MITRA Forum Question Details

The manufacturer's literature lists composition as fatty acid methyl esters derived from vegetable oil. There were no CAS numbers or anything more specific. If I can locate any literature on effects on drying oil films, I will share info here. I am unfamiliar with this product myself, however.​

While I am technically not back moderating until Monday, I had to chime in. It is a major “red flag” when manufacturers use terminology like “non-volatile” for diluents.  We are supposed to interpret this as “there is nothing to breath in” and therefore it is safe. Even sidestepping all of the vagaries about what is non-toxic, harmless, and more importantly, based on what tests, there are issues here.

As Michael also alluded to, if it were truly non-volatile, it would dilute the paint but would also remain in the film, likely weakening and preventing the formation of a solid oxidized oil film. On the other hand, perhaps it is not a diluent but an additive. Canola oil can be produced organically, is derived from vegetables, does not appreciably volatilize, would be harmless to the user, and would create a non-drying mess if used with oil paint. Solvent-free alkyd mediums will thin paint, are not especially volatile, and are relatively harmless to humans, but they are not diluents, they are additives or mediums.

Now this is assuming that it is intended as a diluent that will not remain in the film. Perhaps it is meant more as a medium rather than a diluent. I am unfamiliar with this product and it may be that the advertising is misleading but that the product has virtues. However, without more compelling information, I would stay away from using it for painting anything other than throw away sketches.

Having written the above, I do not know everything about what is available today and will be happy to be proven wrong about this product.

Additionally, the whole use of the term “natural” as it relates to harmless drives me crazy. There is enough cyanide in 50 "natural" wild bitter almonds to kill an adult and a child can die eating only a handful. Try munching on random mushrooms without the proper knowledge if you want to see the folly of the idea that natural=healthy.   

​This product is also available in the U.S. from several manufacturers, such as Stepan, which markets it under the tradename STEPAN C-65. It is a methyl ester derived from soybean oil, or also known as methyl soyate (methyl esters of C16-18 unsaturated fatty acids, CAS No. 67762-38-3). This is mostly used in industry as a surfactant, but is widely touted as a replacement for solvents in oxidative paint systems, such as alkyds. It is in a category of substances known in the paint industry as “reactive diluents”.

Reactive diluents are substances that dilute or thin a resin system (which is the function of solvents in paint systems) and that also reacts or oxidizes with the resin and/or oil. These products have the advantage of lowering or eliminating VOCs in paints to regulatory limits. Their practical use by artists is limited. They cannot be used to clean brushes or tools, and when added to dilute oil paint, they do not hasten drying unless driers are dosed correctly, which is impractical for artists.

While they are derived from natural sources, such as soy and tung oils, and they are not considered hazardous, we should not assume they are non-toxic. This is the problem of using such designations as “non-toxic”, since we cannot assume it is not toxic in all applications.

​As always, thanks George. You are always on top of things.