“The best painters seem to have left nothing undone
to render oils as colorless as possible before they were used”
– Sir Charles Eastlake (1793-1865), President of the Royal Academy
IMPORTANCE OF CLEANING OIL
Since the Renaissance, artists have sought superior, pale oils that will dry fast and not yellow.
Most oils sold to artists today are either uncleaned containing a large amount of fat, mucilage and unnecessary discoloration; or are cleaned using chemicals and heat.
HOW OILS WERE CLEANED HISTORICALLY
Historically cleaning oils for the use of painting had several steps.
First, a pure cold-pressed oil was used rather than one extracted by heat or chemicals ensuring better acidity and less impurities. Less oil is extracted when it is cold pressed, but it is a superior oil.
Second, the oil was washed with water, separating out the mucilage and fat which would normally make the oil less transparent and impede the drying.
Third, the oil was filtered with various natural minerals removing impurities which cause the oil, particularly linseed oil, to appear yellow.
Oil is sensitive to sun light and will darken when not exposed. The same is true for dried oil paintings which is why historically it was reported that some painters and owners would bring their paintings out in the sun to “brighten”. Some of the yellowing is caused by impurities in the oil that can be removed by cleaning and purifying the oil.
Overall the cleaning of the oil can take approximately 2 months, and result in 1/3 of the oil being lost during the process.