In obtaining the best sorts of liver oils, e.g. codliver oil, the livers are taken from the fish as soon as they are caught, and are heated in steam-jacketed vessels until the cell membranes burst and the oil exudes. The oil is then separated by pressure.
Inferior qualities of oil are obtained by treating putrid livers in the same way at the end of the voyage. These tainted liver oils are unfit for medicinal purposes, but are used in large quantities in the leather industry.
Blubber (which is from 8 to 20 ins. thick) is stripped from the whale as soon after capture as possible. Generally the dead whale is made fast alongside the whaler, a deep, spiral cut is made round its body, and the blubber is stripped off and hauled aboard. This is then cut into pieces, chopped up in mincing machines and fed into melting pans and heated with steam, often under pressure. The oil gradually exudes and collects upon the water, the cell membranes, etc.—the greaves—settling to the bottom. At the conclusion of the boil, the oil is drawn off from above the aqueous (gluey) layer, and is clarified by straining through sieves or filters. The “greaves” is placed in hair or woollen bags and submitted to hydraulic pressure, by which means a further quantity of oil is obtained.
Fish oils, unless specially purified for medicinal purposes, are dark-coloured liquids, with a characteristic, unpleasant, fishy smell, due to the presence of small quantities of fishy decomposition products, for example trimethylamine.
When cooled, many samples of fish oil deposit solid masses of fish tallow (fish stearine).
Fish oils, and, to a less extent, the marine animal oils, e.g. whale, seal, porpoise, are drying oils like linseed oil, that is they possess to a very marked degree a capacity for absorbing oxygen from the air, and so become thickened and viscous. This thickening is generally induced by blowing air through the warm oil. Oils that have been thickened in this way are known as “blown” oils.
Blown fish oils are mixed with mineral oils for use as lubricants for heavy machinery. They have been used as vehicles for paints in place of linseed oil, but with somewhat disappointing results. They are used successfully in place of linseed oil in the manufacture of printers’ ink, and in making paints for painting smoke stacks. Such paints resist successfully the action of heat and light.
More particularly, they are used in the leather industry. Fish oils are used chiefly in the manufacture of chamois leather. Ordinary chamois or wash-leather is made from the flesh-splits of sheep skins. The skin is well washed and softened, and freed from hair by treatment with lime. It is then split, and the loose and fatty middle layer removed by a sharp knife. The lime is removed by a short bran-drench and the superfluous moisture is pressed out. The skin is thus rendered porous and easily able to absorb the oil. It is stretched on a table and oiled with fish or whale oil. The oiled skin is folded up and worked for two or three hours in the faller stocks and then shaken out and hung up for a short time to cool and partially dry. The process is repeated a number of times, until all the water originally present in the skin has been replaced by oil. The oiled skins are then piled in a warm place. The oil gradually oxidizes—probably owing to some fermentation process—and the skins become yellow and very hot. From time to time the skins are strewn on the floor to cool and then re-piled, the process being repeated until the oxidation of the oil is complete. In France the freshly-oiled skins are hung in hot stoves, and the oxidation of the oil is completed in one operation.
The skins are then dipped in water and passed through hydraulic presses, by which the surplus oil is removed. This surplus thick, oxidized oil is known as “degras” or “moellon,” and is used for stuffing leathers that have already been tanned. Stuffed leathers are supple and impervious to water, and are used for harness, belting, etc. A further quantity of oil may be removed from the “chamoised” leather by treating it with potash or carbonate of soda, “sod” oil being recovered from the extract by neutralization with sulphuric acid. The value of sod oil for oiling dressed leather is due to a resinous acid of unknown composition, that is soluble in alkali but insoluble in petroleum ether.
Enamel or patent leather is generally coated, after tanning, with a linseed oil varnish, boiled with prussian blue, and dried in a steam heated chest at 70° to 80°C., the process being repeated until a sufficiently thick coat is produced. Fish oils are now used successfully in place of linseed oil. The enamel leather produced, although not quite so glossy as that made with linseed oil, is said to be more pliable.