FISH GLUE AND ISINGLASS 525 



and the yield greater than in glues made from most other fish. Many species of 

 fish have thin skin and small bones, and therefore yield such small quantities of 

 glue that it is not economical to use them; examples of this type of fish are 

 herring, mackerel, and menhaden. Moreover, since these fish contain large quan- 

 tities of fat, a special procedure must be followed to remove it during the glue- 

 making process. Many fish which would otherwise be used are not caught in large 

 enough quantities in any one locality, and consequently the supply of fish waste at 

 any particular point is not sufiicient to justify the establishment of a glue factory. 

 Other fish are caught only for short seasons; as a result, the glue factories would 

 be idle most of the year. 



The ground-fish waste ordinarily is divided into three general classes: (1) fish 

 heads; (2) waste from salted fish (strictly speaking salt-fish trimmings and bones, 

 but usually considered trimmings and bones from salted cod, cusk, hake, pollock, 

 and haddock, and haddock and pollock skins); (3) skin stock (usually considered 

 only skin from salted cod and cusk) . The fish heads are fresh stock and are obtained 

 from the wharves where the ground fish are cleaned before salting or marketing. 

 With the exception of the exported salt fish most of the dried salted fish are 

 trimmed and skinned before being packed for shipment. Most of them are also 

 boned. These skins, trimmings, and bones constitute waste and skin-glue stock. 

 Cod and cusk skins are thicker and furnish larger amounts of a better grade of 

 glue than pollock and haddock skins; cod and cusk heads are also more valuable 

 as glue stock than pollock and haddock heads. Because of this the larger fish-glue 

 factories pay higher prices for cod and cusk skins and heads than for similar had- 

 dock and pollock stock; therefore, many of the larger fish salters keep these two 

 classes of glue stock separate. 



Methods of Manufacture 



Skin Glue. The glue stock, regardless of whether it is obtained from fresh 

 or salted fish, must be freed from salt (freshened) before being made into glue. 

 It is ordinarily agitated in running water until a chloride analysis of the wash water 

 indicates that practically all the salt has been washed out. In the larger factories 

 this is accomphshed by placing the stock in a circular tank about 30 feet in 

 diameter and 5 feet deep, covering it with water, and rolling it with a heavy 

 wooden truncated-cone-shaped roller, which revolves as it travels arotmd the tank. 

 These washers are efficient, for the roller squeezes out a portion of the water in the 

 stock as it rolls over it; the stock then takes up more water, which is squeezed out 

 on the next revolution of the roller. Unfortunately, the stock tends to collect at the 

 center of the washers and from time to time it must be pitched from the center 

 toward the outside. A small stream of water is run through the washers continually, 

 and at intervals all the water is withdrawn and the tanks are refilled with fresh 

 water. The fish skin and waste stock, being waste products of the salt-fish in- 

 dustry, contain a much greater percentage of salt than the fish heads; hence more 

 care must be used in freshening them than in freshening fish-head stock. 



Washing the stock requires at least 12 hours and often much longer. The usual 

 practice is to weigh the stock as it is received from the fish wharves and dump it 

 into the washers. After distributing a weighed quantity uniformly in a washer the 

 stock is covered with water and the agitator (roller) is started. Washing is usually 



