Term: Processed Oils

Processed oils include a variety of drying oils that have been treated to improve their painting characteristics (such as, reduced drying time, non-yellowing, and so forth). The most common types of processed oils and their properties are briefly described below:

• Sun-Thickened Oil - Sun-thickened oil is a created by exposing it to the sun in an open container for several months. This oil has a thick, syrup-like consistency and is somewhat bleached in color. It dries quickly and hardens to a tough, non-yellowing film.
• Sun-Bleached Oil - Like sun-thickened oil, this oil is created by exposing it to the sun but in a closed container (to minimize the oxidation). The result is an oil that is lighter in color and has less tendency to yellow.
• Stand Oil - Stand oil (or polymerized oil) is made by heating linseed oil to a high temperature for several days in the absence of oxygen. The oil has a pale clear color and is rather thick. It produces a tough, enamel-like, non-yellowing film but it takes longer to dry (about a week to be dry to the touch although it will remain tacky for some time thereafter).
• Boiled Oil - Boiled oils have been heated and mixed with a dryer to give them a faster-drying time and a glossy finish. These oils tend to yellow and darken with age, so they should be used only in lower layers of paint and with darker colors.
• Refined Oil - The term "refined" can be used to describe all types of processed oils but it is used more commonly to refer to an oil that has been bleached and purified. The refining process is called alkali refining and involves mixing the oil with soda and lightly heating it until the impurities can be isolated and removed.