(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987 revised 1998)
No one likes to open a tube of oil paint and find oil coming out of the tube instead of paint. However, the separation of oil and pigment in an artist‑grade paint does not necessarily indicate poor quality; it often means that little or no stabilizer was used. However, it can indicate the lack of aging before packaging, lack of resin (natural or synthetic), or lack of modern grinding techniques, none of which would necessarily be serious drawbacks.
Separation commonly occurs in pigments containing cobalt, manganese, cadmium, and lead and appears to be due to an aging process whereby the surface tension of certain pigments is reduced with age resulting in a lesser amount of oil being needed to coat each particle. The excess oil is naturally expressed and causes the apparent separation. It is not uncommon to find an “old time” painter looking for old tubes of such colors, rather than “fresh” tubes, because the aging process improves the quality of oil paint (as long as the container is closed). Some of the finer quality paints are aged up to six months to allow this separation to occur before packaging.
The separation of oil and pigment in lesser quality oil paints is cause for alarm and often indicates that too much oil was used in the manufacture. This would be especially true of paints in which stabilizers are commonly used to compensate for excessive oil.