532 MARINE PRODUCTS OF COMMERCE 



Permanence. Another point of first importance in the consideration of the value 

 of any liquid glue is its permanence. Not only must it retain its adhesive power 

 indefinitely, but it must retain its original appearance (i.e., no coagulation, pre- 

 cipitation, or discoloration should appear after storage for long periods of time. 

 The glue must, therefore, be preserved to prevent bacterial and mold growth, and 

 it must not contain any suspended matter that will settle out on standing. The glue 

 should be neutral to litmus, but slightly acid to phenolphthalein. One of the best 

 tests that may be applied to a liquid glue to determine its permanence is storage 

 at a slightly elevated temperature (98.6° F [37° C]). If liquid glue remains free 

 from bacterial decomposition and other changes during a month's storage at this 

 temperature, it is likely that it will keep indefinitely at ordinary room temperatures, 

 provided it is properly preserved to prevent mold growth. 



Interpretation of Analytical Results 



If a liquid glue is to be used for joining work, the properties of prime importance 

 are permanence, adhesiveness or strength, and hygroscopicity. If the glue is lack- 

 ing in any one of these, it is of no value for use in joining wood. Other properties 

 such as gel point, viscosity, color, and odor are of minor importance in most 

 cases. Where the glue is to be used for special work, such as joining glass, other 

 properties, for instance color and elasticity, become of importance. 



In buying glue, viscosity, moisture content, and speed of set should be taken 

 into consideration, for these properties determine the relative cost of liquid 

 glues. Obviously, more glue of a low viscosity and high moisture content must 

 be applied to joints than glue of high viscosity and low moisture content. If the 

 liquid glue sets too quickly after it is applied, it will be impossible to spread 

 uniformly; therefore, more glue must be applied in making the joint than would 

 be the case if a slower setting glue were used. 



By-products 



The residue left in the cookers after the final cooking and draining contains a 

 considerable quantity of glue liquor. This residue is placed in canvas bags in a 

 hydraulic press and pressed; this accomplishes the dual purpose of recovering the 

 glue hquor and of partially drying the chum. The recovered glue liquor is added 

 to the main bulk of the liquor and evaporated. The residue in the hydraulic press 

 is dried further, either by direct or indirect heat in a long, revolving cylindrical 

 drier, and is then sold either as feed or as a fertilizer. At one time a large propor- 

 tion of the dried chum was used for the preparation of fertilizer, but the demand 

 for use as a poultry feed grew rapidly and now nearly the entire production is 

 sold to poultry men. The dried chum is an excellent poultry feed as it contains 

 about 50 per cent protein in an easily digestible form; fish head and waste chum 

 contain a high percentage of calcium phosphate. The latter constituent supplies 

 calcium for the egg shell and phosphorus for the yolk. 



Chum produced from "skins" contains (water-free basis) on the average 12.2 

 per cent nitrogen, calcium and magnesium phosphates 25.5 per cent; from 

 "waste," 11.1 per cent nitrogen, calcium and magnesium phosphates 30.0 per cent; 

 and from "fish heads," 10.0 per cent nitrogen, calcium and magnesium phosphates 

 36.6 per cent. The average composition of the mixed chum prepared for chicken 

 feed is as follows: 



