In Mediaeval times panes of glass were often made by the Crown glass process. A lump of molten glass was rolled, blown, expanded, flattened and finally 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. Other techniques of forming glass panes have been used but it is only the relatively recent float glass processes which have produced good quality flat sheets of glass. There is still much about the molecular physics and thermodynamics of glass which is not well understood, but we can give a general account of what is thought to be the case.
Many solids have a crystalline structure on microscopic scales.
The molecules are arranged in a regular lattice. As the solid is heated the molecules vibrate about their position in the lattice until, at the melting point, the crystal breaks down and the molecules start to flow. There is a sharp distinction between the solid and the liquid state which are separated by a first order phase transition , i.e. a discontinuous change in the properties of the material such as density. Freezing is marked by a release of heat known as the heat of fusion . A liquid has viscosity , a measure of its resistance to flow.