The oils used in oil paint serve two purposes. First, the paints are made by grinding pigments together with oil, which acts as an adhesive that attaches the pigment to the support and keeps the particles in suspension. Second, oils are used as a medium to modify paints, either by themselves or mixed with turpentine or resin. Locks the pigment particles into a film, protecting them from the environment and physical damage and from being disturbed by additional applications of paint. Oils are classified as either drying or semi-drying.
Linseed oil is a drying oil and the most widely used.
It has been utilized by painters since the 15th Century for both grinding pigments and as a paint medium. Safflower, sunflower, and poppy oils are all semi-drying. Because they are paler and have less of a tendency to yellow, they are often used for grinding white pigments. Semi-drying oils are often used to slow down the fast drying times of some pigments and can be used either by themselves or mixed with a drying oil. When using them, care should be taken to make sure that they are completely dry before applying the next paint layer.