MISCELLANEOUS INDUSTRIAL PRODUCTS 251 



Food fish skins are seldom used as a source of leather because of their 

 small size and fragile characteristics. At one time a process was developed 

 for preparation of a leather from salmon skins \ but this product is not 

 produced at present. 



Glue and Isinglass. The formerly large domestic salt cod industry was 

 the basis for a substantial by-products industry for the manufacture of 

 fish glue and isinglass. With the disappearance of the salt cod industry 

 on the Pacific Coast and the greatly reduced operations on the Atlantic 

 Coast, the magnitude of this industry has greatly declined in the United 

 States. 



Glue is manufactured from bottom fish skins, and sometimes heads and 

 bones in the New England area, in a manner quite similar to that em- 

 ployed in the preparation of other animal gelatin glues. Where the raw 

 material comes from the salt cod industry, a considerable preliminary 

 freshening of the skins is required to remove salt. This may be accom- 

 plished by a thorough agitation of the skins in large tanks of water for a 

 period of about 12 hours or longer. This may be followed by maceration 

 with 0.2 per cent caustic alkali or saturated lime followed by an extended 

 treatment with 0.2 per cent hydrochloric acid for removal of alkali and a 

 final washing in running waters 



The processing to make glue from fish skins consists essentially of a 

 cooking of the freshened skins with dilute acid in steam-jacketed cookers 

 in two stages for five to ten hours each, with glue being withdrawn after 

 each stage. The approximately five per cent glue stock then goes to the 

 evaporators (usually vacuum triple-effect type) where the moisture con- 

 tent is reduced until the desired viscosity is reached. Preservatives are 

 added generally before cooking to minimize bacterial decomposition. 

 Essential oils are added to provide a pleasing odor. 



Fish skin glue, when made from the best skins and under the most 

 careful conditions, can be used as photoengraving glue. Glues of less 

 desirable characteristics are prepared from bones and other waste from 

 the fish industry or from fish heads. 



The word 'isinglass" comes from the German word hausenblase mean- 

 ing sturgeon's bladder, which was the original raw material from which 

 the product was made. It is the purest form of fish gelatin which can be 

 prepared, and it is used as a clarifying (filtering) aid primarily in wine 

 manufacture. One ounce of isinglass will clarify 200 to 300 gallons of 

 wine''. The sounds or air bladders from bottom fish are used in the United 

 States for isinglass manufacture, those from hake being considered the 

 most choice. The sounds are first washed thoroughly and then air-dried. 

 They are then moistened with water and drawn into a ribbon which is 

 rolled on spools. A fairly complete description of manufacturing methods 

 for both fish glue and isinglass is given by Tressler and Lemon^. 



