The oldest known extant oil paintings date from 650 A.D., found in 2008 in caves in Afghanistan's Bamiyan Valley, "using perhaps walnut and poppy seed drying oils. Though the ancient Mediterranean civilizations of Greece, Rome, and Egypt were familiar with vegetable oils, there is little evidence to indicate their use as media in painting. Indeed, linseed oil was long rejected as a medium because of its tendency to dry 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 from the Baltic Sea area, but advocated against the use of olive oil due to its excessively long drying time.
As early as the 13th century, oil was used to add details to tempera paintings. In the 14th century, Cennino Cennini presented a painting technique utilizing tempera painting covered by light layers of oil.
The modern technique of oil painting was created circa 1410 by Jan van Eyck. Though van Eyck was not the first artist to use oil paint, he was the first who is known to have produced a stable siccative oil mixture which could be used to bind mineral pigments. Van Eyck’s mixture probably consisted of piled glass, calcined bones, and mineral pigments boiled in linseed oil until reaching a viscous state.
Antonello da Messina later introduced another improvement to oil paint: he added litharge, or lead (II) oxide, to the mixture. The new mixture had a honey-like consistency and increased siccative properties. This medium was known as oglio cotto—"cooked oil."
Leonardo da Vinci improved the technique even further by cooking the mixture at a low temperature and adding 5 to 10% beeswax, which prevented dramatic darkening of the finished paint. Giorgione, Titian, and Tintoretto each slightly altered this recipe for their own purposes.
Since that time, experiments to improve paint and coatings have been conducted with other oils. Today, oils from bladderpod, sandmat, ironweed, and calendula plants are used to increase resistance or to decrease 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. Artists were no longer bound to the studio. They could work outside in direct sunlight, misty fog, at dawn or twilight. 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.
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.
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.
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.
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.
Chemistry of Oil Paint