Can glass change shape? Seems impossible that glass could move at all as it is so inflexible. What if we were to introduce heat? What do you think will happen now? The amount of heat is a huge factor in the way glass behaves. Here is some insight on how glass can behave while exposed to heat and, how much heat creates those changes.
First rule of glass - it wants to be 6mm or 1/4" by the rules of nature. What does this mean? It means that if you have a piece of glass that is an 1/8" or 3mm, and you heat it up until it is almost molten, it will shrink until it becomes 6mm thick. If that piece of glass was square in shape, it would most likely have a 'dog-bone' shape when it got to room temperature. The dog-boning is where the sides pull in towards the center creating a dog-bone shape.
What happens if the glass is greater than 6mm thick? Well....while the thinner piece of glass pulls in, the thicker glass will spread out until it reaches the 6mm thickness.
If a piece of glass is 6mm at the start of fusing, it will mostly maintain it's shape and thickness while going through fusing temperatures and back to room temperature.
Most of the pieces I create are many layers of glass. I have to look at the thickness and decide how I want that piece to behave while heated so I get the result I'm looking for.
What temperature does glass start changing or moving? That all depends on the desired result. Typically, and this can slightly vary based on your kiln, glass will start to soften and move at approximately 1200 deg F. If you have a piece that you want to make into a shaped plate for example, you will go to this temperature, usually in a mold. This is called slumping.
In order to get two or more pieces of glass to fuse into a solid piece, you expose it to 1480 deg F. This will cause the glass to melt together. This is also the stage where you need to be concerned with the thickness so you don't get surprised when you open the kiln. This is call full fuse.
There are many other stages the glass goes through depending on the desired outcome and it is a learning curve for the artist to understand how glass and their kiln behaves together.
So all said, we now know that glass is a shape shifter and that only imagination and the control of heat are our limitations!