The word resin when used in its most specific sense is a hydrocarbon secretion of many plants, particularly coniferous trees. The resin produced by these plants is a viscous liquid, composed mainly of volatile terpenes. Oleoresins are naturally occurring mixtures of oil and resin. Other resinous products in their natural condition are a mixture of gum or mucilaginous substances and are known as "gum resins." Mastic gum is a good example of a gum resin.

Balsam is a term used for various plant products and the plants that produce them. True balsams are oleoresins, but contain benzoic acid or cinnamic acid and their esters, obtained from the exudates of various trees and shrubs. Hence, all balsams are "oleoresins" or less specifically "resins," but not all resins are balsams. Interestingly, Canada balsam, larch balsam and copaiba balsam are terpenes, containing the characteristic resin in solution, and are not regarded as true balsams.

Larch turpentine or sometimes referred to as "larch balsam" is the exudate of the Larix europaea (European Larch).

Venice turpentine is a product of the European Larch or other closely-related larch species. When pure, it is limpid, with a yellow color, sometimes having a green tint, tenacious, and thick like molasses. It can be heated with other resins and combined with linseed oil and turpentine to make an oil varnish. It requires an exposure to the air for many years before it becomes hard and brittle, hence it is used only as a plasticizing resin in varnishes and mediums.

Venice turpentine is often mixed with gum rosin and turpentine spirits to make it less expensive. The product sold under the name of Venice turpentine (especially in tack shops) is often a factitious product, consisting entirely of gum rosin (colophony) and turpentine. There are tests that can identify adulteration of Venice turpentine with rosin, and we have used them on several brands of Venice turpentine sold in tack shops.

Just to let you know how confusing and contradictory the nomenclature for these materials can be, Gettens and Stout (Rutherford Gettens, George Stout. Painting Materials, A Short Encyclopedia [New York: D. Van Nostrand & Co., 1942]) identify Venice turpentine as a balsam, but Frederic Hyde, PaB. (Solvents, Oils, Gums, Waxes and Allied Substances [London: Constable & Company, Ltd., 1913]) lists it as a solvent. (He also writes that it is "closely related to Canada balsam.") In reality, it is both a resin and solvent, because it contains both mixed together.

I do not recommend Venice turpentine in oil painting for the sole reason that it usually contains rosin, which oxidizes and darkens upon exposure to the atmosphere. Pure larch oleoresin would be preferable, but only as a plasticizer in the medium. Most natural resins are susceptible to darkening and embrittlement, and should be used quite sparingly in a painting.