FISH GLUE AND ISINGLASS 531 



determination is made on each batch of glue manufactured. The hquid glues are 

 then graded according to the percentage of chlorides they contain. The best fish 

 glues should not contain more than 0.02 per cent of chlorine as chloride. 



Drying Properties. The relative rate of drying may be judged fairly by com- 

 paring the drying qualities of a sample of liquid glue when spread on a glass 

 plate with those of a standard sample. The time required for a layer 0.2 cm in 

 depth to dry, at a given humidity and temperature (e.g., 77° F [25° C]) and 70 

 per cent humidity, is noted and compared with the standard. The best grades of 

 liquid glues dry to transparent, glossy films which are strong and brittle. If the 

 dried film is easily powdered, the glue is of little value for most purposes. 



Hygroscopic Properties. If the completely dried film of glue is placed in a 

 constant-temperature, constant-humidity room, or in a humidor over a sulfuric 

 acid solution, it absorbs some moisture. The gain in weight is an index of its 

 hygroscopic properties. The absorption of moisture softens the glue film to a 

 greater or less degree. If the dried films are placed over 80 per cent sulfuric acid 

 at 77° F (25° C), those which become sticky and of the consistency of molasses 

 are very hygroscopic and are of little value for joining work; if the film becomes 

 flexible, the glue possesses slight hygroscopic properties. 



Joint Strength. In the manner in which strength tests are ordinarily carried out 

 on glues, they are of little value in grading the glues. Even accurate experimenters 

 are unable to check their results closely. The weather conditions— temperature and 

 humidity— have an important influence on the strength of all glues (hide, bone, 

 fish, etc.). To obtain uniform results testing of the strength of glues must be 

 carried out in constant-temperature, constant-humidity rooms. Fair results are 

 obtained by drying the joints in a humidor at 77° F (25° C) over 90 per cent 

 sulfuric acid. Three different strength tests are used in determining the joint 

 strength of glues: shearing, tensile, and breaking. Of these only the shearing 

 strength test has been generally recognized as a standard method. The procedure 

 recommended by the U. S. Forest Products Laboratory is as follows: 



Well-seasoned hard maple wood is sawed into two pieces, 12 by 2.5 by 1 inches. 

 The contact surfaces should be planed just before the glue is applied. The glue 

 is spread on both of the contact surfaces; then the blocks are allowed to stand to 

 permit the glue to soak in for at least 15 hours, after which they are put in a 

 rack and allowed to stand from 6 to 9 additional days. Each of the joints is 

 sawed into four smaller joints. The blocks are broken apart by bringing a shearing 

 strain to bear on the layer of glue between the two wooden halves of the joint in 

 an Olsen Universal Testing Machine. 



This method, when carried out under uniform conditions of temperature and 

 humidity, gives admirable results with the weaker glues. As the stronger glues 

 pull a larger or smaller area of the wood out, the test may not indicate the full 

 shearing strength. For such glues the tensile- and breaking-strength tests give 

 more uniform results. 



Color. The color of liquid glues is most easily determined by clarifying the 

 liquid glue with egg albumen and then matching the color with the glasses in a 

 Lovibond Tintometer. Very light-colored glues are required for use on light 

 woods. Darker glues may be used on dark woods without danger of making a 

 conspicuous joint. Light-colored glues have a more pleasing appearance and 

 therefore command a higher price. 



