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.
APPLY HEAT INDIRECTLY
Heat never should be applied directly, as this results in burning, or scalding, the glue.
Some glue-melting appliances have been constructed in which steam is turned directly upon the glue mass. This is bad practice of the most harmful kind.
Steam never should come into direct contact with glue. The temperature of steam is always at least 212° F.—under pressure it is much higher—and consequently it cooks the glue and destroys the fibres. Live steam burns glue just as it burns your hand if turned directly upon it.
The destructive effect of live steam upon glue may not be noticed at once, but work on which overheated glue has been used will eventually pull apart on account of the destruction of the glue-fibres.