A dispersion of pigments in a drying oil that forms a tough, coloured film on exposure to air. The drying oil is a vegetable oil, often made by crushing nuts or seeds. For paints, linseed oil is most commonly used, but poppy, sunflower, safflower, soya bean and walnut oils have also been used. Drying oils initially cure through oxidation leading to cross linking of the molecular chains; this is a slow process affected by film thickness and paint components. Artists have used turpentine or mineral spirits to dilute oil paint. A heavily diluted layer dries relatively quickly, being tack-free in a few days. Thicker layers, containing more oil, take longer. Oil paint continues to dry, getting harder with age over many decades. Pigments and extenders will also affect the rate of drying, so different colours may dry at different speeds.