According to our instructor:
There are 3 rules of glass. One: it is always a liquid. It is very slowly moving at room temperature. Two: it wants to be 6mm thick. Three: Gravity forces it straight down. So when it is melted, glass on top goes straight down, not puddling over the edges. When the glass on top goes down, the beneath is displaced. She also said that the stained glass windows in old European churches were getting thinner on top and that silica is one of the most readily available resources on the planet.
Art Is Good 