Rosin, the residue after the distillation of the volatile oil from the turpentine of different species of pines. It is rather an incidental product of the preparation of the oil of turpentine, which, though amounting to only 10 to 25 per cent. of the turpentine (and the rosin constituting the large remainder), is by far the most valuable product. (See Turpentine.) The rosin while still liquid is drawn off into metallic receivers coated with whiting to prevent adhesion, and from these it is transferred to the casks for shipment. When the distillation is stopped at the proper point, the product is the yellow rosin, which contains a little water; or this may be expelled, and the product is then transparent rosin. By continuing the heat the residue in the stills is made brown or black, a variety which in Europe is sometimes known as colophony. Rosin melts at 276° F., and becomes completely liquid at 306°; at 316° it emits bubbles of gas, and at a red heat it is entirely decomposed. Its specific gravity varies from 1.07 to 1.08. It is insoluble in water, but dissolves easily in alcohol, ether, wood spirit, and both fixed and volatile oils. Strong acids dissolve and decompose it.
Chemically it is for the most part a mixture of several resinous acids, viz.: picric, which forms the principal part, sylvic, and colopholic; sometimes also pimaric acid. These acids are isomeric, having a common formula, C20H30O2. They are perhaps formed by oxidation of oil of turpentine. When quickly heated in a retort, it distils partly undecomposed, and partly resolved into gases and volatile oils, leaving a small residue of carbonaceous matter. When the distillation is performed on a larger scale, the gases evolved are air, carbonic acid, carbonic oxide, and carbides of hydrogen; at a higher temperature the oxygen disappears. The first portion of the liquid distillate is yellow and mobile; later a viscid, fluorescent oil passes over, called rosin oil. At a red heat rosin yields a mixture of gases, burning with a very luminous flame, which are largely used in villages and isolated buildings instead of coal gas. - Many attempts have been made to bleach the common sorts of rosin, which would add materially to their value. By the process of Messrs. Hunt and Pochin, the rosin is distilled at a temperature below that by which it would be decomposed, the process being conducted with steam under a pressure of ten atmospheres.
The maximum temperature allowed is about 600°. The rosin and steam are collected and condensed in a suitable receiver kept as cold as possible by the application of water, and free from the moisture of the condensed steam. Instead of steam, carbonic acid, or a mixture of carbonic acid and nitrogen, or hydrogen gas, etc, is introduced to decolor the rosin. The product is white and almost transparent, and is greatly preferred to the crude article by soap and varnish makers. - Rosin is employed for a variety of useful purposes. It is an ingredient in varnishes, and is united with tallow in the preparation of cheap candles. It answers to some extent as a substitute for fixed oil or fat in the manufacture of yellow soap; but, without glycerine in its composition, it possesses no true saponifying properties. (See Soap.) Rosin is also used in perfumery, and in various pharmaceutical preparations, as plasters and ointments. In caulking the seams of ships it is used in a melted state to fill them, and by oakum makers it is intermixed in a pulverized state with the oakum to increase its weight. It enters into the composition of some fireworks, and is used as a reducing agent in soldering.
Another well known use of it is for covering the bows of violins, to prevent them from slipping over the strings without producing vibration. In France rosin oil is largely used as an ingredient in printers' ink, and elsewhere in the composition of coarse lubricating oils. Nearly all the rosin of commerce is furnished by North America.