Oil paint comes in a variety of grades, from student oils, which contain less pigment and more oil, also known as binders or vehicles, to artist's oil colors, which usually contain higher grade materials, more pigment and finer grades of oil.
Depending upon the brand and if the paint has dryers mixed in, the type of oil added to paint will vary. Linseed oil is found in most common brands, however it is not unusual to find walnut or poppy seed oil as part of the mixture.
Hundreds of years ago, it was common for artists or chemists to grind pigments and binders between stones in order to create paint. Now, commercial paint manufacturers will mill the mixture together on large steel rollers, which is then tested prior to filling paint tubes and labeling for sale. It is not uncommon for mixtures of paint to remain in a plant for months while testing continues to make sure the paint has not changed color before going to the consumer market.
Linseed Oil
Derived from ripe dried seeds of the Flax plant (Linum usutatissimum, Linaceae), Linseed oil is a drying oil with a yellowish tint. It is high in acidity, which helps the color of the oil remain stable. This means that the yellowish color of the oil will not affect paint color over time. Linseed oil is also known to create a durable and flexible film when the paint dries. Cold-pressed linseed oil is suitable in a raw state for human consumption.
There are two methods of creating linseed oil for painting, cold pressed on stone mills in a windmill and industrial cold-pressed linseed oil. The windmill produced oil, is most similar to what the old masters used. It is a long process and results in costly oil. Old Holland, with a line of professional quality artist paints, claims to use this process to produce their line of artist's colors. Industrial pressed oil is a bit lighter in color, but similar in use as a vehicle for pigment and is used in most other brands that can be found in art stores.
Poppy Oil
A popular alternative to linseed oil, poppy oil is non-yellowing. It is especially popular for uses in manufacturing whites and other light colors. It immediately makes paint look brighter directly from the tube. It also produces a buttery paint consistency which is revered by artists who like to paint straight from the tube. The drawback to poppy oil is its tendency to crack. It is also slower to dry than linseed oil and its durability over time is not as good as linseed oil's.
Walnut Oil
Leonardo da Vinci's favorite oil paint was created using walnut oil. It is not as commonly used today as poppy oil. There isn't a tremendous amount known about walnut oil, more than likely because it is costly to produce. It is considered inferior to linseed oil when tested for durability. It is about the equivalent of poppy oil, however, for longevity and drying time.
Stand Oil
When heated to 300 degrees Fahrenheit in an oxygen-free environment, linseed oil is referred to as stand oil. The heat causes the molecular structure to change, while the composition remains the same. Stand oils dry slowly and yellow less than the cold-pressed oils and form a strong film that is not water-soluble. Stand oils are used as a painting medium, to be mixed in with paint from the tube to dilute thick paint and make it easier to work with or to help dry paint faster. Stand oil is not recommended for the manufacturing of paints, although it is highly recommended for use in glazing techniques in which the artist mixes a small amount of pigment into the oil and creates thin layers, exposing under painting through transparent color.
Mediums
Linseed, poppy, walnut and stand oils all function as mediums as well. This means that they can be mixed into paint in order for paint to flow easier. There is a rule in oil painting "fat over lean," meaning that thinner, faster drying layers of paint should be used first and then worked up to fatter layers or layers with more oils.
