Varnishes are solutions of the various resins, commonly called gums, in either oil, turpentine, or alcohol. The gums principally applied are amber, anime, copal, lac, sandarac, mastic, damar and common resin. The varnishes are all applied to the surfaces of the woods, metals, or other materials, while in the fluid state, and the solvent is afterwards evaporated, leaving a thin glossy coat of the different resins as a defence from the action of the atmosphere, or from slight friction.

Sometimes the resins are used separately; at other times two or more are combined according to the qualities required in the varnish.

The Gums And Their Qualities

Amber. - The durability of the varnishes is of course mainly dependent upon the comparative insolubility of the resins; their hardness, toughness, and permanence of color. In these respects amber excels all other resins used for varnishes; it resists the action of all ordinary solvents, and can only be dissolved for making varnish by fusion at a high temperature; it is hard and moderately tough, and its color is but little influenced by the atmosphere; but, unless very carefully selected, it is too yellow for delicate works of light colors. Amber is, however, but little used in making varnishes, principally on account of its high price, but partly because the varnish dries slowly, and does not attain its full hardness for many weeks.

Anime is nearly as insoluble and hard as amber, and the best is of a very pale color; but it is not nearly so tough as amber. The varnishes made from anime dry quickly, but are very liable to crack, and the color becomes deeper by exposure to light and air. Anime is, however, extensively used in making oil varnishes, and most of those called copal varnishes contain a considerable proportion of anime, which is substituted principally on account of its quick drying qualities.

Copal is next in durability to amber; when very carefully selected it is almost colorless, and becomes rather lighter by exposure; it is more easily dissolved by heat than either amber or anime, and although softer than these resins, is too hard to be scratched by the nail. Copal is, therefore, a most excellent material for varnish, and numerous attempts have been made to employ it as the basis of a spirit varnish, but hitherto with only partial success. Pure alcohol has little effect on copal; with the addition of a small quantity of camphor, the greater portion of the copal is dissolved, but the camphor impairs the durability of the varnish. Copal may be perfectly dissolved by ether, but this spirit evaporates too rapidly to allow of the varnish being uniformly applied. The essential oils of spruce and lavender have been occasionally employed as solvents of copal, but not with sufficient success to warrant its general adoption in spirit varnishes.

Oil Varnishes

Amber, anime, and copal are usually dissolved for making varnish by fusing the gum, and adding linseed-oil heated nearly to the boiling point. They are then amalgamated by stirring and boiling, and the varnish is reduced to the required degree of fluidity by the addition of oil of turpentine. They constitute the more important of what are called oil varnishes, are the most durable of all, possess considerable brilliancy, and are sufficiently hard to bear polishing. They are therefore employed for works of the best quality, that are exposed to the weather or to much friction; as coaches, house decorations, and japanning.