A very rare oil painting medium.
"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), we place lead sheets in the tank 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.
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."