INDUSTRIAL ESPIONAGE

industrial glass solid material that is normally lustrous and transparent in appearance and that shows great durability under exposure to the natural elements. These three propertieslustre, transparency, and durabilitymake glass a favoured material for such household objects as windowpanes, bottles, and lightbulbs. However, neither any of these properties alone nor all of them together are sufficient or even necessary for a complete description of glass. Defined according to modern scientific beliefs, glass is a solid material that has the atomic structure of a liquid. Stated more elaborately, following a definition given in 1932 by the physicist W.H. Zachariasen, glass is an extended, three-dimensional network of atoms that form a solid which lacks the long-range periodicity (or repeated, orderly arrangement) typical of crystalline materials. Normally, glass is formed upon the cooling of a molten liquid in such a manner that the ordering of atoms into a crystalline formation is prevented. Instead of the abrupt change in structure that takes place in a crystalline material such as metal as it is cooled below its melting point, in the cooling of a glass-forming liquid there is a continuous stiffening of the fluid until the atoms are virtually frozen into a more or less random arrangement similar to the arrangement that they had in the fluid state. Conversely, upon application of heat to solid glass, there is a gradual softening of the structure until it reaches the fluid state. This monotonically changing property, known as viscosity, enables glass products to be made in a continuous fashion, with raw materials melted to a homogeneous liquid, delivered as a viscous mass to a forming machine to make a specific product, and then cooled to a hard and rigid condition. This article describes the composition and properties of glass and its formation from molten liquids. It also describes industrial glassmaking and glass-forming processes and reviews the history of glassmaking since ancient times. In doing so, the article focuses on the composition and properties of oxide glasses, which make up the bulk of commercial glass tonnage, and on conventional thermal-fusion, or melt-glass, methods of glassmaking. However, attention also is given to other inorganic glasses and to less conventional production processes. For a detailed treatment of the physics of the glassy state, see the article amorphous solid. For a full-length treatment of the various artistic uses of glass, see stained glass and glassware.