What is a kiln, and why does glass need to be fired?	back

A kiln is a kind of high temperature oven most often used to fire harden ceramics. In glass, it is used to remove the stress that occurs to pieces as they are being made, and to prevent stress from occurring as the piece cools (this process is known as 'annealing'). We have seen unfired pieces spontaneously crack, and even explode for no apparent reason. Properly annealed pieces on the other hand are surprisingly strong, and we have seen many of them bounce off of concrete (4 ft drop) again and again, with no detectable damage (not that you should try this with your glass, even fully annealed pieces can break on the concrete).