Tung oil is a drying oil obtained by pressing the seed from the nut of the tung tree (Vernicia fordii). Tung oil hardens upon exposure to air, and the resulting coating is transparent and deep almost wet-look. Used mostly for finishing/protecting wood, after numerous coats the finish can even look plastic like. The oil and its use are believed to have originated in ancient China and appear in the writings of Confucius from about 400 B.C. Raw tung oil tends to dry to a fine wrinkled finish; the US name for this is gas checking: this property was used to make wrinkle finishes, usually by adding excess cobalt drier. To stop this, the oil is heated to gas-proof it, and most oils used for coating are gas-proofed. Thus, to avoid the wrinkling, all tung oil available for finishing today is "boiled".
Tung oil is very popular today because of 2 properties: First it is natural or "green" product when it has dried. Secondly, after it cures (5 to 30 days, weather/temperature related), the result is a very hard and easily repaired finish. This is why it is used on boat decks and now on floors. The oil is often diluted with hydrocarbon thinner so that the viscosity is very low and enables the oil to penetrate the finest grain woods. This thinning vehicle evaporates within 15 to 20 minutes and results in a totally green residual finish. As mentioned above, when applied in many fine/thinner coats over wood, tung oil slowly cures to a satin "wetted wood" look with slight golden tint. Tung oil resists liquid water better than any other pure oil finish and does not darken noticeably with age and is claimed to be less susceptible to mould than linseed oil. Most importantly, of all the oil finishes, tung oil is the only drying oil that will polymerize 100% (completely harden). Linseed oil, for example, never completely hardens.
While tung oil has become popular as an environmentally friendly wood finish, it should be noted that many products labelled as "tung oil finishes" are deceptively labelled: polymerized oils, wiping varnishes, and oil/varnish blends have all been known to be sold as tung oil finishes (sometimes containing no tung oil at all), and all the above contain solvents and/or chemical driers. Product packaging will usually clearly state if it is pure tung oil.