A furnace such as that described above will carbonize four charges of bones in one day, each charge beiug more than half a ton in weight. With careful work, the bones will yield 60 per cent. of bone-black, or more than one ton daily.
If it be required to condense the volatile gaseous products of the carbonization, this process is conducted in retorts similar to those used in the manufacture of acetic acid from wood; these are so arranged that the whole of the gaseous products are condensed and collected. The aqueous portion of the distillate is usually evaporated down to obtain salts of ammonia ; the uncon densable gases may be employed for illuminating purposes. The manufacture of bone-black is usually carried on in the neighbourhood of large towns, where a good supply of bones may be readily obtained.
The principal use of hone-black, or animal charcoal, is to decolorize various solutions, parti cularly syrups ; inferior qualities aro used in the manufacture of blacking. The decolorizing pro perties of this substance are extensively made use of by sugar refiners in the purification of their different syrups (see Sugar). When it has become unfit for the clarification of the sugar liquors, the charcoal is pinified for re-use by processes which will be described in the article on Suga r.
Ordinary bone-black has about the following composition: Phosphate and carbonate of lime, and sulphide or oxide of iron, 88 parts; charcoal, containing a small quantity of nitrogenous matter, 10 parts ; silicated carbide of iron, 2 parts. The decolorizing properties of bone-black are due solely to the presence of the charcoal.
When intended for use as a deodorizer or decolorizer, bone-black should be kept carefully ex cluded from the air, for by exposure it loses this power to a great extent, and becomes almost inert. That which has been freshly burnt is therefore best for these purposes.
The cost of production of bone-black may be calculated as follows :— (Fn., Noir de Francfort ; GER., Frankfurter Schwarz.) Frankfort-black is a black powder obtained from dried vine-twigs, carbonized to a full black and then ground very fine. On a large scale, it is prepared from a mixture of vine-twigs, wine lees, peach stones, bone shavings, and ivory refuse. It varies in shade according as the animal or
vegetable oharcoal is in excess ; when the latter predominates, the powder is of a bluish colour ; but when there is an excess of animal charcoal, it has a brownish timge. It is customary to wash the powder well when first made, in order to remove any soluble inorganic impurities. The finest Frankfort-black is probably the soot obtained from the combustion of the materials mentioned above. It makes an excellent pigment, and ifs extensively used by copperplate engravers in the preparation of their ink.
(Fa., Noir d'Ivoire ; GER, Elfenbein-Schwarz.) Ivory-black is a beautiful black pigment prepared by carbonizing waste fragments and turnings of ivory. These are exposed to a red heat for some hours in crucibles, great care being taken to avoid overheating or burning. When quite cold, the crucibles are opened, and the contents pulverized, the richest coloured fragments being kept apart for the best quality. The powder is then levigated on a porphyry slab, washed well with hot water on a filter, and dried in an oven. The product is of a very beautiful velvety black colour, superior even to that obtained from peach kernels, and quite free from the reddish tinge which so often characterizes bone-black. Ivory-black, like Frankfort-blaek, is employed by copperplate printers in the preparation of their ink. Mixed with white lead, it affords a rich pearl-grey pigment.
(Fa., Noir de Fain& ; GER., Kienruss.) Lamp-black is an exceedingly light, dull-black powder, formed by the imperfect combustion of oils, fats, resins, 8tc. It may be prepared on a small scale by suspending a small tin-plate funnel over the flame of a lamp fed with oil, tallow, or crude naphtha, the wick being so arranged that it shall burn with a large and smoky flame. Dense masses of this light carbonaceous matter gradually collect in the funnel, and may be removed from time to time. The funnel should bo furnished with a metal tube to convey the gases away from the room, but no solder must be used in making the con nections.