In Focus: Fact or Fiction?: Glass Is a (Supercooled) Liquid

It seems so. The term amorphous comes to my mind. Supercooled liquid is a term which seems to describe it very well. I seem to have heard glass called a liquid some distant time in the past. Maybe that classification was more popular 30 years ago.

In Mediaeval times panes of glass were made by the Crown glass process. A lump of molten glass was rolled, blown, expanded, flattened and spun into a disc before being cut into panes. The sheets were thicker towards the edge of the disc and were usually installed with the heavier side at the bottom. There is no evidence at all that glass 'runs'

Is glass a liquid?

A liquid has viscosity, a measure of its resistance to flow. Usually when a liquid is cooled to below its melting point, crystals form and it solidifies. Sometimes it can become super cooled and remain liquid below its melting point because crystallisation initiation does not take place. If the viscosity rises enough as it is cooled further, it may never crystallise. It will form thick syrup and eventually an amorphous solid. The molecules then have a disordered arrangement, but sufficient cohesion to maintain some rigidity. In this state it is often called an amorphous solid or glass.

Some people claim that glass is actually a super cooled liquid because there is no first order phase transition as it cools. In fact, there is a second order transition between the super cooled liquid state and the glass state, so a distinction can still be drawn.

The situation at the level of molecular physics can be summarised by saying that there are three main types of molecular arrangement:

crystalline solids: molecules are ordered in a regular lattice

fluids: molecules are disordered and are not rigidly bound.

glasses: molecules are disordered but are rigidly bound.

From a common sense point of view, glass should be considered a solid since it is rigid according to everyday experience.

The hardness of glass by itself seems to support the idea it is a solid, too. I guess that those medieval windows should be thickest at the bottom corners.