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. In the latter case, when it is required to obtain only bone-black, the apparatus employed is of a very simple nature, and the amount of fuel needed is comparatively small. The carbonisation is effected in fire-clay crucibles, 16 in. high and 12 in. diameter. These are to be preferred to crucibles made of iron, 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 more closely together in the furnace, less air can penetrate, and therefore less of the charcoal is consumed by oxidation. The furnace is an ordinary flat hearth, having a superficial area of about 40 square yards, 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 eighteen crucibles. The crucibles, filled with the coarsely broken bones, are covered with a lid luted on with clay. To economise fuel, the furnaces should be in a row, and placed back to back.
The arrangement of the furnace and pots is shown in Figs. [1] and [2]. A is the fireplace; B, the crucibles, eighteen in number, spread over the floor of the furnace in a single layer; c, d, e, and f are the flues for conducting away the heated gases arising from the calcination of the bones, as well as the waste heat itself; the last portion of the flue is fitted with a damper g. The furnaces are intended to be built in fours, back to back, the waste heat serving in a great measure to conduct the operation of the revivifying apparatus placed in the centre of the group, and marked C.
Figs. 1 and 2.—Bone-black Furnace.
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 six or eight hours. The combustible gases are evolved and consumed in the furnace 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 carbonisation is complete, the doors are taken down and the crucibles are removed to cool, their places being immediately filled with fresh ones. The heat must be kept as uniform as possible throughout the process: 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, on the other hand, the temperature be raised too high, the bone-black will become dense and compact, whereby its efficacy as a decoloriser is much reduced. When the charcoal in the crucible has become perfectly cool, it is removed and crushed. When required for decolorising or deodorising 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 carbonised bones are crushed to fragments between them, but are not reduced to powder. They are passed successively through six of these mills, the cylinders of each couple being nearer to each other than the last. Finally the crushed bones are carefully sieved; 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 above will carbonise four charges of bones in one day, each charge being 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 carbonisation, 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 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.
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 decolorising properties of bone-black are due solely to the presence of the charcoal.