There are quite a number of these, drying, semi-drying and non-drying, and not a few have their special uses in paint. The most important of the drying variety is China wood oil or tung oil, which has been fully described as to its origin, characteristics and uses in Chapter XIV (China Wood Or Tung Oil. A General Description Of Its Origin, Production, Physical And Chemical Properties And The Great Importance Of Its Use In The Manufacture Of Varnishes. Origin) and requires no further reference here.
Poppyseed Oil is, next to linseed oil, most prominent for grinding the finer grades of zinc white and artists' colors, and must be classed among the drying paint oils for the reason that when pressed from ripe seed it dries very nearly as rapidly as raw linseed oil. The reason for the use of poppyseed oil in colors or paints is due to the non-darkening of this oil and its free spreading. Its advantages over linseed oil, however, are overestimated by some paint makers.
Bombay Nut Oil was at one time largely offered at a price somewhat lower than poppyseed oil, was very clear, almost water white, and while the specific gravity of poppyseed oil hovered about .926 this walnut oil was slightly heavier, averaging .932, and its drying property fully equal to that of bleached linseed oil. However, this oil has not been heard from in the market for some time.
Sunflower Seed Oil is also classed among the drying oils, but it has not found its way into general commerce and therefore nothing more is known about it than has been ascertained in an experimental way, which is that it has the specific gravity of poppyseed oil and nearly all of its other characteristics.
Hempseed Oil also belongs to the class of vegetable drying oils, but this seed being raised principally in Russia and a few other localities in Europe, it is used mostly there as a paint oil; and if any is brought to this country it comes as an admixture with linseed oil. Its specific gravity runs a little below that of linseed oil - between .926 and .930; its drying quality is slightly deficient as against that of raw linseed oil, but the chief objection to its use is its darkening tendency, which makes it serviceable only in outside paint in the darker colors. Russian authorities, however, claim that for wearing property it is far superior to linseed oil. This may be due to climatic conditions.
Other Drying Vegetable Oils, as nigerseed oil, tobacco seed oil, Scotch firseed oil, etc., that are not readily obtainable in commerce, are not at all interesting to the paint maker and color grinder.
Another vegetable drying oil that has been largely imported for some time into this country and Europe, under the name of Candle Nut Oil, by soapmakers and is known to science as Kukui Oil, is now being tested by progressive varnish and paint manufacturers. It bids fair to be a strong competitor of linseed oil when its characteristics become better known to the trade and it is prepared in a more scientific manner than it is now. When expressed from the kernels it has a rather dark color of reddish character, but when extracted it is light yellow in color and the odor is not so strong as in the expressed article. In the issue of the Oil, Paint and Drug Reporter of July 14,1913, Volume 84, No. 2, page 34, will be found a very interesting article on the "Chemistry of KukuiOil" by Alice R. Thompson, where the values of this oil are given as follows: - Specific gravity, .92 at 15.5 Centigrade; saponification value, 179.1; iodine number 155.5; Hebner value, 89.9; soluble acids, 1.71; Reichert Meicel number, 2.86. This would indicate that this oil is so far not quite as good a drying oil as linseed, but considerably better in drying than soya bean oil. It is just possible, however, that by more rational treatment its quality in that respect may be much improved.
E. V. Wilcox, special agent in charge of the United States Agricultural Experiment Station at Honolulu, in a letter dated August 2, 1913, says:-
"While Mr. W. M. Hoogs, of the Algaroba Feed Co., and two or three other parties, are preparing to extract kukui oil, it is not yet on the market as a commercial product. I believe, however, that it will be placed on the market during the present year. I have received numerous requests for the oil in large quantities.
"We find that about 80 per cent of the oil is readily recoverable by pressure, and that the nuts can be gathered and transported to town from the neighboring forests for about $10.00 per ton. Since the press cake is so valuable as a fertilizer I estimate that the nuts have a total value of $30.00 per ton ($20 for oil and $10 for fertilizer)."
Another drying oil of animal origin is the fish oil known as Menhaden Oil. This is barred out, however, from use in many paint materials, especially in interior paints, because of its offensive odor, and is made use of only in special outside paints, as in roof paints, some stack paints and by some manufacturers in other specialties.