Tung oil, historically known as China Wood Oil, is a natural substance derived from pressing the nut of the tung tree. This vegetable oil is classified as a “drying oil” because it hardens significantly upon exposure to air. Its unique chemical structure provides properties that have made it a valuable commodity for centuries, particularly for durable and water-resistant coatings.
The Biological Origin of Tung Oil
The source of this oil is the seed, or nut, of the tung tree, which belongs to the genus Vernicia. The vast majority of the world’s commercial supply comes from the species Vernicia fordii, though Vernicia montana is also cultivated in more tropical regions. The oil-rich seeds are encased within a thick, woody fruit that drops from the tree once mature. While the oil is highly valued for coatings, all parts of the tung tree, including the seeds, are toxic if consumed. For this reason, the substance has always been used industrially rather than culinarily.
Historical Production and Geographic Spread
The tung tree is native to the southern and central regions of China, particularly around the Yangtze River basin. Evidence suggests the oil was in use as far back as 400 B.C., referenced in the writings of Confucius. For millennia, the Chinese utilized tung oil for its remarkable ability to repel water, using it to caulk and paint wooden ships, preserve textiles, and waterproof paper.
The oil remained largely a Chinese product until its introduction to other parts of the world in the late 19th and early 20th centuries. Commercial cultivation eventually spread to other subtropical and temperate zones, most notably in South America, with Argentina and Paraguay becoming significant producers. Historically, the southeastern United States also saw a period of extensive cultivation, peaking around the time of World War II, when the oil was designated a strategic material for defense applications.
Key Physical Properties and Extraction
Tung oil’s performance is directly linked to its high concentration of alpha-eleostearic acid, which typically constitutes 80% or more of its composition. This fatty acid contains three conjugated double bonds, making it highly reactive to oxygen in the air. This rapid reaction, known as polymerization, transforms the liquid oil into a solid, durable film.
The oil is extracted from the harvested nuts primarily through mechanical pressing, which yields the raw oil. This raw form tends to dry with a fine, wrinkled texture, a property once exploited for specialized finishes. To produce a smoother, more uniform coating, the oil is often heat-treated, or “boiled.” This process partially polymerizes the oil and prevents the wrinkling effect. Heat treatment increases the oil’s viscosity and improves its film-forming quality, resulting in a harder, more water-resistant finish than other natural drying oils.
Primary Uses as a Finishing Agent
The combination of water resistance, flexibility, and hardness makes tung oil a premier choice for protective coatings. Its primary application is as a penetrating finish for wood, where it seeps into the grain and cures to form a tough, hydrophobic barrier within the wood itself. This results in a matte or light satin appearance that enhances the wood’s natural grain without creating a thick, plastic-like surface layer.
The cured oil does not darken noticeably over time and offers superior resistance to mold compared to alternatives like linseed oil. Beyond wood finishing, the oil’s fast-drying and durable qualities made it an ingredient in the formulation of varnishes, lacquers, and traditional printing inks. Its natural properties allow it to provide a protective, zero-VOC finish, which is highly valued for items that may come into contact with food, such as cutting boards.