The waste is washed in running water to free it from salt. Sometimes the waste—particularly the heads—is decomposed with hydrochloric acid and afterwards neutralized with lime. It is then charged into a cooker provided with a perforated, false bottom. The stock is covered with water and heated with steam. The glue is extracted and gradually concentrates in the water. When this glue liquor is sufficiently concentrated (from 5 to 6 per cent), it is run off (the first run) and more water is added to the waste and the cooking continued. After about 10 hours cooking, nearly all the glue has been extracted and the liquor is again run off (the second run). The cooked waste is then withdrawn, and any remaining glue liquor is pressed out of it and added to the second run. From 2 to 4 per cent of phenol or boric acid are added to prevent decomposition by bacteria. The glue liquor is evaporated down to a concentration of 32 per cent in open vats or closed evaporators, and is bleached with sulphurous acid. A small amount of some essential oil, e.g. cassia, clove, wintergreen, is added to check mould growth and mask the fishy odour. Glue is also made in a similar way from the “greaves” obtained from whale blubber. Fish glue is manufactured in three grades. GRADE I is made from skins, only the first run being used. It is used for photo-engraving work, for the production of half-tone plates. GRADE II is made from second run skin liquors and fish waste. It is sold in small cans and bottles for general repair work. 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).