The Truth About Turpentine

Q. I use turpentine as a thinner and a medium for my oil paints. What’s the difference between the expensive, small cans of turpentine available in art supply stores and the cheaper gallon cans available in hardware stores? Also, why is turpentine considered such a toxic substance? In the 1800s it was used topically for medicinal purposes, and I consider some of the odorless petroleum distillates and many pigments to be far more toxic than turpentine, which is a product of pine trees.
William F. Martin
Glendale, AZ

A. The gum turpentine in the small, expensive cans in your local art supply store is usually a purer and more refined version of the gum turps in the larger cans at the hardware store. So if you’re concerned with purity, stick with the art store product. On the other hand, if the solvent at the hardware store is labeled “pure gum spirits of turpentine” (the proper name for this product), you can expect the same material found in the smaller can. You might also consider that gum turpentine deteriorates with long storage, and the stock turnover in a hardware store is likely to be quicker than in an art supply store.

Despite its prior medicinal uses, gum turpentine is considered toxic because some of the species of pine trees from which it’s distilled can produce a solvent that causes skin irritation and allergic reactions in some people. Its concentrated vapors can also cause headaches. For the sake of tradition, many artists still prefer to use gum turps for diluting their oil paints, but regular mineral spirits are just as good for this purpose—and for all solvent purposes in your studio except for the dilution of damar varnish. Mineral spirits vapors are considered toxic as well, so whether you use gum turpentine, mineral spirits or odorless mineral spirits, be sure your studio is well ventilated and has a source of fresh air.

You need to be careful, however, about using any of these solvents as a “medium,” by which I assume you mean that it’s the sole material you’re adding to the paint. If you over-thin oil paint with a solvent, you can easily affect the binding capacity of the paint’s vehicle. In other words, you’ll make a paint that doesn’t stick to the ground very well. I recommend that oil paints be substantially thinned with a solvent only in the first layer of the painting. In subsequent layers, much less thinner should be used—only enough to allow the paint to be spread out.

If your paintings are more than three layers thick, use a medium containing linseed oil (plus a little thinner), or one of the alkyd painting mediums such as Galkyd (from Gamblin) or Liquin (from Winsor & Newton).

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