Tung oil

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".

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.

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.

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.