Since the Bronze Age, about 3000 BC, glass has been used for making various kinds of objects. It was first made from a mixture of silica (from sand), lime and an alkali such as soda or potash, and these remained the basic ingredients of glass unto the development of lead glass in the seventeen century. When heated, the mixture becomes soft and malleable, and can be formed by various techniques into a vast array of shapes and sizes. The homogenous mass thus formed by melting then cools to create glass, but in contrast to most materials formed in this way (metals, for instance), glass lacks the crystalline structure normally associated with solids, and instead retains the random molecular structure of a liquid. In effect, as glass cools, it progressively stiffens until rigid, but does so without setting up a network of interlocking crystals customarily associated with that process. This is why glass shatters so easily when dealt a sudden blow, why glass deteriorates over time (a process call devitrification), especially when exposed to moisture, and why glassware must be slowly annealed (reheated and uniformly cooled) after manufacture to release internal stresses by uneven cooling.

Another unusual feature of glass is the manner in which its viscosity changes as it turns from a cold, rigid solid into a hot, ductile liquid. Unlike metals which flow or ‘freeze’ at specific temperatures, glass progressively softens as the temperature rises, going through varying stages of malleability, until it flows like a thick syrup. Each stage of malleability allows the glass to be manipulated into various forms, by different techniques, and if suddenly cooled the object retains the shape achieved that that point. Glass is thus amenable to a greater number of heat-forming techniques than most other materials.

Chemically, glass is a generic term for a substance manufactured from an endless number of recipes. Although most glassware contains the same basic ingredients, the term glass does not refer to a specific chemical compound, since different types of glass have a different chemical composition and exhibit different chemical and physical properties. As a result, glass has the widest applications in modern times, both in industry and the home. While some types of glass are most suitable for window-panes or services of cut ‘crystal’, others are designed to be spun into fibres or formed into the heat shields of spacecraft.

Ancient Techniques

In antiquity, silica (silicon dioxide or SiO2) was the major constituent of glass, with soda added as a flux to facilitate the melting of the batch, and lime as a stabilizer making the glass less vulnerable to the adverse effects of water. To this primary mixture chemical compounds (metallic oxides) might be added to give the glass colour or remove it, or to make the glass opaque. Four major manufacturing methods (with many variations, both major and minor) were standard. These were rod- and core-forming; casting with open and closed moulds; free blowing; and blowing moulds and forms of various kinds. These techniques have been deduced from physical examination and scientific analysis of ancient pieces, analogy to contemporary practices, and modern attempts to reproduce glassware in the ancient manner. Despite such studies, however, a good deal remains to be learned about the actual workings of ancient glass factories and about specific glass-making methods. Tableware was reproduced in numerous centres and cultures over many centuries, and so it is unlielly that there were uniform procedures or a common development. Indeed, most recent research has revealed an unexpectedly rich variety of techniques, too, and practices in diverse times and places.

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The Late Bronze Age

Although the place and time of the discovery of glass is uncertain, the earliest known industry Bronze Age by the middle of the third millennium BC. This industry, first founded in western Asia, probably in the Mitannian or Hurrian region of Mesopotamia, was the natural outgrowth of experimentation with vitreous glazes, used to embellish pottery, tiles and other objects, or with faience. The earliest archaeologically datable objects are beads, seals, inlays and plaques. Vessels appeared later, probably by the close of the sixteenth century BC. Shortly afterwards, knowledge of the manufacture of glass spread quickly to northern Syria, Cyprus, Egypt, and the late Aegean, where the palace-dominated civilizations of the Late Bronze Age used glass as a semi-precious material.