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Oil painting is the process of painting with pigments that are bound with a medium of drying oil - especially in early modern Europe, linseed oil. Often an oil such as linseed was boiled with a resin such as pine resin or even frankincense; these were called 'varnishes' and were prized for their body and gloss. Other oils occasionally used include poppyseed oil, walnut oil, and safflower oil. These 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. Painters often use different oils in the same painting depending on specific pigments and effects desired. The paints themselves also develop a particular feel depending on the medium.
Traditional oil painting techniques often begin with the artist sketching the figure onto the canvas with charcoal or a "clean", which is thinned paint. Oil paint can be mixed with turpentine, linseed oil, artist grade mineral spirits or other solvents to create a thinner, faster drying paint. Then the artist builds the figure in layers. 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. There are many other mediums that can be used in oil painting, including cold wax, resins, and varnishes. These additional mediums 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. When looking at original oil paintings, the various traits of oil paint allow one to sense the choices the artist made as they applied the paint. For the viewer, the paint is still, but for the artist, the oil paint is a liquid or semi-liquid and must be moved 'onto' the painting.
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. Scraping may also be used to smooth a portrait before scumbling and glazing. Many oil paintings reveal evidence of scraping on close inspection, particularly when the surface itself is examined. Oil paint dries by oxidation, not evaporation, and is usually dry to the touch in a day to two weeks. 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.
Traditional artists' canvas is made from linen, but the less expensive cotton fabric has gained popularity. The artist first prepares a wooden frame called a "stretcher" or "strainer". The difference between the first and second is that stretchers are slightly adjustable, while strainers are rigid and lack adjustable corner notches. The canvas is then pulled across the wooden frame and tacked or stapled tightly to the back edge. The next step is for the artist to apply a "size" to isolate the canvas from the acidic qualities of the paint. Traditionally, the canvas was coated with a layer of animal glue (size), (modern painters will use rabbit skin glue) and primed with lead white paint, sometimes with added chalk. Panels were prepared with a gesso, a mixture of glue and chalk.