Walnut Oil for Wood Finishing
This article was written by Eugene Dimitriadis in reply to Wellwood's question as to why walnut oil does not go rancid when used for wood finishing.
Of course, our walnut oil does not go rancid in the bottle if stored in a cool dry place before opening, and in the refrigerator after opening. This account is aimed at explaining what may be happening when oils are used on wood and when they go rancid (in the bottle).
As walnut oil is reasonably high in unsaturated fatty acids, it would probably (for this reason) "dry", i.e. polymerise quickly, as a thin film on a porous surface like that of wood. This polymerisation, once complete, "uses up" the free (available) unsaturated portions in the oil which are the centres for rancidity formation. Once touch-dry (i.e. polymerised), the leathery film is therefore no longer prone to rancidity. If you ever tasted an old, i.e. rancid, walnut you quickly know what rancidity is like. Rancidity is an oxidative process caused by oxidation from light, air and induced by, what are called, free radicals. These involve oxygen attacking unsaturated sites in the fatty acid chain of the oil (resulting in the unpleasant hydroperoxides, chain breakdown (shortening), aldehydes etc. being formed in the oil. Some plant oils have natural radical scavengers, (called anti-oxidants) eg a- tocopherol, B-carotene, in them which extend their shelf life in the bottle, "naturally". ( US Black walnut has a radical scavenger, called juglone, but I'm unsure if any of this ends up in the oil. Perhaps it's found mostly in the exterior seed coat, that dark bit that cracks, dries and falls off the shell.) Looking at interactions with wood:The problem of rancidity in the bottle may never arise when this or other fresh seed oils are used on a wood surface. Some woods have chemicals (eg phenols, polyphenols or tannins) and sometimes even traces of minerals which may either retard or accelerate the drying rate of surface finishes, including seed oils.