Whale Oil (Fr. Huile de baleine; Ger. Wallfischthran) is extracted from the blubber of various species of whale, and often contains traces of spermaceti, the substance which characterises the oil from the sperm whale. This yields on saponification higher alcohols, which are found in the unsaponifiable matter; but in ordinary whale oil the total unsaponifiable matter seldom exceeds 11⁄2 to 2 per cent. Whale oil is largely used on the Continent for “[chamoising]” (q.v.), and is consequently a constituent of dégras. It is much less oxidisable than cod.
Seal oil (Fr. Huile de phoque; Ger. Robbenthran) is obtained from the common rough-coated seal, abundant in the Arctic regions. It bears a strong resemblance to both whale and fish oils, and cannot be detected in mixtures of these. The Swedish “Dreikronenthran” (Three Crown Oil) is a mixture of seal and fish oils. As genuine seal oil only contains about 1⁄2 per cent. of unsaponifiable matter, its adulteration by mineral or resin oils may be detected by a determination of the matter extracted by petroleum ether after saponification of the oil (see L.I.L.B., [p. 178]).
There is no simple test by which the purity or otherwise of a sample of oil can be determined, as the dealers know all the best tests which the users could try, and fake up their oils accordingly. For instance, if petroleum is to be added surreptitiously to a cod oil, the decrease in specific gravity of the oil caused by this addition would be corrected by the addition of a suitable quantity of soap or rosin oil, which would scarcely affect the colour, taste or odour of the sample. The only satisfactory method of detecting adulteration is to submit the oil to a complete chemical examination, and for this purpose L.I.L.B., pp. 156 et seq., or the larger text-books already named may be suitably consulted.
Menhaden Oil (Porgie oil, Straits oil) is largely used in certain districts as an adulterant or substitute for cod oil. It is obtained from the Alosa Brevoordia or menhaden, a member of the herring family, about a foot long. The fish is caught on the Atlantic coast of America, and is so plentiful that it is very doubtful whether cod oil can ever compete with it successfully in price. The fish are boiled in steam kettles, the oil squeezed by hydraulic presses, clarified, and bleached by exposing to the sun in shallow glass-covered tanks. An inferior grade is known as “Bank oil.” Menhaden oil is chiefly characterised by its very high “specific temperature reaction” (L.I.L.B., p. 169) which is about 306. It is not a good leather-oil, being very liable to “spue.”
Many other varieties of oil extracted from the bodies, and not from the livers only of fishes, are classed as fish oils. Menhaden oil is the principal of these; but Japanese oil, sardine and herring oils, and those obtained from the refuse of other fish are scarcely less important, though as they are derived from such different sources it is not possible to quote any definite characteristics by which they may be identified when mixed with more valuable oils. They are usually very liable to “spue.”
Fish Tallow, which, according to Eitner, is a good and cheap substitute for dégras, is the solid grease obtained from different kinds of fish oil by subjecting them to a low temperature and separating the matter which is thus precipitated, or (as in China and Japan) the solid fat which is extracted at the same time as the oil from the body of the fish. Formerly fish tallow was only obtained from and with Japanese train oil, but it is now obtained from whale blubber. This latter yields a very pure form of the tallow, which does not need any rectification; but the Japanese variety, which is obtained from fish of the herring family, contains a sort of fish glue, which greatly deteriorates the quality of the product. By careful purification, however, this glutinous matter may be removed, and the refined product has none of the leather-staining properties so characteristic of the crude tallow. The refined tallow is sold in square flat cakes, melts at 42° and is not quite so stiff as ox tallow.
Dégras and Sod Oil are products of chamois-leather dressing ([p. 378]) which are used in currying. Skins are treated with marine animal oils, and submitted to oxidation, and the surplus and partially altered oil is recovered. In the French method, whale and seal oils as well as liver oils are used, and the oxidation is slow and gradual, and the residual oil, being liquid, is recovered by pressure, and constitutes moellon, of which the first pressing (première torse) is the best. This is never sold for currying in its original purity; but, mixed with further quantities of fish oils, tallows, and sometimes wool-fat, it constitutes the ordinary dégras of commerce. The additions, though they lower the value, are not to be considered as simple adulterations, since the moellon alone would be less suitable for the purpose. After removal of as much oil as is possible by dipping in hot water and pressing, a further quantity is recovered by washing with solutions of potash or soda, from which it is separated by addition of acid, and constitutes a lower quality of degras. The moellon is of such value as a currying material, that factories are run in which chamoising is carried on solely for its production, the skins being oiled and oxidised repeatedly, till reduced to rags.
In the English method of chamoising, liver oils are almost exclusively used, and the oxidation is much more rapid and intense, the skins being packed in boxes or piled, and allowed to heat. The product obtained in this way is much more viscous, and can only be recovered by scouring with alkalis; and the product, recovered with acid, constitutes sod oil. In many English factories, a modified method is now adopted, and a product recovered by pressure, which scarcely differs from moellon.
An important peculiarity of dégras and sod oil is its ready emulsification with water, which from its mode of preparation, it always naturally contains, and which should be present in a good dégras to the extent of not less than 20 per cent. Such a mixture, containing water, is a sort of natural fat-liquor and is absorbed much more perfectly by the skins than an oil alone. Sod oils, however, are frequently “evaporated,” or deprived of water by heating above 100°, with the object not only of effecting a fancied improvement, but of getting rid more completely of the sulphuric acid which the water is apt to contain. This makes them more homogeneous, and consequently much darker in colour. It is not easy to neutralise the acid in an aqueous sod oil by direct addition of alkali; possibly ammonia is best adapted for the purpose; or a suggestion, I think due to Eitner, may be adopted, of incorporating a small quantity of a suitable soap. In any case, very complete mixture is required. If the sulphuric acid used in recovery has been insufficient for complete neutralisation of the alkali, the dégras or sod oil will naturally contain soaps, and sometimes also free alkali. Free acid and free alkali are both injurious to leather, the former if anything the more so, darkening the colour, and even rendering the leather tender. When dégras is used in mixture with other fats, care should be taken not to raise the temperature of the mixture so high as to drive off the water, to which a good deal of its special efficacy is due.
The chemical changes which take place during the chamoising process are as yet incompletely understood. A large proportion of the glycerine is dehydrated during the “heating,” forming acrolein (acrylic aldehyde), to the action of which it is very possible that the actual conversion of the skin into leather is due, while the fatty acids also undergo oxidation. Dégras therefore always contains considerable quantities of oxidised fatty acids, which are sometimes associated with nitrogenous products from the skins, and which are soluble in alcohol, but insoluble in petroleum ether. To these products Simand gave the name of Degrasbildner (dégras-former, Fr. dégragène), and it has been considered a measure of the quality of the degras, but its exact value and function is rather doubtful. According to Simand, a genuine dégras should contain not less than 15 to 20 per cent. of the dégras-former as estimated by his method, calculated on the dry oil, and a smaller percentage is also present in the original fish oils. (For method of estimation see L.I.L.B., p. 182).