FISH ISINGLASS AND GLUE. 13 



MANUFACTURE AND USES OF FISH GLUE. 



In the manufacture of fish glue the fish wastes are first washed 

 thoroughly with cold water to remove dirt and blood from the fresh 

 fish and salt from the salted fish. The washed material is allowed to 

 drain, the washings being discarded, and then is subjected to the 

 action of hot water or steam. 



In the older methods of preparing glue the crude material was 

 treated with water and the mixture boiled in open glue kettles for 

 several hours until the collagen had all been converted into gelatin 

 which dissolved in hot Avater. This method yields a fairly good 

 glue if the raw materials are clean and fresh, but because of the 

 lengthy time required for complete extraction the liquor obtained is 

 usually dark colored and contains in solution many other protein 

 substances than gelatin. Glue thus prepared is often a poor adhesive 

 and is malodorous. 



Newer methods of fish-glue manufacture involve heating the stock 

 with steam under pressure in an autoclave so that the extraction pro- 

 ceeds rapidl}^ and there is less time for decomposition of the fish 

 protein to occur. In some plants the stock is placed in tall iron cyl- 

 inders, steam-jacketed, and heated for several hours until the whole 

 mass is thoroughly digested. By a better method, the stock is placed 

 within the inner, perforated section of a double boiler. Steam enters 

 the inner vessel from the outer, and the whole is heated under pres- 

 sure. The glue liquor filters out of the inner vessel and may be 

 drawn off from the outer jacket continuously. Sometimes an alter- 

 nate action of steam and cold water on the stock is brought into play, 

 and this process repeated until the extract is too dilute to be profit- 

 ably worked up into glue. 



The digested fish wastes may be filter pressed and the residue 

 dried. The resulting product, containing 45 to 55 per cent protein 

 matter, and 1 to 2 per cent oil, is a. valuable by-product ; in fact, on 

 account of the demand for it, the scrap can be considered to be the 

 main product of the industry and the glue to be of only secondary 

 importance. At any rate, the manufacture of glue alone would not 

 pay. The better grades of scrap are used for poultry food under the 

 name " chum," while second grades are sold for fertilizer, for which 

 there is always a good market. 



The solutions running from the autoclaves or the filtrate from the 

 filter presses are run into vacuum condensers, since the excess moist- 

 ure in the glue liquor must be distilled off at as low a temperature 

 as possible in order to prevent unnecessary decomposition of the 

 dissolved gelatin. In general, vacuum evaporators consist of a 

 spherical or C3dindrical iron vessel, steam-jacketed and provided in- 

 ternally with steam coils immersed in the glue liquor. Sometimes, 



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