A type of ink vehicle—the fluid carrier of the ink pigment—that dries by oxidation (the absorption of and chemical reaction with oxygen) of the vehicle, rather than by evaporation or absorption into the substrate. Once an ink vehicle has absorbed oxygen, it hardens by a process called polymerization, or the combining of simple molecules to form a long, chain molecule called a polymer. Linseed oil is the drying oil most commonly used in inks, which must be boiled to impart the desired viscosity to the vehicle. Other oils used for drying oil vehicles include castor oil, tung oil, cottonseed oil, fish oil, petroleum drying oils, rosin oil, soybean oil, and various types of synthetic oils. Synthetic resins and other substances may be added to the vehicle to prevent absorption of the vehicle by the substrate, as high-gloss inks require a high degree of ink holdout in order to dry with the desired level of gloss. Paper that has a low degree of ink absorbency is required for these types of inks. Drying oil vehicles are commonly used in letterpress and offset lithographic inks. (See Vehicle.)
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