When intended for use as a deodoriser or decoloriser, bone-black should be kept carefully excluded from the air, for by exposure it loses this power to a great extent, and becomes almost inert. That which has been freshly burned is therefore best for these purposes. The cost of production of bone-black may be calculated as follows:-- £ _s._ _d._ 4 tons fat bones at 4_s._ per cwt 16 0 0 27½ bushels coals 1 3 9 2 firemen 0 4 9 4 workmen 0 8 0 1 carman 0 2 4 2 horses 0 5 7 Breaking up the bones 1 5 4 Rent and taxes 0 8 0 Interest, repairs, and wear and tear 0 7 2 Contingencies and transports 0 2 4 ------------- £20 7 3 Produce:-- Black, 60 per cent., say 38 cwt. in grains, at 14_s._ 3_d._ 13 10 9 10 cwt. fine, at 5_s._ 6_d._ 1 7 8 Fat, 6 per cent., say 5 cwt., at 31_s._ 8_d._ 7 18 4 --------- £22 16 9 22 16 9 -------- Profit £2 9 6 Bone-black never has the depth or brilliancy of lamp-black, but it mixes well with either water or oil, and though a slow drier as an oil paint, is permanent and not high priced. =Frankfort-black or Drop-black.=--This is a black powder obtained from dried vine-twigs carbonised 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 charcoal 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 tinge. 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 is extensively used by copperplate engravers in the preparation of their ink. Drop-black is simply Frankfort-black ground exceedingly fine, mixed with a little glue water, and dried in pear-shaped drops for sale.