Tung oil or China wood oil is a

drying oil Drying is a mass transfer process consisting of the removal of water or another solvent by evaporation from a solid, semi-solid or liquid. This process is often used as a final production step before selling or packaging products. To be conside ...

obtained by pressing the

seed In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a ''testa''. More generally, the term "seed" means anything that can be Sowing, sown, which may include seed and husk or tuber. Seeds ...

from the nut of the tung tree (''Vernicia fordii''). Tung oil hardens upon exposure to air (through

polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...

), and the resulting coating is transparent and has a deep, almost wet look. Used mostly for finishing and protecting wood, after numerous coats, the finish can even look plastic-like. Related drying oils include

linseed Flax, also known as common flax or linseed, is a flowering plant, ''Linum usitatissimum'', in the family Linaceae. It is cultivated as a food and fiber crop in regions of the world with temperate climates. In 2022, France produced 75% of the ...

safflower Safflower (''Carthamus tinctorius'') is a highly branched, herbaceous, thistle-like annual plant in the family Asteraceae. It is one of the world's oldest crops; today, it is commercially cultivated for vegetable oil extracted from the seeds. ...

poppy A poppy is a flowering plant in the subfamily Papaveroideae of the family Papaveraceae. Poppies are herbaceous plants, often grown for their colourful flowers. One species of poppy, '' Papaver somniferum'', is the source of the narcotic drug ...

, and

soybean The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean. Soy is a staple crop, the world's most grown legume, and an important animal feed. Soy is a key source o ...

oils. Raw tung oil tends to dry to a fine, wrinkled finish. This property was used to make wrinkle finishes, usually by adding excess cobalt drier. To prevent wrinkling, the oil is heated to gas-proof it (also known as "boiled"). "Tung oil finish" is often used by paint and varnish manufacturers as a generic name for any wood-finishing product that contains the real tung oil or provides a finish that resembles the finish obtained with tung oil.

History

The word "tung" is etymologically derived from the Chinese 桐 ''tóng''. The tung oil tree originates in southern China and was cultivated there for tung oil, but the date of cultivation remains unknown. The earliest references for Chinese use of tung oil are in the writings of Confucius (551–479 BCE) around 500 to 479 BC. During the Song dynasty (960–1279), tung oil was used for

waterproofing Waterproofing is the process of making an object, person or structure waterproof or water-resistant so that it remains relatively unaffected by water or resists the ingress of water under specified conditions. Such items may be used in wet env ...

on ships.

Composition

The primary constituent is a

fatty acid In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an ...

with a chain of 18 linked carbon atoms or methylene units containing three conjugated double bonds. They are especially sensitive to

autoxidation Autoxidation (sometimes auto-oxidation) refers to oxidations brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark. The term is usually used to describe the gradual degradation of organi ...

, which encourages cross linking of neighbouring chains, hence hardening of the base resin.

Uses

left, Showing the golden effect of polymerized tung oil versus bare wood Tung oil is a common traditional wood finish, used typically for two main properties: first, it is a naturally derived substance. Second, after it cures (5 to 30 days, depending on weather/temperature), the result is a very hard and easily repaired finish, so it is used on boat decks and now on floors. The oil is often diluted with hydrocarbon thinner so its

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

is very low and enables the oil to penetrate the finest grain woods. This thinning vehicle evaporates in 15 to 20 minutes. When applied in many fine/thinner coats over wood, tung oil slowly cures to a matte/light satin look with slight golden tint. Tung oil resists water better than any other pure oil finish and does not darken noticeably with age. It is claimed to be less susceptible to mould than linseed oil. It is considered safe to be used on sculptures made near waterways. Heating tung oil to about 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 Limonene () is a colorless liquid aliphatic hydrocarbon classified as a cyclic monoterpene, and is the major component in the essential oil of citrus fruit peels. The (+)-isomer, occurring more commonly in nature as the fragrance of oranges, ...

and D-limonene are less toxic alternatives for mineral spirits. The oil-paper umbrella is the traditional umbrella used in

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

, and other countries in the

East Asian cultural sphere The Sinosphere, also known as the Chinese cultural sphere, East Asian cultural sphere, or the Sinic world, encompasses multiple countries in East Asia and Southeast Asia that were historically heavily influenced by Chinese culture. The Sinosph ...

, and was brought to Western countries through the

Silk Road The Silk Road was a network of Asian trade routes active from the second century BCE until the mid-15th century. Spanning over , it played a central role in facilitating economic, cultural, political, and religious interactions between the ...

. 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.

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, lint-free cloth such as tee-shirt cotton. Diluents range from traditional spirits of

turpentine Turpentine (which is also called spirit of turpentine, oil of turpentine, terebenthine, terebenthene, terebinthine and, colloquially, turps) is a fluid obtainable by the distillation of resin harvested from living trees, mainly pines. Principall ...

to any of the new citrus-based thinners to

naphtha Naphtha (, recorded as less common or nonstandard in all dictionaries: ) is a flammable liquid hydrocarbon mixture. Generally, it is a fraction of crude oil, but it can also be produced from natural-gas condensates, petroleum distillates, and ...

. 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, buffed smooth with very fine sandpaper and 0000

steel wool Steel wool, also known as iron wool or wire sponge, is a bundle of very fine and flexible sharp-edged steel filaments. It is the most common type of wire wool and is often the type meant when wire wool is mentioned. It was described as a new pro ...

. The surface is wiped clean with a moistened rag and 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).

References

{{Reflist Chinese inventions Painting materials Vegetable oils Wood finishing materials