Glass can be classified as a ceramic. It is an amorphous solid; it lacks the long-range order which characterises crystalline materials like metals, so there is no regularity in the arrangement of its molecular constituents on a scale larger than a few times the size of these groups (Doremus 1994: 1). This means that its constituents (largely oxides) are only arranged in a regular way in small areas, as revealed by early work by Zachariasen (1932). A solid is a rigid material and does not flow when it is subjected to moderate forces. Although Jones (1956: 1) defines glass as 'an inorganic product of fusion which has been cooled to a rigid condition without crystallisation, Doremus (1994: 1) states that glasses can be prepared without cooling from the liquid state. Nevertheless ancient glasses all fall into the definition proposed by Jones, and to avoid crystallisation of the glass would have been very important to ancient glass-workers, just as it is today. Glass is also a supercooled liquid. This means that when sodalime-silica glass is made it is cooled from temperatures above 1100 °C to a temperature at which the rate of cooling must be controlled carefully (the transition temperature, Tg). If the melt is allowed to cool too slowly a silicate can be produced which consists of a series of crystals (following the cooling curve AB-C-D in Figure 3.1); by cooling the glass at an appropriate rate this can be avoided, and a glass which contains no crystals is formed (cooling curve A-B-E-Gl in Figure 3.1). It can be seen in Figure
3.1 that the rate at which the temperature falls between Tg and T is especially critical and is reflected in the volume of silicate material produced.