FISH PRODUCTS 129 



the skins of non-oily, demersal fish, for example cod, 

 haddock, soles, plaice and hake. 



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 The cooked waste is then withdrawn, and any remaining 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. 



