(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987 revised 1998)
A medium is a recipe that includes, in specific proportions, two or more of: drying oil(s), resin(s), drier(s), wax(es), and thinner(s). A medium, in addition to regulating the consistency of the paint, thinning or thickening it, will also regulate the drying time, transparency, gloss, thickening and leveling characteristics, as well as improve the overall aesthetic appearance.
Oil painting without media, or with only linseed oil and turpentine, is like going to a French restaurant to order boiled duck, no sauce. An appropriate medium does for oils what a French chef does to an otherwise bland and greasy dead bird, and the results, like Duck l’Orange, are exquisite. (This is discussed further in the section on oil media.)