The oil in an oil stick is a drying oil, such as linseed oil, which means that it will form a hard permanent paint film. The oil and wax of the stick form a gel in which the uniformity of the wax encapsulates the oil, even when the oil is the predominant ingredient. When the stick is crushed on being applied to the painting surface, the wax structure breaks down into particles which are partially encapsulated by the oil which, when it dries is insoluble in painting solvents. The unencapsulated waxes, however, are partially soluble in these solvents.
Oil pastel generally contains more wax than an oil stick and employs a non-drying oil, such as mineral oil or castor oil. Work done in oil pastel, therefore, will not dry and needs to be protected by glass.