The glass soaks in heat, typically about 960° F, to give the molecules a chance to settle down. The thicker the glass the longer it should anneal. Typical annealing times for 1/4" plate glass might be a few hours. Improperly annealed glass is more subject to fracture with subsequent temperature changes.

i think its "annealing"... if so, it is a process where both metal and glass are heated in order for them to change their/its properties.

annealing for steels

spherodizing, normalizing, full annealing, process annealing, and stress relief

Any brand will do, as long as it has a timer and decent heat control for annealing glass, and can put out enough heat to handle the metal clay.