Oil paint is a type of slow-drying paint consisting of small pigment particles suspended in a drying oil. Oil paints have been used in England as early as the 13th century for simple decoration, but were not widely adopted for artistic purposes until the 15th century. The most common modern application of oil paint is domestic, where its hard-wearing properties and luminous colors make it desirable for both interior and exterior use. Its slow-drying properties have recently been used in paint-on-glass animation.
Traditional oil paints require an oil that will gradually harden, forming a stable, impermeable film. Such oils are called siccative, or drying, oils, and are characterized by high levels of polyunsaturated fatty acids. One common measure of the siccative property of oils is iodine number, the number of grams of iodine one hundred grams of oil can absorb. Oils with an iodine number greater than 130 are considered drying, those with an iodine number of 115-130 are semi-drying, and those with an iodine number of less than 115 are non-drying. Linseed oil, the most prevalent vehicle for artists' oil paints, is a drying oil.
When exposed to air, oils do not undergo the same evaporative process that water does. Instead, they oxidize into a dry solid. Depending upon the source, this process can be very slow, resulting in paints with an extended drying time.
This earliest and still most commonly used vehicle is linseed oil, pressed from the seed of the flax plant. Modern processes use heat or steam in order to produce refined varieties of oil, which contain fewer impurities, but cold-pressed oils are still the favorite of many artists. Other vegetable oils such as Hemp, poppy seed, walnut, sunflower, safflower, and soybean oils may be used as alternatives to linseed oil for a variety of reasons. For example, safflower and poppy oils are paler than linseed oil and allow for more vibrant whites.
Once the oil is extracted additives are sometimes used to improve its chemical properties. In this way the paint can be made to dry more quickly if that is desired, or to have varying levels of gloss. Modern oils paints can, therefore, have complex chemical structures; for example, affecting resistance to UV or giving a suede like appearance.
New carriers for paint were developed out of organic polymer technology in the twentieth century. In many cases, such as acrylic paint, a different binder is substituted for oil. These new binders have different properties than oil paint, such as faster drying times and increased mechanical strength of the paint film. They require different (though overlapping) techniques and provide new possibilities that are not available to oil painters, such as the building of heavy texture and impasto, the use of collage, and the sculpting of the paint surface. Contemporary thinking therefore recognizes the new materials as separate mediums.
Some manufacturers, in an attempt to produce a medium that is oil-based but avoids toxic cleaners and thinners, have managed to produce water miscible oil paints. The vehicle for such paints is an oil with a surfactant molecule chemically bonded to it which allows oil to mix with water in much the same way dish soap does, but with greater sophistication.
The color of oil paint derives from the small particles mixed with the carrier, this is known as the pigment. Common pigment types include mineral salts such as white oxides: lead, now most often replaced by less toxic zinc and titanium, and the red to yellow cadmium pigments. Another class consists of earth types, e.g sienna or umber. Synthetic pigments are also now available. Natural pigments have the advantage of being well understood through centuries of use but synthetics have greatly increased the spectrum available, and many are tested well for their lightfastness.
Many of the historical pigments were dangerous. Many toxic pigments, such as emerald green (copper(II)-acetoarsenite) and orpiment (arsenic sulfide), to name only two, have fallen from use. Some pigments still in use are toxic to some degree, however. Many of the reds and yellows are produced using cadmium, and vermilion red uses natural or synthetic mercuric sulfide or cinnabar. Flake white and Cremnitz white are made with basic lead carbonate. The cobalt colors, including cobalt blue and cerulean blue, are made with cobalt compounds. Some varieties of cobalt violet are made with cobalt arsenate. Manufacturers advise that care should be taken when using paints with these pigments. They advise never to spray apply toxic paints. Read the health warnings on the label. Some artists choose to avoid toxic pigments entirely, while others find that the unique properties of the paints more than compensate for the small risks inherent in using them.
Zinc white and titanium white may carry a California health label for lead content. Those paints contain far less lead than the lead whites. Some manufacturers put the text "California only" above the warning.
Thinners such as turpentine and white spirit are flammable. Some of them, particularly the poor grades of turpentine, have a strong odor. Both turpentine and odorless mineral spirits can be harmful to the health if used inappropriately. Thinners made from D-limonene are thought by some to have some potential for risk. The EPA has not made that determination, however.
Generally speaking, these risks are minor if the materials are used as intended. Solvents can be made safer by painting in a well-ventilated area, and paint is likely only dangerous in the hands of small children.