Oil paint became widely used in the 15th and 16th centuries, although oil was mixed with egg tempera in medieval painting. Oil painters gained the advantage of blending and mixing paints directly on the support rather than carefully layering them, as was the case with egg tempera. Because oil paint dries slowly, surfaces can be worked for hours or sometimes days. The key is to keep the paint fresh and "clean" by not overworking and creating muddy color. Artists can easily change areas by wiping with a turpentine rag or scraping with a painting knife. Oil paint must be used on top of a prepared surface (see Grounds), in order to create a barrier between the acidic oil paint and the support.
The traditional solvent for oil paint is turpentine. Turpentine is used in traditional oil painting mediums, especially those that contain damar varnish. Unfortunately, turpentine produces noxious fumes. Odorless, artist quality mineral spirits (OMS) is now available, which dramatically cuts down on noxious fumes (see solvents). Hardware store mineral spirits or paint thinners also produce dangerous fumes and are not suitable for fine art oil painting.
Oil paints cannot be mixed with water or water based paints, including acrylics. Some artists, however, layer oil paints on top of dry acrylic paint or gesso. Acrylics cannot, however, be used on top of oil paints because they will not properly bind to the oil surface.
Drying Oils - Vegetable oils like cold-pressed safflower oil or refined linseed oil are used in artist's oil paints because they are non-yellowing, quick drying, and durable. Boiled or raw linseed oil available in hardware stores is unsuitable for artists' oils, because they are heat-treated and will yellow the paint in a relatively short period of time.
When using oil paint, the first layers should contain less drying oil than the subsequent layers. This is the principle of fat over lean. A painting should be planned so that the underpainting is lean or low in oil content. These initial layers should be thinned with a medium composed of larger amounts of solvent with smaller amounts of oil. Oil expands and contracts as it dries (oxidizes). If lean (low oil) paint is brushed over fat or oilier paint, the oily flexibility underneath may cause the lean (and more brittle) layer on top to crack. Underlayers which are not completely dry also release gases that will crack a more brittle, less oily overlayer.
Homemade oil mediums
Oil paints may be thinned with a mixture of oil and turpentine. However, it is best to premix an oil painting medium beforehand to insure consistency. A traditional medium that can be used for both thinning paints and glazing can be mixed with 1 part oil, 1 part damar varnish, and 1 part turpentine. If you are using odorless mineral spirits, you should eliminate the damar varnish, because it will not dissolve properly (1 or 2 parts oil to 1 part OMS). Replacing the linseed oil with sun-thickened will make a richer, faster drying medium. Using stand oil rather than linseed oil will also create a thicker medium. Stand oil also gives the paint a glossier finish. (If you use the stand or sun-thickened oils, a thinner medium can also be made using 1 part of these thicker oils to 2 parts OMS.)
Ralph Mayer's glazing medium - Ralph Mayer's, The Artist's Handbook of Materials and Techniques, includes several glazing recipes used by oil painters. A standard recommended glazing medium for an oil glaze painting is composed of 1 ounce stand oil, 1 ounce damar varnish, 5 ounces turpentine, and 15 drops of cobalt dryer. The cobalt dryer is added to accelerate the drying time of the slow drying stand oil, but if too much is added, the medium will coagulate. Ideally, a glaze should dry rapidly so artists can proceed with layering subsequent transparencies. The best glazes will dry overnight.
Impasto wax and oil medium -- An impasto wax medium for oil paint can be made by gently and slowly melting 1 part bleached beeswax into 2 parts linseed, sun-thickened, or stand oil. Pour the oil in a coffee can and place this into a pot of hot water (basically, a double-boiler). When the oil is hot enough to melt the wax, place small pieces of beeswax into the oil until all the wax is melted. Stir thoroughly and allow to cool. If the mixture is too hard or too oily, reheat and add either more wax or oils as necessary. A drop or two of cobalt dryer can be added to help the medium dry more quickly. The final consistency should be similar to lard. Store oil painting mediums in glass jars with lids.
Making homemade sun-thickened oil - The process of thickening oil is described in The Materials and Techniques of Painting, by Jonathan Stephenson. In the summer (or the tropics), pour about 1/2" - 1" thick linseed oil onto a flat, glass dish. The flat dish creates a large surface area for exposure to the sun and the air. Place the dish outside (on a ledge or roof) in an area that will be exposed to the maximum amount of sunlight. Thickening oil happens fastest in warm, sunny climates. Cover the dish with a slightly opened glass lid to allow for air flow. Stir the oil daily. After about a week or two, stir occasionally to prevent a skin from forming over the surface layer of the oil. Continue the process until the oil has thickened to a syrupy consistency. Impurities will settle to the bottom, so the oil on top can be poured off. Store in a glass jar with a lid.
Many art supply companies produce a variety of painting mediums for use with oil. In addition to traditional oil mediums (such as copal painting medium), many oil painting mediums are made with fast-drying alkyd resins which are soluble in odorless mineral spirits. Because a weaker solvent (OMS) can be used with alkyds, vapors are far less harmful.
Alkyd based translucent impasto gels are also available, such as Gamblin's Galkyd gel, Utrecht's Flex and Alkyd Gels, and Winsor Newton's Oleopasto. Gamblin also produces a Cold Wax Medium for opaque impasto applications.
Alkyd mediums are far less toxic than traditional oil mediums. They dry glossy and transparent. To extend the "open" time of fast drying alkyd mediums, small amounts of drying oil may be added. To prevent wrinkling, not more than 10% is recommended by Gamblin. (Gamblin has terrific technical support, by the way). Alkyds dry within 24 hours, so a change in painting process might be necessary. However, an alkyd medium mixed with oil paint works well if you use a lot of transparencies and glazes, because layers can be applied overnight.
Alkyd mediums, such as Galkyd or Liquin are thick (though not as thick as a gel). Galkyd Lite has been formulated as a thinner alkyd medium, although its open time is similar to Galkyd #1. Gamblin also makes Galkyd Slow Dry which dries slower than either Galkyd or Galkyd Lite.
Damar Varnish Preparation - Artists can make their own damar varnish by dissolving damar crystals into turpentine. Damar crystals can be purchased at art supply companies like Utrecht or Daniel Smith. The crystals are placed in a cheesecloth bag. (You can make a bag from a piece of cheesecloth tied at the top with a string. I've found pre-made cheesecloth bags at homebrewing supply stores). Suspend the bag into a measured amount of turpentine - usually about 1 part crystals to 3 parts turpentine by volume. The damar crystals do not dissolve properly in odorless turpentine. The regular hardware store gum spirits variety is best. Don't let the bag sink to the bottom. The turpentine will dissolve the crystals in a few days. Expect some of the crystals to remain undissolved.