This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
All bones (and ivory is bone in a sense) consist of a framework of crystallized matter or bone earth, in the interstices of which organic matter is embedded. Hence if bones are heated red-hot in a closed vessel, the organic matter is destroyed, leaving carbon, in a finely divided state, lodged in the bony framework. If the heat is applied gradually the bone retains its shape, but is quite black and of much less weight than at first. This bone black or animal charcoal is a substance which has great power of absorbing coloring matter from liquids, so that it is largely used for bleaching such liquids. For example, in the vast industry of beet-sugar manufacture the solutions first made are very dark in color, but after filtration through animal charcoal will give colorless crystals on evaporation. Chemical trades require such large quantities of bone charcoal that its production is a large industry in itself. As in breaking up the charred bones a considerable amount of waste is produced, in the form of dust and small grains which cannot be used for bleaching purposes, this waste should be worked up into a pigment. This is done by dissolving out the mineral with hydrochloric acid, and then rinsing and drying the carbon.
The mineral basis of bones consists mainly of the phosphates of lime and magnesia, salts soluble in not too dilute hydrochloric acid. A vat is half filled with the above-mentioned waste, which is then just covered with a mixture of equal volumes of commercial hydrochloric acid and water. As the mineral matter also contains carbonates, a lively effervescence at once ensues, and small quantities of hydrofluoric acid are also formed from the decomposition of calcium fluoride in the bones. Now hydrofluoric acid is a very dangerous substance, as air containing even traces of it is very injurious to the lungs. Hence the addition of hydrochloric acid should be done in the open air, and the vat should be left by itself until the evolution of fumes ceases. A plug is then pulled out at the bottom and the carbon is thoroughly drained. It is then stirred up with water and again drained, when it has fully settled to the bottom. This rinsing with clear water is repeated till all the hydrochloric acid is washed away and only pure carbon remains in the vat. As for pigment-making purposes it is essential that the carbon should be as finely divided as possible, if is as well to grind the washed carbon in an ordinary color mill. Very little power is required for this purpose, as when once the bone earth is removed the carbon particles have little cohesion. The properly ground mass forms a deep-black mud, which can be left to dry or be dried by artificial heat. When dry, the purified bone black is of a pure black and makes a most excellent pigment.
Bone black is put upon the market under all sorts of names, such as ivory black, ebur ustum, Frankfort black, neutral black, etc. All these consist of finely ground bone black purified from mineral matter. If leather scraps or dried blood are to be worked up, iron tubes are employed, closed at one end, and with a well-fitting lid with a small hole in it at the other. As these bodies give off large volumes of combustible gas during the charring, it is a good plan to lead the vapors from the hole by a bent tube so that they can be burnt and help to supply the heat required and so save fuel. Leather or blood gives a charcoal which hardly requires treatment with hydrochloric acid, for the amount of mineral salts present is so small that its removal appears superfluous.