Oils are widely used as a thinner for the oil paint because of their ability to dry by oxidization, which makes a paint layer resistant and durable.
How the oil dries?
In contrast to water and turpentine, which dry by vaporizing, drying oils harden by taking the oxygen from the air and creating a durable solid film.
Fat Over Lean
Artist who uses oils as a thinner should always remember the principle called Fat Over Lean – apply paint with more oils over less oily paint to create a reliable structure for the painting.
The reason is simple – thick layer of paint dries longer, but artists tend to apply next layer before the previous completely dries. Oil from the upper layer leaks to the lower layer, so the upper layer tarnishes.
That is why some artists recommend not using oil as a thinner when you do the underpainting. Oil should be added only when needed and in very limited quantities.
Excess oil in the upper layers leads to:
- settling of oil and formation of wrinkles
- oil could stand on the surface and lighten the tone of the paint, making it whitish and muddy
Linseed oil
- made from the seeds of the flax plant. Linseed oil has been one of the most usable drying oil through the oil painting history. It is considered that the layer by linseed oil is quite stronger than by poppy.
Pros
- dries comparably fast (under the permanent light and increased temperature it takes 3 days to dry, under diffused light – 5-6 days, in the dark – 60 days or even more)- the layer it creates has a good endurance
Cons
- yellowing. The film has a tendency to change the color towards more yellow or darker tone
Stand Oil
- made by heating linseed oil at a high temperature in the complete absence of air. The product, which is highly viscous. Has a slower drying time (a week to be dry to the touch). Perfect for glazing, because it produces a smooth finish so the brush marks will not be seen. The film it creates is not yellowing.
Sun-bleached oil (Sun-Refined)
- is created by exposing the oil to the sun for bleaching. Usually, it takes 3-4 months.
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| Photo from WetCanvas forum. The discussion about sun oil is here |
Sun-Thickened oil
- If the oil was also exposed to air (no lid on the container while the oil was under the sunlight), the oil is called Sun-Thickened. It is thick, syrupy and its qualities are similar to stand oil. Both sun-thickened and sun-bleached oil has an almost non-yellowing film. The sun-thickened oil is one of the most recommendable medium for oil painting.
Cold-Pressed Linseed Oil
When you buy linseed oil in art stores it is already refined and bleached. Manufacturers sometimes use strong chemicals to make it clearer and more transparent. Some artists say that it may destroy the good properties of the linseed oil. The Cold-Pressed Oil is made without heat or chemicals of any kind, so it can be called the best quality linseed oil. However, it still has a tendency to yellow over time.
Poppyseed oil
- is made from a poppy seed. Nowadays, this oil is as popular as a linseed. It gives oil paint consistency similar to soft butter. Was widely used by French Impressionists.
Pros
- very pale, more transparent and less likely to yellow than linseedCons
- takes longer to dry (5-7 days), ideal for working wet-on-wet (alla prima)- the layer created with a poppyseed oil is inferior in endurance
Walnut oil
- is a pale yellow-brown oil (when newly made it's a pale oil with a greenish tinge) that is extracted from English walnuts (also known as Persian walnuts). It was one of the most important drying oil around Renaissance. It is considered that Leonard da Vinci, Jan Van Eyck, Albrecht Durer, Titian and others prefered to use walnut oil, because it yellowed and cracked less.
Pros
- doesn't change its color- dries fast (4-5 days)
- has better endurance
Cons
- tends to rot when stocked at shops for long time- more expensive than others
Safflower and Sunflower oils
- have come into use only recently. Have the same characteristics as poppyseed oil, but dry a bit faster. They are made from safflower and sunflower seeds respectively. Recently this oils are frequently used as binder of tube oil paint instead of poppy or linseed oils. Although at this time it appears that both sunflower and safflower oil can be safely used in the manufacture of some colors, they are not recommended for use as a medium.
TIPS
- As linseed oil has a tendency to yellow as it dries, avoid using it in whites, pale colors, and light blues. While poppyseed oil is a very pale oil, more transparent and less likely to yellow than linseed oil, it is often used for whites, pale colors, and blues. The walnut oil is formerly used for bright colors.
- Be careful painting with a linseed oil on a poppy oil layer. They have a very different drying speed and if the upper layer will dry faster then previous the cracks may appear.
- Drying of the oils (and oil paints) under direct sunlight is not recommended, since at the same time with film formation starts the process of decay. Drying in dark place leads to yellowing and darkening, it especially increases with high humidity.
CONCLUSION
I've tried to keep the article small, but somehow it has grown to an impressive scale, so I need to sum up everything.The best oil to work alla prima is poppyseed oil.
If you prefer to work with layers and grisaille, walnut or linseed oil will be perfect for you. In most cases the choice between them is pretty simple – walnut oil is expensive, so if you can spend extra money on oil choose walnut. If not, choose one of the linseed. The perfect choice will be sun-bleached or sun-thickened (if you prefer a viscous one) since they have no problem with yellowing. It can be hard to find a real sun exposed oil, so, anyway, stand oil or a cold-pressed linseed oil are a good choice.
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