70. Varnish is a solution of certain gums or resins in alcohol, turpentine, linseed oil, or the like, and is applied to produce a hard, shining, transparent coat on the surface. The oil dries, but the alcohol and turpentine evaporate, leaving, in either case, the fine transparent film of resin over the surface varnished.

To estimate the quality of a varnish, the following points are to be considered: (1) quickness in drying; (2) hardness of film or coating; (3) toughness of film; (4) amount of gloss; (5) permanence of gloss; (6) durability on exposure to weather.

The value of a varnish depends almost entirely upon that of its constituents, but much care and skill are demanded in mixing and boiling the ingredients. Varnish is used to give brilliancy to painted surfaces and to protect them from the action of the atmosphere or from any slight friction. Varnish is often applied to plain unpainted wood surfaces in the roofs, joinery, and fittings of houses, to intensify and brighten the ornamental appearance of the grain. It is also applied to painted and papered walls. It is, in the former case, sometimes flatted, to give a dead appearance, similar to that of a flatted coat of paint.

The resins from which varnishes are most usually made are, in general appearance and characteristics, very much the same, but differ in name, according to the tree or shrub from which they are gathered, and, in most cases, give their names to the varnish into which they enter. Thus, we have copal varnish, dammar varnish, mastic varnish, shellac varnish, etc., each of which is made from the gum its name indicates. Before considering the various applications of these varnishes, let it here be understood that they are all more or less interchangeable for all purposes, the specific applications being governed as well by the quality of work required as by the cost of the varnish.

11. Copal varnish is thus prepared: In an iron pot, over a slow fire, eight parts of powdered copal gum are melted with two parts of balsam copaiba, previously warmed. The melted gums are then removed from the fire and reduced to the proper working consistency, by the addition of about 10 parts of oil of turpentine.

Gum copal is more soluble in turpentine, when the gum is powdered and melted, and then permitted to stand for some time in a loosely covered vessel. The varnish is, under this method, best made with 24 parts of powdered copal, 40 parts turpentine, and 1 part camphor.

72. White copal varnish is prepared by dissolving 8 parts of copal and 1 part camphor in 6 parts of white drying oil and 4 parts of the essential oil of turpentine. The powdered copal is mixed with the camphor and drying oil, then heated, the turpentine added, and the varnish strained.

73. Gum mastic affords a base for a varnish nearly as hard as copal varnish, but better adapted to certain classes of work, where subsequent rubbing is not required. It is particularly useful to the artist in varnishing pictures, and is incidentally used, for the same purpose, by the fresco painter, when his works are executed in oils.

Mastic varnish is prepared as follows: A vessel containing 10 ounces of gum mastic and 1 pint of turpentine is heated over a sand bath until the mastic is completely dissolved. The varnish is then strained through a sieve and is ready for use unless too thick, in which case its consistency may be reduced by the addition of a sufficient quantity of spirits of turpentine.

74. Magilp is the term applied to the composition resulting from a mixture of equal portions of turpentine and pale dry oil, of linseed drying oil and mastic varnish, of simple linseed oil and sugar of lead, or of boiled oil, mastic varnish, and sugar of lead combined. These ingredients gelatinize when mixed with oil colors and give them a certain amount of body and pulpy transparency.

75. Varnish resisting boiling water and well adapted to the finishing of floors, or other surfaces likely to meet with hard wear, may be made of: linseed oil 1/2 pound, amber 1 pound, pulverized litharge 5 ounces, powdered white lead 5 ounces, minium 5 ounces. Boil the linseed oil in a copper vessel, suspend in it the litharge and minium in a small bag, which must not, however, touch the bottom of the vessel; continue the ebullition until the oil has acquired a deep brown color; then take out the bag and put in a clove of garlic; this to be repeated seven or eight times during the continuance of the boiling. Before the amber is added to the oil, it is to be mixed with 2 ounces of linseed oil and melted over a hot fire; when the mass is fluid, it is to be poured into the linseed oil. The mixture is again boiled and stirred continuously for two or three minutes. It is afterwards filtered and preserved in a bottle well corked. This varnish is to be used only on wood previously well polished and covered with a thin coat of soot and oil of turpentine. When this coat is dry, some of the varnish is applied with a sponge, care being taken that it is equally distributed in every part. This operation is to be four times repeated, and attention always paid that one coat be well dried before the application of another. After the last coat of varnish, the wood is thoroughly dried and afterwards polished.

76. Soft resins are sometimes substituted for mastic, and inferior hard resins also employed in place of copal, in the composition of varnishes known as copal varnishes. Copal is of difficult solution in turpentine and linseed oil, both of which enter into the composition of the ordinary copal varnishes, employed by the coach painter, affording the best varnishes used by the house painter and grainer. Combined with linseed oil and oil of turpentine, copal varnish affords a vehicle superior in texture, strength, and durability to mastic or magilp, though in application a less attractive instrument and more difficult of management.

As copal swells while dissolving, its solutions and varnishes contract and consequently split in drying, making unsightly fissures, against which linseed oil is the essential preventative. The mixture of copal varnish and linseed oil is best effected through the medium of oil of turpentine, heat being for this purpose sometimes requisite.

77. White hard spirit varnish is a composition used very largely for japanned work, and may be prepared by dissolving in 3 pints of rectified spirits (65 over proof) one pound of gum sandarac, and adding to the solution 6 ounces of turpentine; or, if sandarac cannot be readily obtained, the varnish may be made of 4 ounces of gum mastic and 1/2 pound of gum juniper, dissolved in 4 pints of rectified spirits, adding 1 ounce of turpentine. When the varnish is to be used on metal work, or when polishing is necessary, 2 ounces of mastic in tears, 8 ounces of sandarac, and 1 ounce of gum elemi are dissolved in 4 ounces of turpentine and 1 quart of rectified spirits (65 over proof).

78. A cheap varnish, suitable for ordinary work, can thus be made: Place 3 pounds of powdered resin in a vessel, add 2 1/2 pints of oil of turpentine; shake well and allow the mixture to stand for a day or two, meanwhile shaking it occasionally. Then add 5 quarts of boiled oil, shake the whole and allow it to stand in a warm room till clear. The clear portion is then poured off for use, and may be reduced in consistency by the addition of turpentine. This varnish is intended for protecting surfaces against the effects of exposure to the atmosphere, and has been used with advantage, where no direct impairment by wear is involved. Unable to resist the effect of constant wear and less brilliant than other varnishes, this ordinary work varnish is unfit for fine results or for work subject to continual usage.

79. White lac varnish is prepared by dissolving in alcohol or spirits of wine the lac resin of India, chemically deprived of all coloring matter and purified from gluten, wax, and other extraneous substances with which it is naturally combined. The varnish it affords, without this purifying process, is opaque, dark in color like the japans and lacquers of the East, but when thus purified is brilliantly transparent, very hard, and nearly colorless. This being a spirit varnish, requires a warm temperature, helpful, indeed, to all varnishes; and enjoys the valued distinction of drying rapidly.

White lac varnish being somewhat costly, its place is often filled, in ordinary work, by the common white shellac varnish, consisting of the softer resins dissolved in alcohol with shellac. To prepare it, dissolve 2 ounces mastic, 1 1/2 pounds shellac, 4 ounces seed lac, and 4 ounces sandarac in one gallon of rectified spirits of wine. Gum, benzoin, dragon's blood, turmeric, or other coloring matters may be added as required, but the natural color is nearly white.

80. Spirit varnishes or lacquers are made with softer gums, such as lac and sandarac, dissolved in spirits of wine or pyroligneous spirit. They dry more rapidly, become harder and more brilliant than turpentine varnishes, but being apt to split and scale off, are used for cabinet and other work not exposed to the weather.