MITRA Forum Question Details

Certainly flax and walnut oils of a high quality can be purchased from sources intended for consumption. As you state, these may have been modified or even processed in such a manner as to make them undesirable for drying oils intended for paint binders. Tiny additions of rape seed oil or other seed may have no effect on the quality of a salad dressing but could have disastrous results in a paint binder. Additionally natural or GMO modifications of fatty acid ratios could be very beneficial for health benefits but be problematic for proper oxidation.

I have, however, conversed with individuals who have found high quality, cold pressed drying oils in health food stores. One would need to test any sample for its drying rate and compare that to samples intended for use by artists. At least you could rest assured that the speed of oxidation of a health food store oil would be completely inherent and not a result of a heavy metal catalytic siccative.

On the other hand, all of this assumes that cold pressed oils are inherently superior to commercially available, and generally very affordable, alkali refined artists oils. There is no evidence to support this claim.

​It's my understanding that, as Brian points out, edible flax oil can be derived from modern hybrids with fatty acid ratio optimized for digestibility. Some edible oils have a "foot" of mucilage and other impurities that should not be introduced into paint. Da Vinci's instructions for extracting and clarifying oil from walnut meats makes me think that he might not have considered raw pressed oil sufficiently clean for use in painting.

​I think the modifications go beyond digestibility, The primary difference in the fatty acid profiles of seed oils meant for consumption versus paints is lessening the linolenic and linoleic acids as much as possible while increasing the oleic component, which is non-drying. This limits oxidization, which is perceived by the consumer as  an oil going rancid, which is obviously undesirable. So even if the oil does not purport to have an anti-oxidant added, simply the type of fatty acid profile can dramatically impact its ability to form a film. The food industry is huge - even in terms of natural foods - and so a lot of resources are used to maximized strains of safflower and flax for different purposes. I would also note that recent conservation cases of liquefying paint points to oils being used that were dangerously on the edge of being non-drying oils and so it is critical to only use oil stock where linoleic and linolenic acids are maximized - which is the opposite of what is desirable in a food-grade product.

On walnut oil, I think the main difference might be that - as far as I know - different strains of walnut trees have not been created for different industries and so the oil sold in health food stores and the one used for paint might be essentially coming from the same or very similar sources. 

Finally, as has been suggested, any oil you use from a store would at minimum need to be tested to make sure it dried in a reasonable amount of time.