Oil Varnishes. These, the most durable and lustrous of varnishes, are composed of a mixture of resin, oil, and spirit of turpentine. The oils most frequently employed are linseed and walnut; the resins chiefly used are copal and amber, and some other gums. The drying power of the oil having been increased by litharge, red lead, or by sulphate of lead, and a judicious selection of copal having been made, it is necessary, according to Booth, to bear in mind the following facts before proceeding to the manufacture of varnish: 1. That varnish is not a solution, but an intimate mixture of resin with boiled oil and spirit of turpentine. 2. That the resin must be completely fused previous to the addition of the boiled or prepared oil. 3. That the oil must be heated from 250° to 300°. 4. That the spirit of turpentine must be added gradually, and in a thin stream, while the mixture of oil and resin is still hot. 5. That the varnish be made in dry weather, otherwise moisture is absorbed, and its transparency and drying quality impaired. Of late years it has been practically demonstrated that not only is there no necessity for boiling the oil and gum after incorporation, but that the produce is equally good if the turpentine be added just before the mixture becomes too cold to permit of a perfect amalgamation. In fact, it is now acknowledged that the oil need not be raised to a higher temperature than that at which the gum employed fuses, and that when the two are mixed the lowest possible degree of heat which will insure their incorporation, is sufficient to secure all the results desired. By this method a largo quantity of the turpentine formerly lost in evaporation is saved, and there is, moreover, less risk of fire. The heating vessel must be of copper, of a capacity at least one-third more gallons than the mixture to bo introduced into it, with a riveted and not a soldered bottom. To promote the admixture of the copal with the hot oil, the copal—carefully selected and of nearly uniform fusibility - is separately heated with continuous stirring over a moderate charcoal fire kept constantly supplied with fuel, without disturbing the kettle until the completion of the mixture with the oil. If the copal is melted in the' hot oil, the resulting varnish is more colored and less drying. There is, however, great care required in fusing the copal by it-self; for if the heat is too much prolonged, the resin becomes pitchy, and gives an inferior varnish. Constant stirring is requisite to prevent adhesion to the sides and bottom of the vessel, and consequent scorching. The pieces of copal should bo of uniform fusibility; the different varieties, therefore, should not be fused together, for that which melts first is apt to scorch before the more refractory are fused. If it is desired to mix different varieties, they should be fused separately and then mixed in fluid state. When the resin is thoroughly melted, the hot oil is to bo ladled in gradually during constant stirring. To determine when sufficient oil has been added, a drop must be now and then taken out and cooled upon a glass plate. If on cooling, it is limpid and wax-like, penetrable with the nail without cracking, the proportion of oil is sufficient; if, however, it is hard and brittle, more oil is required. Some resins absorb more oil than others. The spirits of turpentine should bo heated, and added in a thin stream to the oil and resin while still hot. Care must bo taken not to add the turpentine while the mixture is too hot, as too much of the turpentine will bo lost by evaporation; but if the mixture gets too cool it becomes sticky, the addition of turpentine must be stopped, and it must be replaced over the fire and heated gradually up to G00°. Limpidity is thus restored, and, upon removal from the fire, sufficient turpentine should bo added to impart the proper consistence ; but this extra heating injures the quality of the varnish.