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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.

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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

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.

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.

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.

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.

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. Manufacturers advise that care should be taken when using paints with these pigments. They 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 small risks inherent in using them.

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]]

References

General

  • Mayer, Ralph. The Artist's Handbook of Materials and Techniques Viking Adult; 5th revised and updated edition, 1991. ISBN 0-670-83701-6
  1. ^ Charles Eastlake, Materials for a History of Oil Painting, Longman, Longman, Brown, Green, and Longman, 1847.
  2. ^ H. Gluck, "The Impermanences of Painting in Relation to Artists' Materials", Journal of the Royal Society of Arts, Volume CXII 1964

History

Chemistry of Oil Paint

  • “Autoxidation.” McGraw Hill Encyclopedia. 8th ed. 1997.

See also

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