THE GLUING OF WOOD 11 
gluing. For these same two classes of work the ratio of water to 
dry glue for a glue grading No. 6 is about iy 2 to 1 and 2 to 1, 
respectively. 
As the water is added to the dry animal glue the mixture should 
be 3tirred thoroughly. The mixture should then stand in a cool place 
until the glue is completely water-soaked. The softened mixture 
should then be melted. Shredded glue will soften in a few minutes ; 
ground glue in an hour or less; and caked glue will sometimes take 
several hours, according to the size of the cakes. Of the many 
types of melting pots in use, those in which the glue flows away as 
soon as melted or in which the temperature of the mixture can be 
accurately controlled are preferable. 
The temperature at which animal glue is kept liquid has much to 
do with its quality. A temperature of 140° F. is the most desirable. 
It is not usually practicable, however, to maintain exactly this tem- 
perature when commercial glue pots are used. In any event a tem- 
perature range between 140° and 150° should not be exceeded. At 
temperatures lower than 140°, decomposition of the glue caused by 
bacteria or other microorganisms may set in, and at temperatures 
higher than 140°. the deterioration of the glue from chemical action 
is hastened. Numerous tests have proved that both high tempera- 
tures and long-continued heating, even at temperatures lower than 
140°, reduce the strength of animal glue. 13 
RECOMMENDED PROCEDURE FOR FACTORY OPERATIONS 
In factories where the same grade of glue is used in various parts 
of the plant it is good practice to mix and melt the glue at a central 
heater. (PL 1, A.) The melted glue can then be distributed in 
small lots over the plant or run off into pans and allowed to cool to a 
jelly. In the jelly form glue does not deteriorate quickly if kept at 
temperatures ranging from 40° to 50° F., and at these temperatures 
it may be kept throughout the day. In the jelly form, glue can be 
distributed in the quantities desired and can be remelted in a bench 
glue pot at the location where it is needed. If the custom of the 
plant is to keep the stock of melted glue warm, only a few hours' sup- 
ply should be melted at a time. The glue pots, pans, and central 
heater should be thoroughly cleaned each day. Glues remaining 
13 At the Forest Products Laboratory solutions of a high-grade joint glue and a veneer- 
grade glue were heated for 48 hours at 104°, 140°, and 176° F., and samples were tested 
every few hours for strength and viscosity. The deterioration due to heating at all three 
temperatures was evident in both viscosity and joint-strength tests. In the case of the 
joint glue a marked decrease occurred in both viscosity and joint strength and at about 
the same rate. With the veneer glue the loss in joint strength appeared more pronounced 
than the loss in viscosity. The average shearing strength of joints glued with the high- 
grade glue decreased about 20 per cent on heating the glue solution for 7 hours at 176°. 
Under exactly the same conditions the average joint strength of the veneer-grade glue 
decreased about 35 per cent. The greatest loss in strength and viscosity occurred at 176°. 
In the solutions kept at 104° there was a sudden drop at the end of 31 hours of heating 
in the strength of joints made with the high-grade glue, owing possibly to a combination 
of microorganic and chemical action. The veneer-glue joints showed a pronounced but 
more gradual decrease at that temperature. The most favorable of the three temperatures 
tried was 140°, but even at that temperature an appreciable weakening in strength of both 
glues was noted at the end of 7 hours, and a greater loss in strength after longer heating. 
Bogue (6), after heating a 1 to 2% mixture at 176°. found that "the value of the glue 
dropped approximately one grade for each two hours of heating, or from a very high to a 
very low hide (animal) grade in 12 hours. In actual figures the loss in (joint) strength 
averaged about 85 pounds per square inch per hour, or about 1,000 pounds per square 
inch in the 12 hours." Bogue's results expressed differently give a loss in 12 hours of 
33 per cent of the original strength. Linder and Frost (32) found a decrease in strength 
of from about 30 to 45 per cent on heating a glue solution for 20 hours at 150°. 
