Varnish. The greater number of varnishes consist of some resinous material (see LAC, and RESINS) dissolved in linseed-oil, alcohol, or some liquid hydrocarbon such as turpentine and benzole. Those made by dissolving a resin in a non-volatile drying oil, like that from linseed, are called oil-varnishes; and those prepared by using volatile solvents (alcohol, benzole, &c.) for the resins are called spirit varnishes. In the case of the latter the solvent becomes dissipated as the varnish dries, so that when any surface is coated with a varnish of this kind only a film or coating of resin remains, which is apt to crack and peel off; but means are taken either in the preparation of the varnish or in the laying of it on to counteract this tendency. With an oil-varnish, on the contrary, the oil remains as part of the coating, giving it toughness, while the resin gives it hardness.
For shell-lac varnish, the spirit-varnish most largely made, see FRENCH POLISHING; and for the varnish called Lacquer, used for coating brass, see that head. Bookbinders use a spirit-varnish, composed of elemi, mastic, and sandarac resins, together 14 parts, Venice turpentine 3 parts, and alcohol 30 parts. They also use among other lacquers one consisting of shell-lac 10, turpentine 1, and alcohol 30 parts. A good spirit-varnish for woodwork is sandarac resin 40, Venice turpentine 4, and alcohol (methylated spirit) 120 parts.
The best oil or fat varnishes are prepared from hard resins, along with linseed-oil and turpentine. Copal and amber, the two hardest resins, require a preliminary heating or fusing before mixing them with the solvents. The proportions given for a good copal varnish are 30 parts of melted copal resin, 100 parts of linseed-oil, and 70 parts of oil of turpentine, part of the oil in a hot state being first mixed with the melted resin. Three parts of litharge, or of a part of borate of manganese is added to act as a 'dryer.' Throughout the process of preparing the varnish a carefully regulated heat is used, and the temperature of the mixture is allowed to fall to 140° F. before the turpentine is added. A copal varnish is also made on a more limited scale by a newer process, which consists in first treating the resin with a powerful solvent (afterwards drawn off by distillation), such as acetone or ether, and afterwards adding 'manganese' oil (linseed-oil and borate of manganese) and turpentine. There are several kinds of copal resins (see COPAL); but the best kind is obtained from Zanzibar, and is now known to be produced by Trachylobium verrucosum. Inferior 'copal' varnishes are made from kauri and other cheap resins. The use of oil-varnishes which are sold at a low price (some of them at one-half that of good copal) should be avoided, as many of these never dry properly. An elastic varnish, useful for rendering textile fabrics waterproof, can be made with the aid of heat, by mixing together common resin 2 lb., caoutchouc 16 oz., linseed-oil 2 lb. It may be stated here that, from the inflammable nature of the materials used, the making of most varnishes is attended with much risk of fire.
Oil-varnish is extensively used for coating the internal woodwork of houses, either after it has been previously painted, or simply to cover the bare wood without hiding its grain. Good copal varnish is the best kind that can be used for this purpose. If properly put on it forms a fine glossy, enamel-like surface, which is very durable, and stands washing with soap and water repeatedly without injury. To produce such a surface at least five coats of the varnish are required, and each in turn is rubbed smooth with pumice except the last. It is only in the best class of work, however, that more than two or three coats are given. A coating of varnish does not increase the durability of oil-paint on external woodwork.
A pure transparent mastic varnish is used for oil-pictures, but, as it has the defect common to spirit-varnishes of being easily abraded, copal oil-varnish is often preferred for this purpose. A less pure mastic varnish is used for maps. As spirit-varnishes dry very quickly, it is often convenient to use suitable kinds of them for coating metal, wood, &c., in cases where it is inconvenient to give proper time for oil-paint or oil-varnish to dry. Japan varnish is noticed under JAPANNING.
See Braunt's translation of Andres on Volatile and Fat Varnishes (1882), and Church's Chemistry of Paints and Painting (1890).