Tung oil or China wood 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. Related drying oils include linseed, safflower, poppy and soybean oils. 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".
Contents
The name is often used by paint and varnish manufacturers as a generic name for any wood finishing product that contains the real tung oil and/or provides a finish that resembles the finish obtained with tung oil.
History
The tung oil tree originates in southern China and was cultivated there for tung oil, but the date of cultivation remains unknown. During the Song Dynasty, tung oil was used for waterproofing ships. Tung oil is etymologically derived from the Chinese tongyou.
Composition
The major fatty acids in tung oil and their concentration are listed in the table.
The primary constituent is an aliphatic carboxylic acid with a chain of 18 linked carbon atoms or methylene units containing three conjugated double bonds. They are especially sensitive to autoxidation which encourages cross linking of neighbouring chains and hence hardening of the base resin.
Wood finishing
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. For example, Danish Oil has been thought to have tung oil in it. However you go to the manufacturer's material data source page you will see that tung oil is not mentioned. [1]
Heating tung oil to about 500 °F (260 °C) in an oxygen-free environment will substantially increase the viscosity and film-forming quality of the product. Most polymerized tung oils are sold mixed with mineral spirits to make them easier to work with. Limonene and D-limonene are less toxic alternatives for mineral spirits.
Oil-paper umbrella
The oil-paper umbrella is the traditional umbrella used in China, Japan and other countries in the sinosphere, and was brought to western countries through the Silk Road. Tung oil is the "oil" mentioned in the oil-paper umbrella, which is used to protect the paper from getting wet, and to make the umbrella waterproof.
Other uses
As the penetrating power of tung oil is excellent and it will adhere to porous minerals, it is often used by stonemasons on granite or marble installed in kitchens, bathrooms, and other staining fluid environments. Applied at a 1:1 to 1:5 ratio (oil to naphtha or limonene), it gives a finish like that of wet stone. Often several thin layers are applied at a high solvent-to-oil ratio to make the coating more durable and permanent.
Application
The traditional technique for applying pure tung oil is to dilute the oil 1:1 with solvent, then apply a succession of very thin films with a soft non-fuzzy cloth such as tee-shirt cotton. Diluents range from traditional spirits of turpentine to any of the new citrus-based thinners to naphtha. The choice of thinner should be guided by how fast the coating needs to set. Naphtha works well in spray-on applications in well ventilated studios. Primary coats may be laid down at a 1:1 oil-to-thinner ratio, and successive layers, if not absorbed into the wood, at higher solvent to oil concentrations. This technique brings out the deepest color of the wood while maintaining a matte finish.
Tung oil finishes that start with polymerized oils or tung oil preparations are best applied in the fat over lean principle: thinned pure oil is applied to deeply penetrate the surface, to fill pores. Straight oil is then applied moderately to adhere to the surface and provide a good base for the thick gloss layers. The polymerized oil is then applied thickly as a single layer, allowed to fully dry, is buffed smooth with very fine sandpaper, then 0000 steel wool. The surface is wiped clean with a moistened rag, then allowed to dry. A final coat is applied fairly thickly (the oil will smooth itself into a glass-like coating) and allowed to dry for two to three days. Rags soaked with tung oil can spontaneously combust (burst into flame).