Although tung oil has been known for many years in the Orient, it has only recently been discovered by Europeans and Americans as very valuable oil for the varnish maker. This oil differs from linseed oil in several characteristics. It cannot be used successfully in its raw state and requires preparation with driers and heat treatment varying somewhat from that given to linseed oil. If it is gradually heated to about 204.4° C (400° F), the oil changes into an insoluble jelly. Therefore, it must be heated with care to avoid this condition.
To prevent the jellying or polymerization of the oil, small amounts of linseed oil or rosin must be heated with it. It is often given heat treatment with about an equal amount of linseed-oil. The same metallic driers that are selected for preparing linseed oil for use in varnishes on pediment, wood frames, and frieze boards are also incorporated into the oil. One method of preparing a quick drying oil is to use two parts of tung oil to one of linseed-oil.
The linseed oil should be heated first to about 260° C (500° F) with a drier such as manganese borate (MnB2O4), using about one pound of the drier to each eight gallons of linseed oil and stirring before heating the oil. The oil is then added slowly to the hot linseed oil. The combined oils are heated to 270° C (518° F), and then more drier is added, a little at a time, to the extent of one pound of litharge (PbO) to each two gallons of tung-oil. The litharge should have about three per cent of slaked lime mixed with it in order to make the oil less acidic.
An oil of this composition contains a maximum of driers and dries very rapidly. Most prepared tung oils are somewhat similar to the above but usually contain less of the metallic driers. Chinese wood oil (tung oil) varies in color from a pale yellow in its best grades to a dark brown color in oils of poor quality. It seems to have rather remarkable drying properties on wood corner blocks, designer window toppers, and similar hardwood moulding. In this respect it is superior to linseed oil, although its iodine number is slightly lower, being about 163.
Meanwhile that of linseed-oil varies from about 178 to 185 for the North American oil. Varnish makers who are using rosin in large quantities as an important resin in their formulas claim that tung oil gives a rosin varnish certain valuable properties that cannot be obtained in any other way. The makers of rosin varnish are said to use about nine-tenths of all of the tung oil that is used in the United States. Ester gum varnishes also seem to require tung oil as an important ingredient. China wood oil is now used as the only drying-oil in many varnish formulas.
It seems to give varnishes waterproofing qualities and, for that reason, it is found in many spar varnishes. Some rosin and ester gum varnishes which contain tung oil as the only drying-oil are claimed to be waterproof if properly prepared. Many other similar varnishes contain a small amount of linseed-oil with tung oil as the chief drying oil. Tung oil not only dries faster than linseed oil, but films made from it become much harder, which is beneficial for hardwood moulding for wood corner blocks and fireplace accessories.
This added hardness is just what rosin and ester gum varnishes lack to make them more useful and satisfactory. While it is true that varnishes made from cheap gums and tung oil show up exceptionally well when new, tests show that they are not as durable when exposed to the weather as varnishes which contain fossil resins. Tung oil weighs slightly more than linseed oil, having a weight of 7.84 pounds per gallon at ordinary room temperatures. The viscosity of tung oil is very high, 20.5, while that of linseed-oil is only about 4.00, compared with that of water.