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.