Coal Tar is produced by heating coal in close iron vessels, and is a bye product in the manufacture of gas. When itself distilled it produces, in various stages - first, coal naphtha, which is useful for dissolving indiarubber, etc.; then dead oil or creosote, used (see p. 394) for preserving timber; and, lastly, pitch, which is used for asphalte work (see p. 253), also as an ingredient of varnishes, etc.
Wood Tar is produced by the distillation of pine and other resinous trees. It has strong preservative qualities, owing to the creosote it contains. It is imported in barrels containing about 30 gallons, from the north of Europe as Stockholm and Archangel tar, and from the United States as American tar. Of these varieties Stockholm is considered the best; the residue left after distillation is pitch (see p. 253).
Mineral Tar is a natural substance found in Burmah, and also obtained by distilling bituminous shales, such as those found in Dorsetshire and elsewhere. It contains less volatile matter than the other kinds of tar, but is otherwise of similar composition.