“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 symbolic patron of →Tadeusz Piotrowski’s School of Fine Arts — a place where I learned the techniques of the old masters.

How I Made Sun-Thickened Linseed Oil This Summer

This summer, I followed Cennini’s advice quite literally. It took only a few simple but disciplined steps:

– Ordering freshly pressed linseed oil, delivered straight from the oil mill the day after pressing—on July 13.
– Setting a smartphone alarm for 9:00 AM and 3:00 PM.
– Stirring the oil in the sun 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 sun-thickened linseed oil was ready for grinding with pigment.

Why Use Sun-Thickened Linseed Oil?

Sun-thickened linseed oil is a return to the roots of oil painting. The sun-thickening process speeds up drying time, shortens curing, and enhances flow. Once dry, such oil forms a glossy, durable surface rather than a matte one.

If water is used during thickening, impurities separate from the oil — elements that would otherwise remain trapped in the paint film. This gives a cleaner, leaner medium with less yellowing over time.

That’s why Cennini, who knew the limits of standard linseed oil, ended his note by stating that sun-thickened oil “will become the most perfect for painting.”

Today, you can buy many modified oils in art supply stores. But the kind of oil that matches what the old masters used — purified by sunlight, tested over weeks — is rare. You’ll typically only find it in conservation-focused suppliers or by making it yourself.

How to Recognize a Properly Sun-Thickened Oil?

Its price has an extra zero.

And when you pour it, it moves differently — thicker, silkier, clearer.
The surface tension is stronger, the brush drag more fluid, and the drying time is consistent without needing siccatives.

It’s not an oil you use casually. It’s one you make intentionally.

Watch the Process

🎥 You can also watch the full video here:
→Sun-Thickened Linseed Oil: The Old Masters’ Secret – on YouTube

Final Thoughts

There’s something grounding about preparing your own painting medium the way it was done six hundred years ago. Sun-thickened linseed oil is more than a technical improvement — it’s a way to connect with the long, silent lineage of painters who came before.
You begin with a jar of raw oil and end up with something rare, intentional, and alive with history.
It’s one of the few materials in painting where the passage of time becomes part of the process itself — visible, tangible, and quietly powerful.

I still remember the smell of the oil on the forty-fourth day — thicker, deeper, changed. No two jars are ever the same. That’s the nature of this medium: shaped by time, light, and care.d by time, light, and care.