Oil paint

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View of Delft in oil paint, by Johannes Vermeer.

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,^[1] 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 colours make it desirable for both interior and exterior use. Its slow-drying properties have recently been used in paint-on-glass animations.

1 History
2 Practical properties of oil paint
3 Carrier
- 3.1 Non-oil carriers
4 How oil paint dries
5 Pigment
6 Toxicity
7 References
8 See also

[edit] History

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.

Though the ancient Mediterranean civilizations of Greece, Rome, and Egypt were familiar with vegetal 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.

However, 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 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.

During his stay in Italy, Rubens studied the Italian oil paint mixture. He later made his own improvement, using walnut oil warmed with litharge and adding mastic dissolved in turpentine.

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.

[edit] Practical properties of oil paint

Many artists today consider oil paint to be one of the fundamental art media; 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:

the long open time, where paint will not dry for up to several weeks, allowing the artist to work on a painting for several sessions.
the propensity for the paint to blend into surrounding paint allowing very subtle blending of colors.
vivid, high chroma colors

[edit] Carrier

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' 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, made from the seed of the flax plant. The seeds are crushed and the oil extracted. 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.^[2] Other sources of carrier oils exist. Hemp, poppy seed, walnut, sunflower, safflower, and soybean oils are often used as an alternative to linseed oil. Other oils are used for a variety of reasons. Some oils, such as walnut and poppy, are paler and allow for more vibrant whites.

Once the oil is extracted additives are sometimes used to improve its chemical properties. In this manner 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.

[edit] Non-oil carriers

The twentieth century saw the development of new carriers for paint. In many cases, such as acrylic paint, a different binder is substituted for oil. Fully substituting oil based paints by those based on a completely different medium that displays characteristics that are, for the most part, not oil-like, will reduce the artists range of possible expression. Some manufacturers, in an attempt to produce a medium that is oil-based but avoids toxic cleaners and thinners, have managed to produce produce water-based 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.

[edit] How oil paint dries

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 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.

[edit] Pigment

The colour of oil paint derives from the small particles mixed with the carrier. Common pigment types include mineral salts such as white oxides: lead, 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.

[edit] Toxicity

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. Flake white and Cremnitz white are made with basic lead carbonate. The cobalt colors, including cerulean blue, are made with cobalt. 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 odour. 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. [[1]]

[edit] References

^ Charles Eastlake, Materials for a History of Oil Painting, Longman, Longman, Brown, Green, and Longman, 1847.
^ H. Gluck, "The Impermanences of Painting in Relation to Artists' Materials", Journal of the Royal Society of Arts, Volume CXII 1964

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