Raw tung oil is heated for 2 hours at 340 degrees Fahrenheit, then permitted to cool and set aside, when after 2 days the clear oil is drawn off from the sediment and heated to 356 degrees Fahrenheit. After one hour it is allowed to cool down to about 265 degrees Fahrenheit, when 2 per cent litharge is added in powdered form and a small portion of turpentine. When cold, the coloring matter is added." This is merely repeated here as an illustration of how differently experimenters are trying to work out problems. While this formula appears to be very good for clarifying the tung oil for further manipulation in varnish manufacture, the practical varnish maker will not be able to see how this process has produced a floor varnish or even a safe drying oil for a floor paint. The practical varnish maker will still make his floor varnish by fusing his gum with oil and he knows how much tung oil is required to produce the required toughness and hardness, while the paint maker in making floor paints will select whatever varnish best suits his purpose, if he uses any varnish at all along with drying oils and driers.
Rubbing Varnishes with Tung Oil. By omitting part of the linseed oil in the usual formula and substituting therefor a similar quantity of treated tung oil, the varnish maker gained quite an advantage in the manufacture of rubbing varnishes. In the first place the drying was more rapid, the varnishes became clear in a shorter time, they were more easily rubbed to a dead surface and the rubbed surface remained matt, because the occasional sweating had been overcome.
Polishing Varnishes with Tung Oil have from time to time given cause for complaint and the writer knows of more than one instance where table tops and piano tops had been rubbed and polished in the most approved manner, having a stunning appearance when finished, but in a short time a large space in the middle of the table or piano top showed a dead flat surface that could not be explained away by blaming moist atmosphere or improper ventilation, etc. The trouble was with the varnish which contained a moderate portion of treated tung oil. The conclusion arrived at after a thorough investigation was that the finishers did not thoroughly understand the nature of the polishing varnish and were led astray by the rapid setting, taking "dust free" drying for hard drying, which is very apt to occur in the use of tung oil varnishes. The drying of tung oil in comparison with linseed oil shows a remarkable difference and this seems to explain to a large degree the difficulty spoken of in reference to the polishing varnish. When raw tung oil is applied to any surface, such as glass, for instance, it forms a skin inside of 24 to 36 hours that has a greasy feel, is soft and non-elastic, and only becomes really hard after 6 or 7 days. Raw linseed oil, on the contrary does not form a skin until after the fifth or sixth day, but after that rapidly hardens and is solid or firm inside of 8 or 9 days. Authorities assert that when linseed oil has formed a hard film it does not gain further in weight through absorption of oxygen, while tung oil gains 10 per cent in weight after it has formed a skin. This could be explained only by the different chemical combination that has been determined in tung oil in comparison with all other drying oils. While linseed oil consists chiefly of linoleic acid, and oxidizes on drying by absorbing oxygen into linoxyn, tung oil on the other hand consists of elaeomargarin, which polymerizes into elaeostearine. This latter body is firm, greasy and non-elastic, which would account for the forming of the peculiar skin when tung oil is applied in a thin film on glass. Only through the oxidation of the elaeostearine, when a disintegration through the separation of volatile, organic substances takes place, is the process of drying complete and at this time the tung oil film attains the elasticity of linoxyn. The idea that tung oil dries more rapidly than linseed oil is only conditionally correct, but when a line is drawn between "dust free" and "thoroughly dry" there is very little, if any difference. Taking it all in all there is yet a wide field in the uses to which tung oil may be put and the research is by no means completed. There are drying mediums on the market under the name "tungate" that come in liquid form and dry linseed oil within a very few hours, according to percentage added, but the limit is 10 per cent, to exceed which the drier has no further oxidizing action on drying oils, although unlike other siccatives or driers such excess will not seriously interfere with the wearing quality of oil paint. Further uses for tung oil have been found in the manufacture of shellac substitutes for wood polishes, also in producing artificial caoutchouc, and to a great extent it is being employed in the manufacture of varnish for floor oil cloth. The Chinese put two qualities of tung oil on the market, the white and the black oil, the white oil is exported while the black oil, which resembles pine tar in color, is consumed at home for stopping seams in boats. The export value of tung oil from Wuchow and Hankow in the year 1906 was $3,271,000. while in the years 1907, 1908 and 1909 it averaged only $2,100,000 and in 1910 it reached $4,153,000, and while figures for 1911 are not available, it is safe to say that there has been a large increase over 1910.
In conclusion it may be said that tung oil or varnishes, in which it is one of the principal constituents, may be used without risk with all inert or chemically inactive pigments, but it is unsafe for use in connection with lead or pigments containing a lead base, because of its strong tendency to over-oxidation or as painters put it, the tendency to "pudding up." Zinc oxide, when ground in linseed oil, raw or bleached, is also apt, when thinned with tung oil varnish to swell or thicken to a great extent, so much so that the resulting paint cannot be spread and when further reduced to proper consistence the covering capacity is missing. A tung oil varnish, well made with the proper resins and without lead driers, is an excellent material for use with litho-pone white, as has been proven by the success of interior flat wall finishes that have been only really successful since the advent of the grinding liquids or mediums that owe their characteristics to tung oil, while the finishes themselves owe their dead flat appearance and moderate cost to the heavy petroleum spirits, with which they are thinned for spreading. The progress made in these materials within a comparatively short time is wonderful.