Raw tung oil of commerce is usually light straw yellow in color and of a disagreeable odor, that is designated by many as similar to smoked hog fat, which is difficult to disguise and cannot be removed, while the oil is in its crude state. When the oil is heated the odor is less penetrant, but the practical man will discover its presence even in varnish, after it has gone through the treatment required to make its use safe. Only when a varnish containing tung oil has become dry and perfectly hard the peculiar odor will have disappeared. Part of the odor will disappear when the oil is heated to 120 deg. F. and a current of dry air passed through it for several hours.
It required an enormous amount of research by chemists and varnish makers before they were safe in using tung oil in appreciable quantities without serious pecuniary losses. It was necessary to ascertain the causes of the disagreeable properties of the oil, foremost of which is the drying matt - or flatting - also the sometimes drying of the surface coated with the raw oil with a frosted appearance. Next is the tendency to coagulate in boiling at high temperature. Then, again, if the oil freezes and is left in that condition over ten days it becomes insoluble, which is explained by the fact that in freezing it expands to the extent of 10 per cent, becomes porous and permits the taking up of an excess of oxygen. It is in that condition difficult to melt, and on cooling forms a jelly that is insoluble.
While tung oil may be heated to very high degrees of temperature, when it has undergone several refining treatments of which we shall speak later on, yet on account of the uncertain qualities in tung oil, the consumers have found it best to heat it with rosin and thus avoid the risk of great losses. And as the original function of tung oil, when first imported into Europe and the United States, was to enable the varnish-making industry to produce low-priced varnishes that would not scratch or rub up, as they had been in the habit of doing, the practice came in vogue generally to melt rosin, adding a hardening medium, such as a salt of lime or lead manganate, etc., and fuse with tung oil, thinning the fused mass with the usual solvent, thus producing what is termed China wood oil varnish, with which to toughen the cheaper grades of commercial varnishes. This has been a boon to manufacturers and consumers alike, stimulating trade in that commodity. There is this peculiarity in the behavior of tung oil when combined by heat with rosin, that its acid number becomes less the longer it is subjected to heat, while linseed oil, when so treated, gives off free fatty acid and increases its acid number.
For instance, pure tung oil with an acid number of 11.8, when heated to 392 deg. F. with rosin, reduced its acid number to 10.7, and when heated to 472 deg. F. its acid number was only 7. Southern pine rosin heated to 572 deg. F. showed an acid number of 298. When one part by weight of this rosin and two parts by weight of the tung oil in question were heated to 572 deg. F., the mixture did not give the expected acid number of 104.4, but it only showed a number of 66.36. This can only be explained by the theory that a chemical combination is taking place between the rosin and tung oil, whose nature is not yet determined. It may be that this is due to the polymerization, to which tung oil is subject on long continued heating and which shows itself in its disposition of turning into a jelly. At all events this decrease in acid numbers is important in so far that owing to this very fact, tung oil varnishes are miscible with metallic pigments that are sensitive to acid. That tung oil as it comes to us in commerce cannot as a rule be depended upon for uniformity goes without saying, when, as is well known, the Chinese and other Asiatics are not overscrupulous in the selection of the fruits of the various groups of oil trees. On this account it is always safer not to heat the tung oil to over 392 deg. F., although some authorities claim that pure tung oil will not coagulate below 464 deg. F.
Very characteristic is the property of pure tung oil to coagulate when heated to over 400 deg. F. and to remain in that condition on cooling. Tung oil extracted with carbon bisulphide coagulates at a temperature of 212 deg. F. with a melting point of 95 deg. F.
Many trials have been made to avoid the coagulation of tung oil when subjected to high temperatures, and these have been more or less successful. One of these processes that may be mentioned here is patented and consists of the addition of zinc dust or similar strongly-reducing metal in powdered form to tung oil, while it is being heated in an enameled kettle to a temperature of not over 190 deg. F. When the metal has been stirred in and is well taken up by the oil the heat is increased to about 250 deg. or 272 deg. F., and when the oil has been permitted to cool it is filtered. It is not necessary to add more than 0.2 per cent of metal, and the tung oil so treated will dry very slowly, but with the subsequent addition of siccatives, it will dry very rapidly, even when only small percentages are given. The advantage of tung oil so treated, however, is that it may be heated to at least 420 deg. F. without risk of coagulation.
When treated tung oil is added to linseed oil, raw or boiled, or to ordinary varnish, it increases their drying properties and the toughness of their films, but when raw tung oil or imperfectly treated tung oil is used in this admixture it may lead to coagulation. In extreme cold weather raw tung oil is liable to separate particles similar to stearine wax and assume a waxlike consistency.
One peculiarity of tung oil as compared with other drying oils is that in drying it does not form a skin, but dries in a solid film, and this is explained by the fact that drying oils uniformly dry from their absorption of oxygen from the air, while tung oil becomes fixed through polymerization.
Meister (see Chem. Revue fur die Harz u. Fett Industrie, 1910, pp. 150) has determined that linseed oil in drying takes up all the oxygen it is capable of, so that when the film has dried the maximum of absorption has been reached, while tung oil absorbs only one-third of the full amount of oxygen up to the time of drying and later on the other two-thirds, so that it may be said that in the case of tung oil there are two periods, one of drying and another of complete hardening after drying. Because of this behavior it may be said to dry dust free, like a varnish and harden subsequently to the touch. The oil cake or residue from the pressing of the meal is rich in protein, but is utterly useless as animal food stuff because to quite an extent poisonous. Many experiments have been made to free the material from its poison, but so far without success. But it is recommended as an excellent fertilizing material on account of its poisonous character, it being believed that its use would destroy certain insects. It is claimed that raw tung oil is also to some extent poisonous and that people with sensitive eyes are liable to have them inflamed in having accidentally some of the oil, even a drop, get into their eye.
The best and safest method for clarifying and bleaching of tung oil is to place a certain quantity of the raw oil into a tank or kettle, provided with a steam jacket or steam coil and a mechanical stirring device, heating the oil to 250 deg. F. and adding at least 5 per cent, but not over 10 per cent of its weight of best fuller's earth (which is an aluminum magnesium hydro-silicate), keeping the temperature at 250 deg. F., while running the stirring device for at least one hour, then running the material through a filter press, which would serve its purpose best if heated.