If a soldering iron were used to heat the key cup, it would concentrate the heat in a specific spot and would probably burn the lacquer before the heat transferred to the entire key cup. Even is the keys are unlacquered, it would be a very inefficient way to heat the key and melt the glue.
Many techs do use electricity however, in the form of a pad heating tool. Votaw sells a
Pad Cup Heater that is quite expensive. The way it works is that the metal part of the key completes the circuit much like the burner of an electric stove. The advantage is that it heats the key quickly without using a flame that can burn neighboring corks, felts, or pads.
Shown in the picture below are 3 methods I use to heat key cups to install and adjust pads.
-The "poor man's version" of the Pad Cup Heater is a Weller soldering gun with the tip cut off. I use it mostly on clarinet keys which are quite small. It only works on plated or unlacquered keys which will conduct the electricity. It is very important to release the trigger
before pulling the tips away from the key, or the electricity will "arc" and make a permanent mark. (Don't ask me how I know this.)
-The hot air gun is a Wagner digital model with several heat settings with a funnel attachment. It is effective, but still can burn corks, felts, pearls, and pads if the temperature is set too high or the airstream is held in one spot too long.
-The small Blazer Torch which runs on butane is the heat source I use most of the time on saxophones. The flame is adjustable and it is lightweight and easy to direct the heat to the area you want. Any tool you choose requires some practice and common sense to be safe and effective.