"Take of animal charcoal, a pound; hydrochloric acid, water, of each, twelve fluid ounces. Mix the acid with the water, and pour the mixture, by a little at a time, over the animal charcoal; then digest with a gentle heat for two days, frequently stirring. Set aside, and decant the supernatant fluid; then wash the charcoal with water frequently renewed, until all acidity be removed: finally dry it."

The ivory black of the shops is animal charcoal mixed with the earthy matter of bone, namely, phosphate and carbonate of lime, which are taken up by the hydrochloric acid, and the charcoal left unacted upon. The subsequent washings are requisite to free it from the acid.

Pure animal charcoal, when digested with coloured vegetable infusions or solutions, deprives them of much if not the the whole of their colouring matter: it thus becomes a valuable agent in many pharmaceutical processes.