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What is Sun Thickened Oil?

The use of Sun Thickened Oil dates back at least to the 14th century. At one point, someone discovered that a partially dried oil would speed up the drying process, and would increase leveling and overall protective qualities of the oil. It was also useful in glazing, sometimes adding an enamel like gloss.

Historically linseed oil was used for sun thickening, and sun exposure lightened the yellow color – another benefit. However, if the sun exposure process was carried on for a bit too long, the oil could shift to a darker shade. In the historical approach, the process for making sun oil was actually very similar to making Water-Washed Oil. Water and sand or ashes would be mixed with the oil, and the combination would be vigorously shaken before placing it in a larger ceramic container for sun exposure. Sometimes lead strips would be added to the oil, or lead dishes might be used, to further increase the drying speed.

The Art Treehouse Sun Thickened Oil is made from walnut oil. This means that the oil overall is less prone to yellowing than linseed oil. This is a 100% natural oil, so it changes over time, thickening with continued exposure to air. It has a viscosity of 2380 mOa-s at 68 degrees F. (For comparison, honey has a viscosity between 2000-10,000 mPa-s at the same temperature.) A tightly sealed bottle will help the oil last, but sometimes it will develop a surface skin that cannot be avoided. A simple solution is to perforate the skin to access the oil below.

Check out Samuel Shelton’s article on how he uses Sun Thickened Walnut Oil in his work!

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