There are two methods by which carbon can be made for use as a pigment. One has been indicated, the heating of organic materials in the absence of air, i.e., their dry distillation; the second, which produces the best qualities of black, consists in burning substances very rich in carbon with a very small supply of air. The product of this process, commonly known as soot, is very different in external appearance from carbon made by dry distillation. The latter forms an easily powdered mass, which only acquires lustre by grinding with oil, whilst soot consists generally of very light flocks, which, in consequence of their greasy nature, exhibit a peculiar velvety lustre.
In commerce there are many qualities of black, the names of which are generally chosen quite arbitrarily. The so-called ivory black is an example of this. At some period it was observed that ivory produced a very fine black when carbonised; this black was made for a long time from refuse ivory. When it was found that a similar black could be made quite as well from much cheaper materials, it was no longer made from ivory. The name, however, remains in trade to the present day: it has come to be regarded as a description of quality. By the name of ivory black a fine black pigment is understood; it is immaterial to the consumer whether it is made from ivory or not: to him the quality of the pigment, but not its source, is important.
According to the method of production, the black pigments can be divided into charcoal blacks and soot blacks.
Charcoal Blacks.
This term is applied to the pigments obtained by heating organic substances in the absence of air. The charcoal blacks have two valuable properties: they are easily made, and they exhibit a pure black shade which it is far more difficult to obtain with the soot blacks. It is very difficult to convert these pigments into the requisite state of fine division. They cannot be levigated on account of their low specific gravity, which causes them to settle very slowly in water. It only remains to convert the charcoal into a very soft powder by grinding, but even then the pigment has but little covering power, because it is not possible to destroy the organic structure of the substance carbonised. 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 charcoal blacks come on the market under various names—ivory black, bone black, vine black, Frankfort black, Paris black, etc. The ivory blacks are the best quality, but recently true vine black, i.e., made from grape residues, has come into use.
True Charcoal Black.—In charcoal-burning wood is burned in great piles with restricted air supply; the greater part of the carbon contained in the wood remains behind as charcoal, which is used for fuel. The charcoal made from hard wood, such as maple or beech, is not well suited for pigments; the lighter and more porous the wood, the more pure is generally the shade of the charcoal, and the more easily it can be ground. When, therefore, charcoal is to be used as a pigment it should be made from wood of a porous nature; the wood of the lime, black-alder and spindle tree is particularly adapted for this purpose. Spent tan-bark is a very cheap, and at the same time suitable material for this purpose. It generally consists of oak bark, and has lost the greater part of the salts it contained by long contact with water; also the woody substance of the bark has suffered a change which is favourable to carbonisation. This is especially the case when the bark has been stored for some time; a considerable quantity of the humins previously mentioned has formed in it, and these are easily decomposed at a low temperature.
The charcoal, however obtained, is ground to a soft powder. When it has reached the proper degree of fineness it should be repeatedly washed with water to remove the salts. When instead of pure water a very dilute acid, such as hydrochloric, is used, practically all the salts dissolve, and the residue consists of nearly pure carbon.
Vine Black.—In wine-producing countries a good and cheap black can be made from the residues of the wine manufacture; this is the so-called vine black. Either the pressed grapes or the lees separated in the fermenting vessels may be used.
Vine Black from Wine Lees.—The lees always contain a considerable quantity of liquid, they must be thoroughly dried before they are carbonised; this is most simply done by spreading out the pasty mass in a thin layer, and exposing it to a temperature of about 100° to 120° C. In drying the volume considerably diminishes. The lees are changed into a brown mass which is easily powdered; it is packed into barrels while still warm, and can be kept for a considerable time without alteration. Fresh lees can be kept but a short time; they are quickly destroyed by a rapid fermentation.