The bones are coarsely powdered and freed from fat by boiling; they are then generally carbonised in iron retorts, a number of which are built in a furnace in a vertical position; a valve at the bottom serves to empty the retorts. In a well arranged apparatus the products of the dry distillation of the bones, chiefly consisting of ammonium carbonate, are collected. The retorts are carefully closed so that air cannot enter, otherwise a portion of the carbon would burn and the black would acquire a grey shade instead of the pure black which it should possess. Bone black makers who work for sugar factories carbonise by preference the densest bones; these produce a black of the most powerful decolourising properties. This property is of no advantage to the colour maker, who simply requires a very dense black. The bones of young animals, and especially certain bones, contain a larger proportion of cartilage than the hollow bones which produce the best ”char”. In making bone black to be used as a pigment just those bones should be chosen which are of less value for ”char”. On the small scale, bone black can be made by carbonising in crucibles; these hold about 16 kilogrammes of bones, they have a projecting rim, so that when one is placed upon the other it serves as a lid for the first. Piles of these pots are formed, the top one being covered by a well-fitting lid. The piles are heated in a furnace in such a manner that the flames can freely circulate between them. At first a small fire is applied; as soon as dry distillation of the bones begins, which is recognised by pale white flames at the rims of the crucibles, the fire is damped, because the burning products of distillation produce so much heat that the crucibles are soon at a good red heat. When the flames disappear the fire is maintained for about fifteen minutes longer, and when the crucibles have cooled to some extent they are taken out of the furnace and immediately emptied into a sheet-iron cylinder, in which the charcoal remains until quite cold. If the black comes in contact with air whilst still hot a portion of the carbon burns and the product has little value either as a pigment or for decolourising purposes. When quite cold the black is ground and levigated. The bone ash it contains makes these operations easy. Bone black contains at the most 12 to 13 per cent. of carbon; the remainder consists of bone ash and water absorbed by the hygroscopic carbon from the air. Bone black which has been partially burnt in the process has a greyish tinge; it may also have an ugly shade of brown: this is the case when it has not been heated to a sufficiently high temperature, so that it still contains some quantity of organic matter. Such black can be made usable by again heating, but it would always be desirable to test a small portion before the whole was ground and levigated; a uniform heating of the finely-powdered black would be attended with difficulty.