526 MARINE PRODUCTS OF COMMERCE 



started during the afternoon and is continued until the following morning, when 

 the percentage of chlorides in the wash water is determined; if the analysis in- 

 dicates that little salt remains in the stock, the washing is discontinued and the 

 water drained oflF. 



The heavy roller disintegrates a portion of the stock, which passes through 

 the screen with the wash water. To prevent this loss in some of the larger fac- 

 tories the wash water is run into large settling tanks through which it slowly passes. 

 This permits the major portion of suspended matter to settle. The settlings are 

 usually added to the chum or residue from the cooking, which is then pressed, 

 dried and sold as poultry food. 



The freshened skin stock is pitched into the cookers which are usually situated 

 adjacent to the washers. The usual type of cooker consists of a steam-jacketed, 

 false-bottomed, rectangular galvanized iron tank. These cookers are often about 

 12 feet long, 3 feet wide, and 5 feet deep. A layer of excelsior is thrown on the 

 false bottoms when the cookers are prepared for use, and the stock is thrown in. 

 A preservative is added at this stage to prevent decomposition of the glue liquors 

 during storage before evaporation. A small amount of acetic acid or other mod- 

 erately strong acid is added to the stock; this acid acts as a catalytic agent and 

 hastens the hydrolysis of the stock into glue. Moreover, it reduces the slime in the 

 chum so that it may be more readily pressed and more easily dried. The stock is 

 covered with water; the steam is turned into the outer jacket and the cooking 

 is continued for 6 to 10 hours. Then the glue liquor is drawn oflF and pumped to 

 tanks and thence to the evaporators. The partially exhausted stock is again covered 

 with water and cooked for a somewhat shorter period. Usually the skin stock is 

 cooked only twice; the residue from the second cooking is commonly added to 

 the freshened waste stock. Thus no glue is lost. 



The glue liquor is usually filtered through a mat of excelsior as it passes to the 

 pump on its way to the tanks situated above the evaporator; as the liquor flows into 

 the evaporator, it is often strained through muslin. These filtrations remove most 

 of the suspended material. 



Many types of evaporators are in commercial use. Vacuum evaporators are the 

 most eflBcient, but are not commonly used because of the great tendency of fish- 

 glue liquor to foam. Open pans heated with steam coils are used in many of the 

 smaller fish-glue factories. One popular type of evaporator consist of a large copper 

 coil, about 6 feet in diameter, which revolves in a shallow, copper-lined wooden 

 tank. Steam passing through the coil causes the glue liquor to evaporate rapidly 

 on the surface of the coil while it is carried out of the body of the liquor as the 

 coil revolves. 



Evaporation is continued until the glue reaches the proper viscosity, or "body," 

 as it is commonly termed. This stage is determined by comparing the viscosity of 

 a sample of the glue with that of a standard sample, either in viscosity tubes or in 

 a viscosimeter of the Stormer type, which is most satisfactory for the purpose. 

 When the tests indicate that the proper viscosity has been reached, the additional 

 preservatives and essential oils are added and the glue is drawn off ready to be 

 tubed, bottled, or canned. 



Photoengraving Glue. Only the very best grades of fish-skin glue are satisfactory 

 photoengraving glues. The procedure followed in their manufacture does not 

 differ from that outlined above for regular skin glue. Smaller amounts of essential 



