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.