Varnish is a solution of resin which dries to form a hard transparent film. It is widely applied to wood, metal, and masonry to improve surface properties without changing appearance. There are two main types: "spirit varnishes," consisting of natural or synthetic resins dissolved in a volatile solvent such as alcohol; and "oleoresinous varnishes" – more resistant to heat and weather – which are mixtures of resins and drying oils dissolved in turpentines or a petroleum oil. Lacquer, the original wood varnish, is the sap of the varnish tree.