Oxide glasses and glass-ceramics comprise a host of materials of widely diverse compositions and properties. It is important to remember that although the properties of a glass or glass-ceramic are intrinsically a function of composition, other factors can often have important, sometimes overriding, effects on observed properties. Such factors include atmospheric weathering and thermal history. Exposure to weathering conditions obviously can drastically modify the surface chemistry behavior of these materials. Thermal history, on the other hand, can alter molecular structure and phase constitution. Therefore, chemical durability, density, refractive index, electrical resistivity, etc. can be measurably affected by the time-temperature relationship a glass has experienced on cooling from high temperatures. An important instance of the effect of thermal history is found on prolonged heating of certain glasses, e.g., borosilicates, above the annealing temperature, when degradation of the durability is observed, owing to phase separation into two or more glasses, one of which is highly soluble. Glass-ceramics represent an extreme case because the material, originally a glass, is deliberately heat treated to transform it into a new material whose polycrystalline structure gives rise to a totally different set of properties. Because of these dependencies, glass properties are listed for glasses in the annealed state, and the properties of glass-ceramics are given on the basis of the manufacturer's standard production process. In

(if it exists). In Table 6.1-2 properties have been listed primarily according to manufacturer. The materials selected for inclusion were chosen largely on the basis of their commercial importance.