Coal-tar, or GAS-TAR, is a thick, black, opaque liquid, which comes over and condenses in the pipes when coal or petroleum is distilled. Now usually obtained in the manufacture of gas, tar was about 1782 extracted from coal by the ninth Earl of Dundean under a patent, expressly for the purpose of being used for protecting ships from rotting. Coal-tar is slightly heavier than water, and has a strong, disagreeable odour. The amount of tar so obtained of course varies with the nature of the coal employed, but it is also dependent on the average temperature of distillation. With a low temperature, a large quantity of tar is produced, along with a small yield of a highly illuminating gas. At first this tar was regarded as a waste product, or, at most, as a source of pitch; but it soon became apparent that as a source of Benzene (q.v.), and through it of the Aniline Dyes (q.v.), it was a commodity of great commercial value.
When coal-tar is distilled, a large number of volatile substances pass over as the temperature rises higher and higher. At first various gases, ammonia and naphtha, are obtained to the extent of about th part of the original tar, and then distillation ceases, although the temperature gradually rises. After a period of about an hour, more oils, like the former, lighter than water, are obtained, and so on the distillation proceeds, with successive intervals, yielding what are known as Creosote oils, and finally Anthracene oils, the residue in the still being pitch.
At first, when anthracene was of little importance, distillation was not pushed so far, and the anthracene oils were allowed to remain in the pitch; but since the discovery of the process for making artificial Alizarin (q.v.), the heat is pushed as far as possible consistent with the production of a pitch that will sell. The first light oils yield chiefly benzol, carbolic acid, and naphtha. The creosote oils yield creosote and naphthaline, while the anthracene oils produce anthracene and lubricating oils.
After this enumeration of the chief coal-tar products, it will be possible to realise the great importance of this substance. The naphtha, besides being used as a solvent for india-rubber and gutta-percha, is burned to produce a fine variety of carbon for printing-ink. The benzol, including in this term many nearly allied substances, not only yields many brilliant dyes, but is used for cleaning gloves, silks, &c., and other articles which would be injured by washing. The creosote in its crude form is largely used for preserving wood, enabling it to be exposed in damp situations without rotting, while, when burned, its smoke yields lampblack. The naphthaline, besides being a source of many dyes, is employed in the Albo-carbon light to give to ordinary coal-gas very high illuminating power. Finally, the residual pitch is in constant requisition for making roofing felt and asphalt pavement. Besides these primary products of coal-tar, there are of course numerous compounds derived more or less remotely from it. Such are the aniline dyes, the quinine substitutes, antipyin, antifibrin, &c., and the sweetening substance, Saccharine (q.v.), which may be used to replace sugar in many cases. For further references, see ANILINE, ALIZARIN, BENZENE, CREOSOTE, and NAPHTHALINE; also Lunge's Coal-tar and Ammonia (1887).