The varnishes are solutions of the various resins, but which are by varnish makers commonly called gums, and those principally employed are amber, anime, copal, lac, sandarac, mastic, damar, and common resin, dissolved in linseed oil, turpentine, wood naphtha, or spirits of wine. The varnishes are all applied to the surfaces of the woods, metals, or other materials, while in the fluid state, like a thin paint, and the solvent is afterwards evaporated, leaving a thin glassy 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 in the same varnish, and in like manner the solvents are sometimes employed singly, and at other times are combined, according to the qualities required in the varnish.
The durability of the varnishes is of course mainly dependent upon the comparative insolubility of the resins, their hardness, toughness, and permanence of colour. 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 colour is but little influenced by the atmosphere; but unless very carefully selected, it is too yellow for delicate works of light colours. 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 colour; but it is not nearly so tough as amber.
The varnishes made from anime dry quickly, but are very liable to crack, and the colour 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 colourless, 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.
Amber, anime, and copal are therefore 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.
Lac and sandarac are more soluble than the above resins, and are generally dissolved in spirits of wine; but sometimes the pyroligneous spirit, commonly known as vegetable naphtha, is employed as a cheaper substitute. These resins constitute the basis of what are called spirit varnishes, and are employed principally for delicate objects not exposed to the weather, such as cabinet and painted works.
Lac is much harder and more durable than sandarac, and is the basis of most lackers for hardwood and metal, and also of the so called French polish. Of the three varieties, stick lac, seed lac, and shell-lac, the latter is the most free from colour, and the most soluble; it is therefore almost exclusively used in making varnishes and lackers; but the palest shell-lac contains a considerable quantity of colouring matter, that renders it inadmissible for varnishing works of a light colour. In addition, shell-lac also contains a small quantity of wax, and other matters, that are only imperfectly soluble in spirits of wine, and therefore give a cloudy appearance to the varnish, but which is not of great importance in varnishing dark coloured works, and may be in great measure avoided by making the solution without heat, and allowing the more insoluble portions time to be precipitated.
Sandarac is softer and less brilliant than shell-lac, but is much lighter in colour, it is therefore used for making a pale varnish for light coloured woods, and other works for which the dark colour of shell-lac would be unsuited. When hardness is of greater importance than paleness, a portion of shell-lac is added, but when paleness and brilliancy are required, a small quantity of mastic is added. When the varnish is required to be polished, Venice turpentine is added to give sufficient thickness or body.
Mastic is softer than any of the resins previously mentioned, and is dissolved either in spirits of wine or oil of turpentine, the latter is most generally used on account of it cheapness. With either of these solvents mastic makes a varnish of a very pale colour, that is brilliant, works easily, and flows better on the surface to which it is applied than most other varnishes. It is also tolerably flexible, and may be easily removed by friction with the hand; it is therefore much used for varnishing paintings, and other delicate works.