If properly prepared, Turkey-red oil (sulphonated castor oil) will, when largely diluted with water, bear the addition of ammonia to alkaline reaction without showing any turbidity even on standing several hours. If a turbidity is produced, it indicates that the castor oil used was impure and contained some oil rich in stearin.
The alcohol test described on [p. 360] may also be applied, as the oily layer will be entirely soluble if castor oil alone was used in the preparation of the red oil.
Turkey-red oil usually contains about 50 per cent. of fatty acids (Allen).
Linseed Oil (Fr. Huile de lin; Ger. Leinoel) is used by leather manufacturers in the preparation of the japan for making “patent leather,” and to some extent also in currying, for oiling off levants and moroccos, though for these purposes it has been largely superseded by mineral oils. It is obtained from the seeds of the flax plant, Linum usitatissimum, chiefly grown in Russia and India. The Russian oil is usually mixed with the oil from hemp to the extent of about 20 per cent., while that from India, being grown as a mixed crop with mustard and rape, is never perfectly pure. The Baltic oil is considered best for japans, and is improved by storing for a considerable time in tanks in a warm place.
When obtained by cold pressure of the seeds, linseed oil is of a bright yellow colour; if a higher temperature be used in the extraction the oil is more or less brown, and tastes much more acrid. On exposure to air, linseed oil turns easily rancid, absorbs oxygen, and if spread out in a sufficiently thin film it dries to a neutral substance (linoxyn), which is insoluble in ether. This property is the one on which the chief value of linseed and other “drying oils” depends.
Linseed oil is chiefly adulterated with other seed oils, cottonseed being the most often used for this purpose, though menhaden and various other fish oils are occasionally employed. As the density of raw linseed oil varies between 0·932 and 0·936 at 15° C., the addition of other seed oils or of mineral oil would cause an appreciable lowering of this figure, whilst rosin or rosin oil would raise it. A judicious admixture of both mineral and rosin oils would give a product of normal density. Fish oils can be detected by their characteristic smell, especially on warming.
Various methods have been proposed for judging the quality of linseed oil, but none of them are perfectly satisfactory. The best oil is that which dries the most perfectly; but the rapidity of the drying, and the consistency of the dried product, are most important factors which must also be taken into account. The iodine-valve, which is a measure of the drying power, should not fall much below 180.
A satisfactory practical test, recommended by Allen,[162] consists in mixing the oil with three times its weight of genuine white lead, and covering a perfectly clean glass surface with the paint. An exactly similar experiment is made simultaneously with a standard sample of linseed oil, and the rates of drying and the characters of the coating of paint compared.
[162] Commercial Organic Analysis, ii. p. 122.
J. Muter has simplified this test by merely flooding a plate of glass with the oil and then exposing it to a temperature of 38° C. (100° F.) in a good current of air. The time required for drying, to such an extent that the coating will not come off when lightly touched, is noted, and compared with standard samples of oil. By applying the finger at intervals to different parts of the film surface the progress of the drying can be readily observed.[163]