Grade III is prepared from fish heads, and is sold in large cans and barrels for sizing, box making, cabinet making, and general joiner work.
The glue is sometimes made more flexible by the addition of glycerine and glucose. The flexibility of fish glue makes it useful for the manufacture of court plaster, labels, stamps, and in book-binding.
The residue from the press is dried and sold as chicken feed or fertilizer. For the latter purpose it is frequently mixed with Carnallite.
Fish Gelatine. Fish gelatine or isinglass is obtained from the swimming bladder of the sturgeon and also of the cod. The bladders are exported, either opened (pipe isinglass) or washed, split open and dried (purse, lump or leaf isinglass).
Isinglass is the purified and dried inner skin of the bladder. It has but feeble adhesive power. It is used for clarifying wines, ciders and beers, and for making jellies and plasters.
Fertilizers. In many places near the sea, fish are employed whole as manure. Sprats particularly are caught in large numbers and distributed over the fields, and left to decompose. Fresh sprats contain 63·7 per cent of water, 1·94 per cent nitrogen, 2·1 per cent ash (0·43 potash and 0·90 phosphoric acid).
Fish guano or fish manure is generally prepared from the fish waste discarded by the curer. An average sample of this manufactured fish manure will contain 12 per cent water, 60 per cent organic matter, yielding 10 per cent ammonia, 16 per cent of calcium phosphate, and a residue of salt, sand, magnesia and potash, the amount of potash being inconsiderable. Fish guano is mainly valuable as a source of ammonia, the ammonia content ranging from 6 to 11 per cent, according to the kind of fish used and its previous history, e.g. whether fresh or salted.
In many places, such as London, the fish offal from the shops and restaurants is collected, dried and ground up for use as manure. In Germany in 1918 herrings’ heads were removed by the curers to be utilized for the production of oil, albumen, and phosphate of lime. The herring meal contained up to 50 per cent of albumen and calcium phosphate, the latter being obtained from the bones and heads. The albumen was extracted chemically and prepared for human consumption. The oil was extracted with benzol or other solvents, and, after hardening, was used in the manufacture of butter substitutes. Fish waste or offal is fed into a continuous cooker. This cooker consists essentially of a long, cylindrical vessel, through which runs a hollow steel shaft on which are mounted perforated radial vanes in such a way that the whole arrangement forms a spiral conveyor. By means of the hollow shaft and vanes, steam is blown into the mass of fish waste as it travels slowly through the vessel, so that it is completely cooked and disintegrated by the time that it emerges at the other end.
The cooked mass is then fed into a press in which a screw conveyor urges it through a gradually tapering cylinder with perforated sides. In this way the oil is extracted from it, and it is then dried and disintegrated by a rotary drier.