MELTING, OR DISSOLVING, GLUE
After the glue has been soaked in the manner described, the most important part of the process is undertaken—that is, the melting of the glue by application of heat.
The words “most important” are used advisedly. It is safe to say that most of the damage done to glue occurs in the melting process. There are all kinds of ways of melting glue, but many of them absolutely ruin glue for practical work. As this is a very important subject, it is well to get the rudiments thoroughly in mind—and for this purpose the reader should remember what has been said about the nature of glue—that it is made from animal matter; and that it is composed of innumerable small fibres on whose strength the holding power of the glue depends.
Whatever injures and breaks down these fibres inevitably weakens the glue; so that in melting, every care must be observed to avoid the breaking down of the glue-fibres.
The most common destructive agent is heat. Just as the application of heat breaks down the fibres of a roast of beef, rendering it “tender” as the saying is, so the prolonged application of heat, or heat of too great an intensity, will destroy the glue-fibres, and therefore radically impair their value for actual use.
So, it is absolutely necessary to employ no more heat in melting glue than is required to reduce the soaked mass to the proper working consistency.
DO NOT HEAT GLUE HIGHER THAN 150° F.
By actual experience it has been determined that a temperature of 130° to 150° F. is all that is required to melt the glue to the requisite consistency; any greater heat is actually harmful, as it assists just so much more in the process of disintegration.
The term “boiling,” or “cooking,” never should be applied to the process of glue melting. These words imply a temperature of 212° F.—and such a temperature is ruinous to glue. In producing glue from the original stock—from the hides, bones, sinews, etc.,—boiling is necessary, in order to extract the gelatinous matter—but as we have already seen, the longer the stock is boiled, the weaker the product. “First boilings” are always best. In preparing for use, however, boiling is not necessary; therefore, never heat glue above 150° F.