TAR, a thick, black, unctuous substance obtained chiefly from old pines and fir-trees by burning them with a close smothering heat, much used in coating and caulking ships, &c. It is prepared in different parts of Germany, in North America, and in all countries where there is much wood. The wood is inclosed in a large oven, to the quantity of ten or more loads at a time: this stands within another oven called the mantle, the space between them receiving the fire. From the bottom of the inner oven there runs a gutter, by which the tar is conveyed off in proportion as it melts out from the wood. Along with the tar there runs out an acid spirit or juice, by means of which part of the oily matter becomes soluble in water; and it is owing to this that tar infused in water communicates to it a medical virtue. Tar when distilled yields a kind of essential oil called oleum pini and oleum tade, which is greatly valued by painters, varnishers, &c. on account of its drying quality: it soon thickens of itself almost to the consistence of a balsam.
Of late it has been found that the empyreumatic oil produced from pitcoal or culm answers the purposes of tar distilled from wood. There is, however, the same difference between the two that there is between a vegetable and a bituminous empyreumatic oil. The process is much the same as when wood is used.