Damar is easily dissolved in oil of turpentine, and when carefully selected is almost colourless; it makes a softer varnish than mastic; the two combined however form an almost colourless varnish, moderately hard and flexible, and well suited for maps and similar purposes.
Common resin is generally dissolved either in turpentine or linseed oil with heat. Varnish made with resin is hard and brittle, but brilliant, and is principally employed to make cheap varnishes for common purposes in house painting, toys, and cabinet work. It is also added to other varnishes in order to improve their brilliancy, but it should be added in small quantities only, as a large proportion of resin renders the varnishes brittle.
Linseed oil is extensively employed as a vehicle for the harder resins, to which it imparts softness and toughness, but causes the varnish to dry slowly, and unless the oil is of the purest and palest quality, well clarified, and carefully combined with the resin, without excess of heat, it materially darkens the colour of the varnish when first made, and it is also liable to become darker by age after it is applied. Linseed oil intended for the best varnishes is clarified by gradually heating it in a copper pot so as to bring it nearly to the boiling point in about two hours; it is then skimmed and simmered for about three hours longer, when dried magnesia, in the proportion of about one quarter of an ounce to every gallon of oil, is gradually introduced by stirring; the oil is then boiled for about another hour, and afterwards suffered to cool very gradually. It is then removed into leaden or tin cisterns, and allowed to stand for at least three months, during which the magnesia combines with the impurities of the oil and carries them to the bottom, and the clarified oil is taken from the top of the cistern as it is required without disturbing the lower portion, and the settlings are reserved for black paint. A pale drying oil may also be made as above, by substituting for the magnesia, white copperas and sugar of lead, in the proportions of two ounces of each to every gallon of oil.
Linseed oil when rendered drying, by boiling and the addition of litharge and red lead, is sometimes used alone as a cheap extempore varnish. In boiling linseed oil, it is heated gradually to bring it to the boiling point in about two hours; it is then skimmed, and well dried litharge and red lead, in the proportion of about three ounces of each to every gallon of oil, are slowly sprinkled in, and the whole is boiled and gently stirred for about three hours, or until it ceases to throw up any scum, or emit much smoke. It is then frequently tested by dipping the end of a feather into it, and when the end of the feather is burnt off, or curls up briskly, the oil is considered to be sufficiently boiled, and is allowed to cool very slowly, during which the principal portion of the driers settle to the bottom. The oil is afterwards deposited in leaden cisterns screened from the sun and air. When the oil is required to be as pale as possible, dried white lead, sugar of lead, and white copperas are employed instead of the litharge and red lead.
Oil of turpentine is employed as a vehicle for most of the resins, the oil varnishes being generally thinned with hot oil of turpentine. Mastic, damar, and common resin are generally made into varnishes by dissolving them in oil of turpentine alone, either cold or with very moderate warmth. Varnishes made with turpentine only, dry quicker than those made with oil, and are paler coloured, but not so tough and durable. Turpentine varnishes hold an intermediate position between oil and spirit varnishes, and are employed principally on account of their cheapness and flexibility. Turpentine varies considerably in quality, and is greatly improved by age; that intended for varnish should be of the best quality, clear and limpid, and be kept for many months, or even years, before it is used; and when employed alone, as for mastic varnish, care should be taken that it is not passed through an oily measure, as is frequently the case in procuring small quantities.
Alcohol, or spirits of wine, is employed for dissolving sandarac and shell lac, to make the white and brown hard spirit varnishes, and lacker for hardwood or brass, and also French polish. The varnishes made with alcohol dry much quicker, harder, and more brilliant than those made with turpentine; but if the spirit contains more than a minute proportion of water, it will scarcely dissolve the resins, and when the varnish is applied, a very slight degree of moisture in the atmosphere will cause the resins to be precipitated from the solution, giving the varnish a dull, cloudy, or milky appearance. It is therefore of the first importance in making spirit varnishes to procure the alcohol as pure as possible.
Ordinary spirits of wine, however, always contains a considerable proportion of water, and is commonly tested for varnish purposes by saturating a slip of writing paper with the spirit, which is then ignited, and if the flame of the spirit communicates to the paper, and the whole is burned, the spirit is considered to be sufficiently good. But if, as frequently happens, the paper should be so far saturated with the water remaining from the evaporation of the spirit as to prevent its burning, the spirit is rejected as unfit for varnish purposes.
Weighing is, however, a far more exact test, the specific . gravity of absolutely pure alcohol being nearly .8, at a temperature of 60°, it may be easily tested by weighing 10 ounces of distilled water in a glass bottle, marking a line on the bottle to show the exact height of the water, and afterwards filling the bottle with spirit to the same height, and weighing it, when the excess over 8 ounces will show the proportion of water with tolerable accuracy; and should it not exceed 8 1/4 ounces, it may be considered to be of very good quality, spirit being frequently used for making varnish when its specific gravity is equal to .85.