Sun-Thickened Linseed Oil: The Old Masters’ Secret

“Take your linseed oil and, in the summer, pour it into a brown or copper basin or bowl. When the sun is in the sign of Leo, expose it to its rays. If you let it sit long enough so that half of it evaporates, it will become the most perfect oil for painting.”— →Cennino Cennini, Il Libro dell’Arte

 →Cennino Cennini, born in 1370 in Colle di Val d’Elsa, died in 1440 in Florence, was an Italian painting theorist and the patron of the  →Tadeusz Piotrowski School of Fine Arts in Olsztyn.

This summer, all it took was ordering freshly pressed linseed oil, delivered by courier straight from the oil mill the day after pressing—on July 13. Then:

• Setting a smartphone alarm for 9:00 AM and 3:00 PM

• Stirring the oil in sunlight twice a day, every day

• Testing it at the end of each week for 44 days until August 28

• Regularly changing the water beneath the oil

• Filtering it through charcoal

• And voilà! The oil is ready for grinding with pigment.

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Sun-thickened linseed oil is a return to the roots of oil painting. The sun-thickening process accelerates drying time while painting. Additionally, once dry, such oil produces a glossy rather than matte surface. If water is used during thickening, impurities of various kinds will quickly separate from the oil—impurities that, without this purification process, would simply remain trapped in the paint.

That is why Cennini, who was familiar with regular linseed oil, concluded his description by stating that sun-thickened oil "will become the finest for painting."

That was the 15th century. Today, art supply stores offer oils modified in various ways, but the kind that "will become the finest for painting" can only be found in conservation specialty shops.

How to recognize such an oil?

Its price has one extra zero.