When bones are heated in a crucible, the organic constituents are decomposed and carbonized. A mixture of combustible gases is given off, which do not condense on cooling; and others, which condense in the form of a heavy oil called bone-oil, and also much water containing tarry water and ammoniacal salts in solution. The residue consists of finely-divided carbon in intimate mixture with the inorganic constituents of the bones: this mixture constitutes ordinary bone-black. The inorganic portion may be removed by washing the residue in dilute hydrochloric acid.

The process, as worked on the large scale, is carried on in different ways,, according as it is desired to collect the volatile condensable portion of the distillate, or to allow it to escape. When it is required to obtain only bone-black, the apparatus employed is simple, and the amount of fuel needed is comparatively small. Carbonization is effected in fireclay crucibles, 16 in. high and 12 in. in diameter. These are preferred to iron crucibles which were much used at one time, since they do not lose their round form when subjected to a high temperature; in consequence of this they fit closely together in the furnace, less air can penetrate, and therefore less charcoal is consumed by oxidation. The furnace is an ordinary flat hearth, having a superficial area of about 40 sq. yd., and is covered in with a flat arch, all of brickwork. The fireplace is situate in the middle of the hearth; the crucibles are introduced through doors in the front, which are bricked up when the furnace is filled; each furnace holds 18 crucibles. The crucibles, filled with the coarsely-broken bones, are covered with a lid luted on with clay.

To economize fuel, the furnaces should be in a row, and placed back to back.

When the furnace is filled and the doors are bricked up, the heat is slowly raised to redness, at which point it is kept for 6 or 8 hours. The combustible gases are evolved and consumed in the furnace as soon as the bones begin to decompose, and by this means so much heat is produced that only a small quantity of fuel is needed to maintain the required temperature. When the carbonization is complete, the doors are taken down and the crucibles removed to cool, their place being immediately filled with fresh ones. The heat must be kept uniform throughout: if it be not sufficiently high, the bone-black will contain a portion of undecomposed organic matter, which renders it quite unfit for use; if it be raised too high, the bone-black will become dense and compact, whereby its efficacy as a de-colorizer is much reduced. When the charcoal in the crucible p.aa become perfectly cool, it is removed and crushed. When required for decolorizing or deodorizing purposes, it is only roughly broken up into small lumps, in which form it is most readily applicable. The crushing is effected by means of two grooved cylinders, consisting of toothed discs, alternately 10 and 12 in. in diameter.

These are so placed that the 10-in. discs of one cylinder are opposite the 12-in. discs of the other, and thus, in revolving, the carbonized bones are crushed to fragments between them, but are not reduced to powder. They are passed successively through 6 of these mills, the cylinders of each couple being nearer to each other than the last. Finally, the crushed bones are carefully sifted; the powder is placed apart from the lumps, again passed through finer sieves, and sorted out into different sizes. A furnace such as that described will carbonize 4 charges of bones in one day, each charge being more than 1/2 ton in weight. With careful work, the bones will yield 60 per cent, of bone-black, or more than 1 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. The aqueous portion of the distillate is usually evaporated down to obtain salts of ammonia; the uncondensable 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. Its principal use is to decolorize various solutions; inferior qualities are used as pigments.

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.