Varnish. Varnishes may be conveniently divided into two kinds, viz., spirit and oil varnishes. Concentrated alcohol is used as the solvent in the former, and fixed or volatile oils, or mixtures of the two, for the latter. The specific gravity of alcohol for the purpose of making varnishes should not bo greater than 0.820 (that is, not below about 93 per cent). Camphor is often dissolved in it to increase its solvent powers. The oil of turpentine, which is the essential oil chiefly employed, should be pure and colorless. Palo drying linseed oil is the fixed oil generally used for varnishes, but poppy and nut oil are also occasionally employed. Among the substances employed in the manufacture of varnishes are turpentine, copal, mastich, lac, elemi, sandarach, anime, and amber, to impart body and lustre; benzoin to impart scent; gamboge, turmeric, saffron, annotto, and Socotrine aloes, to give a yellow color; dragon's blood to give a red tinge; asphaltum to give a black color and body; caoutchouc to inpart body, toughness, and elasticity. Varnish constitutes a distinct branch of manufacture, and many of them can be advantageously or safely made only on the large scale on premises adapted for the purpose.

2868. Preparation of Linseed Oil for Making Oil Varnishes

2868. Preparation of Linseed Oil for Making Oil Varnishes. In the manufacture of oil varnishes, one of the most important points is the use of good drying oil. Linseed oil for this purpose should be pale, limpid, brilliant, scarcely odorous, and mellow and sweet to the taste. 100 gallons of such oil arc put into an iron or copper boiler capable of holding 150 gallons, and gradually heated to a gentle simmer for 2 hours, to expel moisture; the scum is then carefully removed, and 14 pounds scale litharge, 12 pounds red lead, and 8 pounds powdered umber (all carefully dried and free from moisture), are gradually sprinkled in; the whole is then kept well stirred, to prevent the dryers sinking to the bottom, and the boiling is continued at a gentle heat, for 3 hours longer; the fire is next withdrawn, and, iu 24 to 36 hours, the scum is carefully removed, and the clear oil decanted from the bottom. This forms the best boiled or drying oil.

2869. Clarified Oil for Varnish

2869.   Clarified Oil for Varnish. When boiled oil is used for making varnish, and a still further clarifying is deemed advisable, it is placed in a copper pan holding from 80 to 100 gallons, and heat gradually applied till the scum rises, after removing which the oil is allowed to boil for about 2 hours, when it is dosed with calcined magnesia, in the proportion of an ounce to every 4 gallons of oil, but added by degrees and with occasional stirrings. This being completed, the oil is again boiled briskly for about an hour, and then, the furnace being drawn, allowed to cool. When the temperature is sufficiently reduced, it is removed to leaden cisterns, where it is stored till fit for use.