Oil painting is the process of painting with pigments that are bound with a medium of drying oil. Commonly used drying oils include linseed oil, poppy seed oil etc. Different oils confer various properties to the oil paint, such as less yellowing or different drying times. Certain differences are also visible in the sheen of the paints depending on the oil. An artist might use several different oils in the same painting depending on specific pigments and effects desired. The paints themselves also develop a particular consistency depending on the medium. The oil may be boiled with a resin, such as pine resin or frankincense to create a varnish; often prized for its body and gloss.
Although oil paint was first used for the Buddhist paintings by Indian and Chinese painters in western Afghanistan sometime between the fifth and tenth centuries, it did not gain popularity until the 15th century. Its practice may have migrated westward during the middle Ages. Oil paint eventually became the principal medium used for creating artworks as its advantages became widely known. The transition began with Early Netherlandish painting in Northern Europe and by the height of the Renaissance oil painting techniques had almost completely replaced tempera paints in the majority of Europe.
In recent years, water miscible oil paint has come to prominence, to some extent replacing the usage of traditional oils. Water-soluble paints contain an emulsifier which allows them to be thinned with water (rather than with paint thinner), and allows very fast drying times (1–3 days) when compared with traditional oils (1–3 weeks).
Traditional oil painting techniques often begin with the artist sketching the subject onto the canvas with charcoal or thinned paint. Oil paint is usually mixed with linseed oil, artist grade mineral spirits or other solvents to create a thinner, faster or slower drying paint. (Because these solvents thin the oil in the paint, they can also be used to clean paint brushes.) A basic rule of oil paint application is ‘fat over lean’. This means that each additional layer of paint should contain more oil than the layer below to allow proper drying. If each additional layer contains less oil, the final painting will crack and peel. Fat over lean does not ensure permanence; it is the quality and type of oil that leads to a strong and stable paint film. There are many other media that can be used in oil painting, including cold wax, resins, and varnishes. These additional media can aid the painter in adjusting the translucency of the paint, the sheen of the paint, the density or 'body' of the paint, and the ability of the paint to hold or conceal the brushstroke. These variables are closely related to the expressive capacity of oil paint.
Traditionally, paint was transferred to the painting surface using paint brushes, but there are other methods, including using palette knives and rags. Oil paint remains wet longer than many other types of artists' materials, enabling the artist to change the color, texture or form of the figure. At times, the painter might even remove an entire layer of paint and begin anew. This can be done with a rag and some turpentine for a certain time while the paint is wet, but after a while, the hardened layer must be scraped. Oil paint dries by oxidation, not evaporation, and is usually dry to the touch within a span of two weeks (some colors dry within days). It is generally dry enough to be varnished in six months to a year. Art conservators do not consider an oil painting completely dry until it is 60 to 80 years old.