A variety of red pigments are made from oxide of iron, varying in hue from a pale to a deep brownish-red. They are quite permanent, and may be used under any conditions. The vehicle is a liquid in which the particles of the base are held in suspension, enabling a thin coat of paint to be formed, uniform in color and consistency, and which on drying forms a kind of skin over the surface to which it is applied. For oil paint the vehicles used are oils; for distemper water is employed. The oils used as vehicles are chiefly linseed oil, raw and boiled, and poppy-seed oil. Nut oils are occasionally used for inferior work because they are much cheaper. Linseed oil, the one most commonly used, is obtained from the seeds of flax by warming it and squeezing out the oil under hydraulic pressure. The resultant, which is of a transparent amber color, is known as "raw oil". It is used principally in interiors for light, bright colors, drying somewhat slowly and giving a firm elastic coat. The oil improves by keeping, and is sometimes refined with acids or alkalis. Boiled oil is the raw oil heated with driers, such as litharge or red lead, to a temperature from 350° to 500 °F, at which it is maintained for three or four hours. It is thick and much darker in color than the raw oil, drying much more quickly, with a coat hard and glossy but less elastic than that produced by raw oil. Poppy-seed oil is expressed from the seeds of the poppy plant. It does not possess the tenacity and quick-drying powers of boiled linseed oil, but being of a very light color it is used for delicate colors. Turpentine is used as a solvent, diluent, or thinner, to bring the paint to a proper consistency so as to allow it to be spread in a thin even coat. When a flat dull surface is desired, turpentine alone is used with the base and the oil is omitted. The best turpentine comes from the pine forests of America. French turpentine is next in quality. Russian turpentine is the cheapest, and has usually a strong and unpleasant odor that renders it objectionable to work with. In consequence of the high price of turpentine of good quality, and the increasing difficulty of obtaining it, substitutes are coming into general use. Driers are substances usually added to paint to hasten the process of oxidation, i.e. the drying, of the oil.