tung oil

tung oil, oil obtained from the seeds of a tropical tree, the tung tree (Aleurites fordii) of the spurge family, and from seeds of some related species, all from Indomalesia or W Pacifica. It is known also as China wood oil and nut oil. The poisonous seeds found in the heart of the tung fruit (which is the size of a small apple) contain more than 50% tung oil, readily obtained when the seeds are heated, ground, and pressed. The oil is amber-colored and contains a high proportion of eleostearic acid. Because of its wide use as a dryer in varnishes and paints, it has great commercial importance. While the bulk of the product is utilized by the paint and varnish industry, tung oil has additional uses, e.g., as a component of insulating compounds and in the manufacture of linoleum and oilcloth. China was long the chief producer of the oil, but the tree has been introduced in other areas as well. The U.S. Dept. of Agriculture experimented with tung tree growing early in the 20th cent., and afterward encouraged Southern farmers to cultivate it. In recent years, an increasing portion of the commercial supply has been obtained from trees grown in the United States, particularly in the South.

Tung oil is used as a wood finishing product. It has two distinct meanings, pure tung oil and tung oil finishes, which are often confused.

Major fatty acid composition of tung oil

Palmitic acid	5.5%
Oleic acid	4.0%
Linoleic acid	8.5%
Eleostearic acid	82.0%

Pure tung oil

Tung oil is made from the pressed seed from the nut of the tung tree. Tung oil is considered a drying oil much as linseed, safflower, poppy and soybean oil. When applied, it provides a tough, highly water-resistant finish which does not darken noticeably with age as does linseed oil. This is not to say that it is a colorless finish; it still has a slight golden tint.

Tung oil is also sometimes called “China wood oil”. It has been used for hundreds if not thousands of years in China to seal decorative and marine wood as well as porous masonry.

Polymerized tung oil

Heating or chemical treatment of pure Tung oil will substantially increase the viscosity and film-forming quality of the product. The resulting oil will range in consistency from that of maple syrup to that of motor oil. Some brands, such as Lee Valley Tools, include quick evaporating thinners to make it easier to work with and improve wood penetration.

“Tung oil” finishes

“Tung oil finish” refers to a wetted wood look that a succession of penetrating tung oil applications gives. However, it is also a generic term and has been liberally borrowed by paint-and-varnish makers to describe the expected result of using their preparations — which may or may not actually contain any tung oil at all.

Application

The traditional technique is to apply the diluted oil as a succession of very thin films with soft non-fuzzy cloth like tee-shirt cotton. Dilutants range from traditional Spirits of turpentine to any of the new citrus thinners to VM&P naphtha. The choice of thinner should be guided by how fast the coating needs to set. Naphtha is often too fast, but works well in spray-on applications in well ventilated studios. Primary coats may be laid down at a 1:2 oil-to-thinner ratio, and successive layers (3 or more) at higher oil concentrations, with the final layer consisting of the oil alone. This technique results in the traditional wet-wood look by filling in all open wood cells as water does. The finish is matte by its nature.

Tung oil finishes that start with polymerized pure 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 00 steel wool. The surface is wiped clean with a moistened rag, then allowed to dry. A final coat is again applied fairly thickly — the oil will smooth itself into a glass-like coating — and allowed to dry for two to three days. The resulting finish is almost completely impervious to water.

Food contact and allergic reactions

As the source of tung oil is a nut, people with nut allergies often report adverse reactions to contact with (or even the odour of) tung oil. Reactions can be severe in some cases. Whilst tung oil has been used for many centuries as a finish for kitchen items such as wooden bowls and cutting boards, some individuals must avoid its use.

Other uses

As the penetrating power of tung oil is excellent and it will adhere to porous minerals, it is often the “secret” ingredient that traditional stonemasons apply to granite or marble that is destined to serve duty in kitchens, bathrooms and other staining fluid environments. Applied at a 1:5 ratio (oil to naphtha), it seals the surfaces permanently, while also giving a finish like that of wet stone to the product. Often several thin layers are applied at the high solvent-to-oil ratio, in order to build up the durability and permanence of the coating. The non-yellowing nature of tung oil is vitally important for its satisfactory use in these applications. Clean up, especially of hands, may be difficult without a solvent.

According to local residents in Crestview, Florida, the tung nut was used to make aircraft fuel during World War II. The trees were all over Crestview, but as the nut was poisonous, the townspeople cut them down.

References

External links

• Finishing pine with tung oil
• Oil Finishes: Their History and Use