Walnut Oil for Wood Finishing

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.

Finally, I have wondered if the walnut oils used in the US for wood finishing are also from the European walnut (Juglans regia), like those grown here in Australia for their edible nuts? Perhaps oils from black walnut (J. nigra) or other walnuts (Juglans spp.) are used for surface finishes in the US as they are more prevalent there? Oils from other species may show differences in composition and drying /rancidity formation.

Delving deeper: In modified (usually blended) commercial oils and oil-based finishes e.g. like those used in paints, manufacturers add trace amounts of driers (metal 'soaps' e.g. copper or cobalt naphthenate) which accelerate the "drying" of oil-based surface coatings.

A Safety Warning: Like with all drying oils (e.g. Danish oil), applied as finishes to wood with clean rags, the user must be careful with the disposal of oily rags as there is a real fire risk. Rapid oxidation / polymerisation of a discarded oily rag has been the cause of many fires in workshops.