich in humins that they can be used as pigments. For this purpose sugar, starch, young plant fibres and beet-sugar molasses may be used, which are converted into humins with great rapidity when they are heated with water. In this way the author has obtained handsome colours from sawdust. These humins are most easily made by the following process: Thick beet-sugar molasses are cautiously heated with 5 per cent. of caustic soda in a very capacious iron pan. The already dark mass soon becomes quite black when seen in thick layers, and evolves a considerable quantity of gas; if the heating is too rapid the soft mass may boil over out of a very large vessel. When the evolution of gas diminishes the heat is increased and the mass frequently stirred. With some practice it is possible to tell when the action is finished from the smell, which is at first sweetish but later of a characteristic nature. At first tests should be repeatedly taken from the mass; these are considerably diluted with water until the liquid begins to be transparent. When two tests taken at an interval show no difference of colour the heating is stopped. The whole mass is then poured into water in order to dilute the alkali, so that it will not destroy the strainer; the soft mass, which in the wet state appears quite black, is washed with water until the washings are neutral. The humin brown made in this way, when ground with oil or gum solution, produces a very handsome brown of great covering power and warmth of shade, also distinguished by complete indifference towards chemical reagents.
The pigments we have designated humins contain a very large quantity of carbon, to which they owe their dark colour. When peat or lignite is treated with caustic soda in a similar manner, good shades of brown are obtained; they are, however, surpassed in beauty by the brown from molasses. A handsome but costly brown is obtained when crude spirits of wine are heated with fuming sulphuric acid. Equal volumes of alcohol and sulphuric acid are used; the mixture is heated in a retort connected to a condenser, which is required on account of the combustible vapours evolved from the mixture. When the mass is quite black the heat is withdrawn; the residue is diluted with water, and soda solution added so long as effervescence occurs. On filtering a very soft brown powder is left, which forms a very handsome and durable pigment.
Bistre.—When soot is produced at a very low temperature it is very lustrous, and, in addition to carbon, contains a notable quantity of the products of dry distillation. When this soot is powdered and treated with water the latter substances are dissolved; boiling water should be used. When water is no longer coloured the soot is suspended in a large quantity of water and subjected to a process of levigation. The fine powder obtained by repeated levigation has, when dry, an ugly brown colour, but when ground it acquires an extremely warm shade.
CHAPTER XXXIX.
BLACK PIGMENTS.
Carbon occurs in nature in many different forms—the diamond, graphite (black lead), and purified lamp black are, from the chemical point of view, one and the same substance, namely, carbon. In colour making the non-crystalline form of carbon, which is pure black, is alone used; almost all black pigments used in painting are composed of tolerably pure carbon. 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.
At first sight it appears to be a very simple matter to obtain black pigments from organic materials; it is simply necessary to expose them in the absence of air to a temperature high enough to decompose them, carbon is then left as a residue. In spite of the apparent simplicity of this operation there are many difficulties. The preparation of good black pigments demands considerable practice.