A glass is a substance that is non-crystalline yet almost completely undeformable. This state is called the vitreous state. Vitreous substances, when heated, will transform slowly through stages of decreasing viscosity. As a sample of glass is heated, it becomes more and more deformable, eventually reaching a point where it resembles a very viscous liquid. Ice, on the other hand, does not go through these changes as it is heated. It changes directly from a solid to a liquid. Ice, therefore, is not a vitreous substance. Glasses are only very slightly deformable. Glasses tend to bend and elongate under their own weight, especially when formed into rods, plates, or sheets. Glasses can be either organic or inorganic materials.

Many definitions of glass include the idea that they do not crystallize as they solidify. Since the definition of solidification is the act of crystallization, this idea of glass as...