film-forming substances prepared from vegetable oils or fatty alkyd resins. Drying oils are transparent liquids ranging in color from yellow to cherry red; they are good wetting agents for wood and metal. When a thin coat of a drying oil is applied to a surface, it dries as a result of polymerization upon exposure to atmospheric oxygen, forming an elastic film that is insoluble in water and organic solvents. The drying process may be accelerated by adding desiccants, such as salts of lead, manganese, or cobalt (0.01–0.1 percent metal per unit weight of the oil). The oil film dries completely in less than 24 hr at room temperature and relative humidity of 60–70 percent.

The preparation of drying oils involves chemical processing of oils (for example, partial polymerization at 270°–280°C), the addition of desiccants, and in certain cases, dilution with solvents. Selection of a drying oil for a specific purpose (see Table 1) is made on the basis of the requirements for the coating, as well as economic considerations. Materials composed of alkyd drying oils form the most weatherproof and durable coatings and have an important economic advantage over other types because of their low oil content.