Product Profiles: Vegetable Oils: Food Grade vs Artist Grade

Q: "If flax oil and linseed oil are basically the same thing as nutritional oils from the grocery, why doesn’t art store linseed oil spoil at room temperature? Is something added to keep it from going bad?”

A: The reason artists’ linseed oil hassuch a long shelf life isn’t because ofwhat’s added, but rather what’sremoved from the raw oil. Alkalirefinement (a ‘washing’ process)removes most of the free fatty acids,mucilage and other impurities that canlead to rapid spoilage. That's why artsupply oils can last a really long timewithout any noticeable loss of quality.

Unrefined cold-pressed linseed and walnut oil sold for use in painting might occasionally go rancid in storage, but due to relatively smal lpackage sizes most painters probably just use it up before it gets smelly.

Fresh, unrefined oils vary in shelflife.Food-grade safflower oil keeps forabout 2 years at room temperature,compared to just a few weeks for flaxoil.

Flax seed, the source of linseed oil

How prone a particular oil is to goingrancid is due to fatty acid composition.Drying oils are mostly made up ofunsaturated fatty acids (e.g. linolenicand linoleic acids). These fatty acidsare the components that produce an“off” smell through oxidation, and alsowhat makes oil paint dry. (Bycomparison, saturated fats like palmand coconut oil have a really longshelf life, but they don’t solidifythrough oxidation, and therefore can’tbe used as paint binders.)

Rancidity doesn't really affect whetheran oil is suitable for use in paintingcomparedto solidification when a skinforms, much less material has oxidizedif the oil is still fluid but just smelly.

Leonardo Da Vinci wrote instructions for making walnut oil where nut meats were washed to remove the skins,then left left to settle, exposed to air for as long as a month. It seems likely the resulting product would be partly oxidized.

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