Tar. The nature of tar differs according to the raw material from which it is derived. In nearly all cases it is a thick, black, or dark-brown liquid of complex composition. The tars of commerce have a strong characteristic odour varying with their origin. When wood, coal, or shale is subjected to destructive distillation in close vessels there are produced (1) incondensable gases, (2) a liquid portion consisting of various substances soluble in water, and (3) another portion composed of bodies insoluble in water. This third or last portion is generally called tar. In former days wood-tar was used for much the same purposes as that for which the residual pitch derived from it is employed now. The time has come when any kind of tar is looked upon rather as a mother-liquor from which valuable chemical substances are obtained than as a material to be used by itself in the arts.
Wood-tar.—Different woods yield tars of a so far unlike nature. The tar from pine-wood is not the same as that from oak or beech. Most of the wood-tar produced in Europe is obtained from the Scotch fir (Pinus sylvestris), but it is also got from other pines and from larch. Coniferous wood yields from 14 to 18 per cent. of tar, while from the wood of dicotyledonous trees of temperate regions only from 9 to 10 per cent. is obtained. Wood-tar is still made by the very old process of burning wood in cone-shaped piles covered over with sufficient earth and turf to exclude air, of which just enough is admitted to promote combustion by means of apertures at the bottom where the pile is ignited. The pile stands on a clay floor, and it is fully a week before any tar is collected. By this rude process much charcoal, crude acetic acid, and other products are wasted; but these are now very often saved by distilling the wood in close vessels. Several forms of retorts or kilns are employed to make charcoal and at the same time to yield tar. One kind of kiln is in the shape of an oblong brick chamber, arched over, and with a floor formed into a sort of channel sloping from the middle of the chamber, where it is highest, to each of the end walls. Its internal size is 25 feet long, 12 feet wide, and 15 feet to top of arch. The chamber is closely filled with wood, the only place where a small space is left being where the flame or heat from the fire enters at one side. Air must be as far as possible excluded during the charring of the wood, which in a kiln of this kind may consist of stumps, branches, and any sort of odd pieces. The fire hearth, placed just outside the side wall of the chamber, has no grate, so as to prevent too much draught. As the tar is formed it runs down the right and left inclines of the channel, and at each end, by one branch of a two-way pipe, is led into a tar vessel, while the more volatile acetic acid and wood-spirit pass by the upper branch of the pipe into a condenser. Wood-tar contains (besides the tar proper) a large admixture of watery distillate, and gives on redistillation acetic (pyroligneous) acid, wood-spirit or methyl-alcohol, methyl-acetate, acetone, wood-creasote, and wood-naphtha, consisting of hydrocarbons very often containing methyl groups replacing hydrogen; also heavy oils and crystalline bodies not yet much investigated and that disappear in the pitch. Tar from some resinous woods contains oils resembling turpentine in composition. Before American petroleum became so cheap an oil of this kind was made in Norway and burned in paraffin-lamps.
In medicine tar and tar-ointment are used for cutaneous diseases (such as psoriasis), and much controversy arose as to the virtues of tar-water, regarded by Bishop Berkeley as almost a panacea. Tar-water was the watery liquor obtained when a quart of wood-tar and a quart of water had been well shaken together and allowed to settle. This Berkeley found was used, taken internally, as a preventive and palliative of smallpox in the American colonies. He tried it and found it exceedingly useful, he says, for consumption, fevers, ulcers, pleurisy, gravel, stomachic disorders, &c.; and on it bases his Siris or Treatise concerning the Virtues of Tar-water, a chain of philosophical reflections, which, as Professor Fraser says, connects tar-water with the throne of the Divine Ruler of the universe.
The use of tar or pitch for 'tarring and feathering' obnoxious persons, still understood to be practised in some parts of the United States in execution of mob-law, is of very ancient origin, dating apparently at least from the times of the Crusades. According to Rymer's Foedera and Hakluyt's Voyages, King Richard I. enacted for the royal navy that a thief or felon 'lawfully convicted, shall have his head shorne and boyling pitch powred vpon his head, and feathers or downe strawed upon the same, whereby he may be known.' Scarron (q.v.) was reported to have tarred and feathered his own person as a carnival disguise.
Coal-tar or Gas-tar.—In the making of gas for illumination the coal is distilled at a higher temperature than that used for the production of ordinary coke, the result being that the constituents of the tar from the two processes differ. Gas-tar contains more benzole and other colour-making materials than coke-tar. See COAL-TAR.
Coke-oven Tar.—Until recently the recovery of the volatile products, such as tar and ammonia, given off during the conversion of coal into coke was neglected. Coke-ovens are, however, now constructed which, among other improvements, include the collection of the tar, the nature of which depends upon the temperature at which the coal is distilled. In the Jameson coke-oven the temperature is low at the point of destructive distillation, and consequently the tar produced contains paraffin and the higher phenols. On the other hand, the Simon-Carvés coke-oven, in which a high temperature is reached, produces a tar more resembling that obtained in coal-gas making. It is characterised by the presence of naphthaline and Anthracene (q.v.), and small quantities of benzole and carbolic acid.
Blast-furnace Tar.—In some cases the tar from iron blast-furnaces in which coal is used as fuel is now recovered. It is of an intermediate quality between the two kinds of coke-tar just noticed, but rather more resembling that produced at a high temperature.
Lignite or Brown Coal and Peat were distilled some years ago to yield tars. The former gave a considerable yield of buttery tar containing about 15 per cent. of paraffin. From the latter a tar was obtained, also yielding some paraffin.