The oil was formerly obtained by keeping the livers of the fish in large wooden vats, stirring constantly until so much decomposition has taken place that the cells containing the oil burst, and the oil thus released rises to the surface and is skimmed off with wooden ladles. The crude oil is allowed to deposit any suspended matters by sedimentation in a tank, and is then poured into casks ready for sale. The “brown oil” so often used by tanners is obtained by boiling the solid matter left after extracting the oil as above in iron tanks until all the water has evaporated; the oil thus liberated is then strained off, clarified and put into barrels.
The purer qualities of cod-liver oil are now obtained by boiling the livers with water and skimming off the oil which rises to the surface. Three grades are on the market at the present time: medicinal, or ordinary bright; an inferior “light brown”; and “dark-brown,” or “tanners’ oil.” It is probable that these steam-extracted oils are much more liable to “spue” than those extracted by the old method at a higher temperature, since Eitner[165] has shown that seal oils extracted at a low temperature spue badly, but lose the tendency if heated for some time to 250-300° C.
[165] Gerber, 1880, p. 244.
Genuine cod oil, as suitable for use in leather manufacture, is always more or less brown in colour, of specific gravity about 0·928, and refractive index 1·482. At present prices it can only be adulterated with other fish oils, rosin, or mineral oil, or with water, gelatine or mucilage. Of these, rosin oil and petroleum are the most frequently employed in sophistication.
An inferior variety of oil, known as “coast cod,” made from the livers of various fish, such as ling, haddock and hake, is also sold, but, as it is frequently mixed with oils from other fish refuse, it has a very poor reputation.
Cod oil, together with most of the other oils obtained from fish livers, has the property of producing an intense reddish-violet colour when a drop of strong sulphuric acid is dropped upon ten or fifteen drops of the oil contained in a white porcelain tray or saucer. The reaction succeeds still better, if, instead of the oil itself, its solution in chloroform, carbon disulphide or tetrachloride is employed. This test, although very useful for the detection of liver oils when they are present in oils of a totally different character, such as rape or olive oils, does not in any way indicate whether a sample of fish oil is pure or otherwise. A very similar reaction is given by cholesterol which is present in wool-fat.
Shark-liver Oil (Fr. Huile de requin; Ger. Haifischthran) is obtained from the liver of the “basking shark,” or “ice-shark,” chiefly caught off the coast of Norway; but the livers of the dog-fish and several allied fish also are sometimes substituted.
Shark oil has been employed in tanneries as a substitute for cod-liver oil, but, according to Lewkowitsch, and to Allen, it is no longer employed in England. From its pale colour it is probably principally used to improve the appearance of darker oils. According to Eitner,[166] its use causes leather to “spue” badly if not previously heated.
[166] Gerber, 1886, p. 266.
Shark oil is characterised by the very notable proportion of unsaponifiable matter which it contains, which is of the same character as that of sperm oil, and not easily removed from its soap solution by petroleum ether. It gives a strong violet-blue coloration with concentrated sulphuric acid, the reaction being even more marked than with cod-liver oil itself, and of a bluer violet.