Sun-Thickened Linseed Oil is made by exposing raw, cold-pressed linseed oil to sunlight in shallow tanks covered with clear glass and exposed to the atmosphere for a period of one month. To give the oil more body (higher viscosity), Rublev places lead sheets in the tanks and blow air through the oil for at least three more months. Trace amounts of lead may dissolve into the oil and may be present in the finished oil.

This linseed oil is slightly more yellow than sun-thickened walnut oil, so it can be used with pale colors and white. It is fast drying and will add gloss, leveling and flow out to oil paint. This oil can be used as a medium or added to homemade mediums.

Use turpentine, spike oil or mineral spirits as a solvent or diluent for oil painting mediums made with linseed oil.

Trace amounts of lead may be present but are not added to the oil. Sun-thickened linseed oil may contain trace amounts of lead because metallic lead strips are placed in the tanks where the oil is exposed outdoor for several months. The metallic lead aids in thickening the oil. Typically, the lead does not dissolve into oil, but it cannot be guaranteed that small amounts of lead do not dissolve into the oil. For this reason Rublev places the warning that the product may contain trace amounts of lead.