Varnish applied to a dried out oil painting restores the optical effect of the applied color. Natural resin varnishes tend to yellow, darken, bloom, and become brittle (particularly copal). Copal is an old varnish which dries by oxidation and cannot be removed by normal solvents. Winton Picture Varnish and Griffin picture varnish are made of synthetic resins. A number of artists in the past and today finish their pictures by the use of wax polishes (sometimes composed of beeswax and white spirit). A newly painted picture should not be varnished for at least twelve months -- longer, if it has thick impasto.