Kilnforming is the process of shaping glass in a kiln with heat and gravity. There are many methods and techniques that are related to kilnforming, including glass fusing, glass slumping, kilncasting, and printmaking.
There is a type of glass called “kiln-glass” that’s specifically manufactured for use in a kiln.  One of the properties of this glass is “compatibility”, which allows it to be fused to another disparate piece of glass in a kiln.
When glass is heated in kiln, it becomes soft and the consistency becomes sticky and almost honey-like.  As the temperature continues to increase, the glass becomes more liquid.  Gravity takes hold of the molten glass, the molten glass fills the space in which it’s contained, and fuses with any other pieces of compatible glass.
When the temperature inside the kiln is returned to room temperature (after an appropriate anneal soak phase), the glass will resume its crystalline structure in the shape it assumed at process temperature.