528 MARINE PRODUCTS OF COMMERCE 



Since fish-head glues are widely used for joining wood, sujfficient zinc oxide is 

 usually incorporated to make them nearly opaque. 



Yield of Glue from Stock 



The yield of liquid glue obtained from cod and cusk skin is usually between 60 

 and 80 gallons per ton of stock. Hake, haddock, and pollock skins yield much less 

 glue; the usual amount obtained is from 35 to 45 gallons per ton. Trimmings 

 and bones (waste) from salted ground fish yield about 25 gallons of liquid glue 

 per ton although as much as 30 gallons are occasionally obtained. The best yields 

 of liquid glue obtained from fish heads are 15 to 18 gallons per ton of stock al- 

 though the yield often falls as low as 12 gallons per ton. 



Composition and Properties of Fish Glue 



Composition 



Fish glue of the usual viscosity contains from 50 to 55 per cent of dry glue and 

 weighs from 9.5 to 10 pounds per gallon. In addition to glue it contains about 1 

 per cent preservatives, 0.2 to 0.4 per cent essential oils, and usually a small propor- 

 tion of zinc oxide or some other white pigment; from 0.5 to 1.0 per cent of ethyl 

 or methyl alcohol is often found. 



Comparatively few analyses of the dry matter of fish glue have been made. 

 Dry-fish skin and fish-waste glues have about 1 per cent ash; the amount of ash 

 in fish-head glues varies, depending on the methods of manufacture and the 

 amount of pigment or other inorganic materials added during manufacture. A 

 sample of the ash of fish-skin glue analyzed by Tressler (1921) had the following 

 composition: 



99.2 



Bogue (1920) carried out the most detailed exact analyses of the dry matter of 

 fish glue of any that have been published, but unfortunately he does not state 

 which kind of fish glue he analyzed. His results are given in the table on page 

 259. 



Fish glue owes its property of complete solubility in water at ordinary tempera- 

 tures (e.g., 68° F [20° C]) to its low gelatin content, as is shown by its relatively 

 low percentage of protein nitrogen and its high proteose and peptone nitrogen 

 content; the latter two are completely soluble in cold water. 



