As much as I love Tung oil for my knife handles, it is not mineral oil and it will get rancid over time. No problem for knife handles on which I put only a light film and then wipe it off, but I wouldn't use it on my boards.
Interesting question as to tung oil turning rancid.. I did a quick search and......
http://www.aawforum.org/vbforum/showthread.php?3899-Tung-Oil-Smell
Hopefully this guy is right......
" I am not sure tung oil will go 'rancid' without polymerizing. This is one of the "drying" oils, that, in the presence of oxygen, and heat/time will allow the oil molecules to link to one another to form the durable surface that we desire. Among the "vegetable oils", tung oil produces one of the hardest finishes (better than linseed [flax] oil) and although the drying rate is slow compared to other finishes, it still produces a good surface that protects the wood pretty well. The fact that the oil you are using seems to accomplish the polymerization process correctly, and does not smell badly after it 'cures' lends me to think that there are also volatile compounds in the oil which you find offensive that are released in the drying/curing process. I don't think your oil is rancid, at least in the sense of the partially-oxidized, non-drying oils (like corn oil or "vegetable oil") which can go rancid without altering its physical properties appreciably. I think truly 'rancid' tung oil would be a solid in the bottle!
Tung oil comes from the seeds of a tree that is in the same family as poinsettia (Euphorbiaceae), and this family of flowering plants is known to have a complex chemistry in its tissues. The degree to which the processor has cleaned the native oil as it is prepared prior to packaging likely has a lot to do with how many other compounds are present in the final product. Because other commercially-available tung oil-based products are mixtures, they may not smell as bad (or at all) if: 1. higher purity oils are used, 2. the oil has been diluted by other additives in the finish, or 3. the volatile compounds causing the offending odor(s) is/are masked by other compounds.
I suppose the question comes down to what the meaning of "pure" is....
Hope this helps a bit...
Rob Wallace
Rob Wallace
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