Oil paint is a type of slow-drying paint that consists of particles of pigment suspended in a drying oil, commonly linseed oil. The viscosity of the paint may be modified by the addition of a solvent such as turpentine or white spirit, and varnish may be added to increase the glossiness of the dried film. Oil paints have been used in England since the 13th century for simple decoration, but were not widely adopted as an artistic medium until the 15th century. Common modern applications of oil paint are in finishing and protection of wood in buildings and exposed metal structures such as ships and bridges. Its hard-wearing properties and luminous colors make it desirable for both interior and exterior use on wood and metal. Due to its slow-drying properties, it has recently been used in paint-on-glass animation. Thickness of coat has considerable bearing on time required for drying: thin coats of oil paint dry relatively quickly.
The slow-drying properties of organic oils were commonly known to early painters. However, the difficulty in acquiring and working the materials meant that they were rarely used. As public preference for realism increased, however, the quick-drying tempera paints became insufficient. Flemish artists combined tempera and oil painting during the 1400s, but by the 1600s easel painting in pure oils was common, using much the same techniques and materials found today.
The oldest known extant oil paintings date from 650 A.D., found in 2008 in caves in Afghanistan's Bamiyan Valley, "using walnut and poppy seed oils." Though the ancient Mediterranean civilizations of Greece, Rome, and Egypt used vegetable oils, there is little evidence to indicate their use as media in painting. Indeed, linseed oil was not used as a medium because of its tendency to dry very slowly, darken, and crack, unlike mastic and wax.
Greek writers such as Aetius Amidenus recorded recipes involving the use of oils for drying, such as walnut, poppy, hempseed, pine nut, castor, and linseed. When thickened, the oils became resinous and could be used as varnish to seal and protect paintings from water. Additionally, when yellow pigment was added to oil, it could be spread over tin foil as a less expensive alternative to gold leaf. Early Christian monks maintained these records and used the techniques in their own artworks. Theophilus Presbyter, a 12th century German monk, recommended linseed oil but advocated against the use of olive oil due to its long drying time.
Today's technique of oil painting was created circa 1410 by Jan van Eyck. Though van Eyck was not the first to use oil paint, he was the first artist to have produced a siccative oil mixture which could be used to combine mineral pigments. Van Eyck’s mixture may have consisted of piled glass, calcined bones, and mineral pigments boiled in linseed oil until reaching a viscous state. Or he may have simply used Sun-thickened oils (slightly oxidized by Sun exposure). He left no written statement.
Antonello da Messina later improved oil paint: he added litharge, or lead (II) oxide. The new mixture had a honey-like consistency and increased drying properties. This mixture was known as oglio cotto—"cooked oil."
Leonardo da Vinci later improved these techniques by cooking the mixture at a very low temperature and adding 5 to 10% beeswax, which prevented darkening of the paint. Giorgione, Titian, and Tintoretto each may have altered this recipe for their own purposes.
The use of any cooked oils or Litharge (sugar of Lead) will darken an oil painting rapidly. None of the old Masters whose work survives used these in their paintings. Both ingredients became popular in the 19th century.
Since that time, experiments to improve paint and coatings have been conducted with other oils. Modern oil paints are created from bladderpod, ironweed, calendula and sandmat, plants used to increase the resistance or to reduce the drying time.
The paint tube was invented in 1841 and artists were liberated from the studio. Artists no longer needed to grind each pigment by hand and carefully mix the binding oil in the proper proportions. Paints were made in bulk and sold in tin tubes with a cap. The cap could be replaced and the paints preserved for future use. The manufactured paints had a balanced consistency that the artist could thin with turpentine if he chose. No longer bound to the studio, they could work at whatever place or time they derived inspiration from.
Paint in tubes also changed the way artists applied paint to the canvas. Painting became much more spontaneous. Artists were no longer obliged to paint in careful layers of thinned pigments and varnish, although they could use that time-tested method if they chose. With paint in tubes, a greater variety of techniques could be employed, such as blending the paint on the canvas and painting directly on the raw, ungessoed surface. The effect of paint in tubes was so important that it contributed to the rise of the impressionist style. The artist Renoir said, “Without tubes of paint, there would have been no impressionism.” Thanks to the mobility that paint in tubes provided, artists could capture the light of a fleeting moment of the day, and the impressions that it provided.
Many modern paint in tubes manufactures have extensive color testing done. The results of this test will tell the artist which paint will last the longest without the colors altering after a period of time.
Artists can also buy unused tubes and fill them with paint. The filling is done through the bottom. Then the bottom is folded and crimped. Paint for later use is accessed through the screw cap at the other end — as usual. This can be useful for having on hand a mixed color that is found to be a good starting point for mixing a color within a commonly used range, as concerns an artist's personal preferences.
Many artists today consider oil painting to be the fundamental art medium; something that a student should learn to appreciate, because of its properties and use in previous, very popular artwork. Typical qualities of oil paint include a long "open time," which means that the paint does not dry quickly. Oil paints take several weeks to dry, this allows the artist to work on a painting for many sessions. Oil paint also has a propensity to blend into surrounding paint allowing very subtle blending of colors. This medium also produced vivid color with a natural sheen and distinct contrast. Oil paints have a surface translucency similar to human skin, making it an ideal medium for portraits.
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.
The 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 straight from the tube.
Once the oil is extracted additives are sometimes used to modify 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 media.
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. these paints still contain dryers and are by nature toxic.
Unlike water-based paints, oils do not dry by evaporation. The drying of oils is the result of an oxidative reaction, chemically equivalent to slow, flameless combustion. In this process, a form of autoxidation, oxygen attacks the hydrocarbon chain, touching off a series of addition reactions. As a result, the oil polymerizes, forming long, chain-like molecules. Following the autoxidation stage, the oil polymers cross-link: bonds form between neighboring molecules, resulting in a vast polymer network. Over time, this network may undergo further change. Certain functional groups in the networks become ionized, and the network transitions from a system held together by nonpolar covalent bonds to one governed by the ionic forces between these functional groups and the metal ions present in the pigment.
Vegetable oils consist of glycerol esters of fatty acids, long hydrocarbon chains with a terminal carboxyl group. In oil autoxidation, oxygen attacks a hydrocarbon chain, often at the site of an allylic hydrogen (a hydrogen on a carbon atom adjacent to a double bond). This produces a free radical, a substance with an unpaired electron which makes it highly reactive. A series of addition reactions ensues. Each step produces additional free radicals, which then engage in further polymerization. The process finally terminates when free radicals collide, combining their unpaired electrons to form a new bond. The polymerization stage occurs over a period of days to weeks, and renders the film dry to the touch. However, chemical changes in the paint film continue.
As time passes, the polymer chains begin to cross-link. Adjacent molecules form covalent bonds, forming a molecular network that extends throughout the painting. In this network, known as the stationary phase, molecules are no longer free to slide past each other, or to move apart. The result is a stable film which, while somewhat elastic, does not flow or deform under the pull of gravity.
During the drying process, a number of compounds are produced that do not contribute to the polymer network. These include unstable hydroperoxides (ROOH), the major by-product of the reaction of oxygen with unsaturated fatty acids. The hydroperoxides quickly decompose, forming carbon dioxide and water, as well as a variety of aldehydes, acids, and hydrocarbons. Many of these compounds are volatile, and in an unpigmented oil, they would be quickly lost to the environment. However, in paints, such volatiles may react with lead, zinc, copper or iron compounds in the pigment, and remain in the paint film as coordination complexes or salts. A large number of free fatty acids are also produced during autoxidation, as most of the original ester bonds in the triglycerides undergo hydrolysis. Some portion of the free fatty acids react with metals in the pigment, producing metal carboxylates. Together, the various non-cross-linking substances associated with the polymer network constitute the mobile phases. Unlike the molecules that are part of the network itself, they are capable of moving and diffusing within the film, and can be removed using heat or a solvent. The mobile phase may play a role in plasticizing the paint film, preventing it from becoming too brittle.
One simple technique for monitoring the early stages of the drying process is to measure weight change in an oil film over time. Initially, the film becomes heavier, as it absorbs large amounts of oxygen. Then oxygen uptake ceases, and the weight of the film declines as volatile compounds are lost to the environment.
As the paint film ages, a further transition occurs. Carboxyl groups in the polymers of the stationary phase lose a hydrogen ion, becoming negatively charged, and form complexes with metal cations present in the pigment. The original network, with its nonpolar, covalent bonds is replaced by an ionomeric structure, held together by ionic interactions. At present, the structure of these ionomeric networks is not well understood.
The color of oil paint derives from the small particles mixed with the carrier. 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 Paris 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, and advise never to spray apply toxic paints. Some artists choose to avoid toxic pigments entirely, while others find that the unique properties of the paints more than compensate for the 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 and no empirical evidence exists for this claim. 
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.
Chemistry of Oil Paint