100% Pure?
Ed_McDonnell said:
Bought a bottle of "100% pure tung oil". I can't come up with a good description of this stuff other than to say it really smells bad. I've used other finishes with tung oil in them and they smell nothing like this. Could it have gone rancid? The oil drys as expected and once the tung oil is dry it doesn't have any smell. I suppose I could just not like the smell of Tung Oil..........
For those who have used "100% pure tung oil", what's your opinion of how it smells? How do you know if your oil finish has gone rancid?
Any thoughts?
Ed
Ed:
I have not ever used tung oil in the 100% form - I always dilute it with mineral spirits (at least to 70%, but more often at a 50%, 1:1, dilution) before applying it to wood. Maybe the mineral spirit smell masks any tung oil smell, but I've never found it to be offensive to me.
There is a distinct possibility among various oil processors and finish manufacturers that there is variation in degree of purity of the oil as it is processed and cleaned after being pressed from the tung seeds. As in most natural products, the degree to which other impurities are removed from or are retained in the finished product may have something to do with how the oil smells; that is, how much other organic materials are carried through the cleaning process into the finished product.
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