Mediums for oil paints are made from mixtures of separate products with which an artist plays to develop his/her own recipe. For example, a traditional medium mixture is one part Damar varnish, one part stand oil, five parts turpentine and few drops cobalt or japan drier. Mediums may also be bought in pre-made mixtures for convenience. Commercially made mediums can vary as much as home-spun recipes. Whether homemade or store-bought, mediums can be placed in a lidded glass jar and reused for many painting sessions.
As you experiment with mediums, keep in mind these guidelines:
-Use the minimum amount of medium to serve a particular purpose. Conservators seem to be in agreement that oil paintings done with straight paint (no mediums except a little thinner) form the strongest, most permanent films.
-When using gobs of oil-containing gel mediums to build up texture, there is significant danger of yellowing.
-Yellowing is sometimes rapid and pronounced when a painting is left in darkness; the yellowing will gradually lighten as the painting is exposed to normal daylight.
-Mediums containing copal varnish are likely to darken significantly.