Product Profiles: "Bodied" Oils for Painting

Portrait by Rubens, a Baroque period artist known to have used sun-thickened oils in painting

Ask the Expert: "I'm interested in using thickened oils in painting. I know about Stand Oil and Sun Thickened Oil, but what about Boiled Linseed Oil and other oils from the hardware store? Can they be used in oil painting?"

A: Artists have devised techniques for thickening and speeding up drying of oils for practically the entire history of the craft. Some methods for making thickened or "bodied" oils can be safely and effectively carried out in-studio, and some require industrial processing.

Many historical painters are known to have used thickened oils. Rubens made extensive use of sun-thickened oil, which was made in-house on the roof of his studio. Vasari recorded the use of "oglio cotto" or "cooked oil", a heat-polymerized oil which may have included metallic driers.

Stand Oil and Sun Thickened Linseed Oil are the main "bodied" oils used in modern painting. Stand and Sun-thickened Oil have a different chemistry, though they share some similar properties. Both are thickened vegetable oil products made of linseed oil, and both are used in painting mediums. (Other oils like poppyseed can be thickened too.) The main differences are how processing alters chemical makeup of the oil, and how the resulting physical properties influence handling, drying and aging of the paint film.

Stand Oil is polymerized by heat in a low-oxygen environment, causing cross-linking in carbon double bonds without oxidation. This gives Stand Oil its characteristic "long" body and syrupy consistency. Stand Oil does not undergo partial oxidation or "drying", and as a result retains a lighter color and dries relatively slowly, producing a tough, flexible film. Because of the danger associated with heating oils to very high temperatures, and the difficulty in achieving an oxygen-free cooking environment, artists should not attempt to make Stand Oil in the studio.

Sun-thickened Linseed Oil, on the other hand, can be made easily in the studio, in glass or enameled pans covered with glass or plastic.(Thickened oil can also be made in jars to save space, though it takes a little longer.) Sun thickened oil is produced by exposing linseed oil to air and sunlight at room temperature, causing partial oxidization, polymerization (chaining of molecules) and bleaching, which lightens oil to a pale amber. Because it has undergone partial drying, sun-thickened oil is relatively fast drying.

Hardware Store Products Not all types of linseed oil meet the standards of permanent painting. Oil that is intended for use as a wood finish or nutritional supplement, unlike a paint binder, may not need to form a strong, continuous film. A tendency to change color may be an acceptable or even desirable in a wood treatment, but in paint, darkening and yellowing with age can be a real problem, particularly if the degree of color chage exceeds the masking ability of pigments.

Raw Linseed Oil sold as a wood finish is not suitable for artistic painting because it isn't alkali-refined to remove free fatty acids and mucilage released from the seed. Of course, alkali-refinement wasn't invented in the time of the Old Masters, but historical painters used oil that was cold-pressed, a process which yields a clearer, cleaner oil. Even when using cold-pressed oil, historical artists invented various methods of "washing" vegetable oils to clarify them.

The industrial product "Blown Oil" is no longer manufactured, but the familiar hardware store wood finish Boiled Linseed Oil is very similar. Boiled Linseed Oil is a hardware-grade wood finish that is not suitable for permanent painting. Boiled oil is heated to high temperatures and exposed to currents of air while treated with large amounts of catalytic siccatives (driers). This type of processing and heavy load of chemical driers can induce wrinkling, darkening and cracking if used with artists' oil colors.

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