This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
By burning wood in covered heaps or in closed vessels, in such a manner as to almost entirely prevent the access of air. The oxygen, hydrogen, and nitrogen of the vegetable substance are driven off, and about 20 per cent. of carbon remains, with a small proportion of earthy salts - carbonates of potash and lime, etc. A pure charcoal may be obtained from the combustion of oils or resins with insufficient oxygen, and is known as lamp-black. For medicinal use, either kind may be further purified by ignition in a closed vessel to a red heat.
Wood charcoal occurs as a black powder, or in black, brittle pieces, very light, and retaining the shape and texture of the original wood. It is distinguished from purified animal charcoal by leaving a bulky, white mineral ash, which is the 1 or 2 per cent. of mineral salt.