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Friday, June 10, 2011

Wood Science 101(2) - Tung Oil

If you watched the first of the three videos on the Hanging Temple , you may have noticed a short snippet that also caught my attention. At the 6:38 mark, we learn that
"before processing, the hemlock wood would have been soaked in tung oil to make it resistant to termites and erosion."
What is this stuff that has produces such amazing results on the properties of wood?
Tung oil is produced from the nut of the tung tree, or 
"Vernicia fordii (Tung Tree; syn. Aleurites fordii Hemsl.) This tree is a species of Vernicia in the spurge family, native to southern China, Burma, and northern Vietnam." - Wikipedia
Woodworkers may be interested to know that tung oil, or China wood oil, was commonly used centuries before its use in the Hanging was mentioned in the writings of Confucius nine hundred years earlier. Many today know of and appreciate tung oil as an excellent oil for finishing wood in a light and natural way:
"When applied in many fine coats over wood, tung oil slowly cures to a satin "wetted wood" look with slight golden tint. It resists liquid water better than any other pure oil finish, though it still provides little protection against water vapour exchange or scratches. Tung oil does not darken noticeably with age and is claimed to be less susceptible to mold than linseed oil." - Wikipedia

Since tung oil was in high demand for electronic components, munitions, airplanes, and various other essential products of the time, for "national security reasons" we imported tung seedlings and created a national tung oil industry in the early part of the 20th century. But drought in the 1930's, and finally hurricanes of the 1960's, wiped out the remaining tung plantations in the southern U.S., and we've relied on imported oil ever since. A few areas of Florida still have Tung trees, but they are considered invasive species these days (what's new?) and are the targets of extermination efforts by natural resource managers in that state. Interesting how our perceptions of nature and stewardship have changed over time.

Tung oil is somewhat of a mystery oil, in the sense that its variability results in making it difficult to characterize chemically.
"The exact chemical constituents have not as yet been accurately determined, simply because tung oil has never been found uniform, as will be seen from tables of the results of analyses by various authorities in chemistry. One authority, Cloez, asserts that tung oil is a mixture of 25 per cent of the glycerides of oleic acid and 75 per cent of margarolic acid, and says that the latter separates in crystals on adding an alcoholic solution, while the former remains in solution. The combination with lead oxide is only feasible after long heating of the oil, the lead soap melts at 212 degrees Fahrenheit, but the oil is easily saponifiable with an alcoholic solution of caustic soda. Another authority, Wenghoefer, as early as 1882 in his chemistry of hydro carbons, says that margarolic acid melts at 105 degrees Fahrenheit, and is a member of the group Cn H2N3 - CO.OH with the empirical formula C17H30O2. This acid forms by a change during the heating of over 392 degrees Fahrenheit and has a melting point of 160 degrees Fahrenheit.
Elaeolic acid may be had from elaeomargaric acid (margarolic acid) as well as from elaeostearic acid, when these are heated to about 350 degrees Fahrenheit under exclusion of air. It also forms, when the oil is extracted with carbon bisulphide and heated to 212 degrees Fahrenheit. The acid melts at 38 to 40 degrees Fahrenheit. As all the three acids may be present in mixture and also as glycerides, it stands to reason that the viscosity of the limpid oils, as well as the melting point of the gelatinized oils may be very different depending upon which of the acids is in preponderance. This would explain why tung oil from various shipments will give varying results in practice and shows how necessary it is to test each shipment." - From Uebele, Paint Making and Color Grinding, at

Even with its variability, tung oil seems to be as useful in the industrial applications as olive oil is in the kitchen. From the Waterlox Company, a U.S. based manufacturer of refined tung oil:
"• Tung oil has been used extensively in the paint and varnish industry.
• In the 30’s, tung oil compounds were used to coat cables, telephone wires, generators, fans, and various other types of electrical equipment.
• The automobile industry used large quantities of tung oil. For instance, every brake band manufactured used it as a binding agent to hold it together.
• At one time, more than 2 million pounds of it were used annually to manufacture cosmetic tubes.
• During the War of 1914 it was used extensively in the treatment of airplane fabrics as a water resisting varnish.
• The Chinese have used it for waterproofing masonry, cloth, shoes, clothing, and paper.
• It can be used to seal concrete.
• Tung oil, mixed with lime mortar or burned tung nut residue, was one of the world’s first agents for waterproofing caulking boats.
• It is used by stonemasons on granite and marble to permanently seal stone surfaces to prevent staining.
• A light coat rubbed onto steel is an effective rust inhibitor.
• The shells of tung nuts yield a valuable raw material for the manufacturing of insecticides.
• It is used in the printing of U.S. Paper Currency.
• Tung oil was reportedly mixed in the mortar that made the Great Wall of China." -
I first encountered tung oil while working with a wet-process fiberboard plant early in my career. This type of fiberboard, often call hardboard, was popular in the second half of the last century as an insulation board and exterior siding product. The wet-process fiberboard process resembles paper-making, in that wood fibers are introduced as a wet slurry into a head-box, the water is drained through a wire mesh screen, and the resulting fiber mat is then pressed into a panelized product. The fibers are bonded by the lignin from the wood without additional binder, but vegetable oils, usually linseed oil, was used by most hardboard plants to help catalyze the lignin bonding process during pressing.  While conducting process experimentation to improve the durability properties of the hardboard, we discovered that tung oil was superior to the various other vegetable oils used for in this process. However, the price of tung oil at that time was highly variable, as were the properties of the bulk oil, and we never we able to justify its continued use in the plant.

Nowadays, tung oil is mostly known to us as a great woodworking oil. But it has a colorful history of use as demonstrated by the Hanging Temple video and by this interesting TIME magazine feature story of 1942. Who knows what we may be using it for in the future?

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