The research work in the possible uses for tung oil has not, by any means, been completed, as yet, but we may say that so far the chief object has been to find a substitute for linseed oil in the manufacture of paint and varnish. It may be said, however, that tung oil cannot take the place of linseed oil unreservedly, but should be classed as an auxiliary to it, because tung oil has certain characteristics that are missing in the former, especially toughness of film, and water resisting properties. On the other hand, linseed oil is more tractable and does not require so much care in preparation for use in oil paints for general work.
By the boiling process, moisture and albuminous matter are removed from tung oil and in order to avoid kettles from running over, the boiling should be done over a slow fire at first, until all moisture is driven off, when the temperature is gradually raised to 356 degrees Fahrenheit and held at that point, until a sample of oil taken from the kettle exhibits the proper viscosity. At that stage the kettle is removed from the fire and allowed to cool as rapidly as possible. Drying mediums, such as litharge and manganese salts may be added to increase the drying properties of tung oil, but it is not absolutely necessary, as the oil has great drying energy. So far it has not been found useful in the manufacture of printing inks and lithographers' varnishes, at least there is no record of any one having been successful in that direction. There is a great difference of opinion in the varnish manufacturing trade as to whether comparatively fresh pressed tung oil or oil that has been stored for a long time, produces the best results, when used by the varnish maker. So much appears to be certain, that when made by the addition of tung oil that has been bodied up much in boiling, varnishes become very tough in long storage.
Tung oil can be heated to a point as high as 420 to 435 degrees Fahrenheit without risk of gelatinizing, provided it has been heated at first to not over 272 degrees Fahrenheit until all possible moisture has been driven off, but it must then be kept at the high temperature for about 3 hours and during that time constantly and uniformly agitated, otherwise it will break and coagulate. Tung oil thus treated is claimed to be excellent for use in the manufacture of varnishes from rosin, imparting to them great toughness and resistance to atmospheric influences, almost equal to varnishes, made from fossil gums and linseed oil. When tung oil breaks in boiling without gelatinizing we have tung oil foots, same as we have linseed oil foots, when linseed oil breaks during the boiling process. Attempts have been made to make use of tung oil foots in the manufacture of substitute for rubber and patents have been granted to protect such inventions or processes. In one of these the specifications are as follows: - "Process for making use of the rubber-like mass that is the result of heating tung oil at a temperature of over 392 degrees Fahrenheit for any length of time, consisting in melting the mass with poppyseed or nut oil at a temperature of 572 degrees Fahrenheit until it becomes soluble in such solvents as spirits of turpentine, benzine, benzol, acetone, amylacetate, etc., and miscible with drying oils, varnishes or pyroxylin solutions." The inventor melts the partly gelatinized portions of tung oil foots in about equal portions by weight with either of the oils described in the specifications, the best policy, however, for the consumer of tung oil to pursue is to avoid by all possible means the breaking as well as the gelatinizing risk and it is recommended wherever it is practicable for the varnish maker to use tung oil in its pure state without admixture with other oils. Furthermore, it should be avoided to add to varnishes that are made with tung oil as much driers as would be given to a varnish of similar nature when made with linseed oil, because it would set too quickly and be soft underneath, thus having a tendency to become tacky. It is also best to use manganese or cobalt resinates rather than lead resinates.
Tung Oil in Enamel Varnishes. Of especial value has this oil proven in the manufacture of varnish for moderate priced enamel paints, where high luster, great hardness, drying properties and wear are features that before the advent and exploitation of tung oil could be had only by the use of selected, high priced, pale, hard gums, that are now so high in cost as to be prohibitive in the manufacture of enamel paint. Of course, it depends on careful manipulation and the selection of the pigments, whether the paint maker meets with success.