Glass is a brittle, transparent material formed by the rapid cooling of certain molten liquids so that they fail to crystallize but retain an amorphous structure. Glasses are in fact supercooled liquids which, however, have such high viscosity that they behave like solids for all practical purposes. Some glasses may spontaneously crystallize or devitrify. Few materials form glasses, and almost all that are found naturally or used commercially are based on silica and the silicates.
Natural glass is formed by rapid cooling of magma, producing chiefly obsidian, or rarely by complete thermal metamorphism (see also tektite). The earliest known manufactured glass was made in Mesopotamia in the 3rd millennium BC. Glass was shaped by molding or core-dipping, until the invention of glass-blowing by Syrian craftsmen in the 1st century BC. Essentially still used, the process involved gathering molten glass on the end of a pipe, blowing to form a bubble, and shaping the vessel by further blowing, swinging, or rolling it on a surface. They also blew glass inside a shaped mold; this is now the chief process used in mechanized automatic glassblowing.
Modern glass products are very diverse, including windows, bottles and other vessels, optical devices, building materials, fiberglass products, etc. Most are made of soda-lime glass. Although silica itself can form a glass, it is too viscous and its melting point is too high for most purposes. Adding soda lowers the melting point, but the resultant sodium silicate is water-soluble (see water glass), so lime is added as a stabilizer, together with other metal oxides as needed for decolorizing, etc. The usual proportions are 70% SiO2, 15% Na2O, 10% CaO.
Crown glass, used in optical systems for its low dispersion, is of this type, with barium oxide (BaO) often replacing the lime. Flint glass, or crystal, is a brilliant clear glass with high optical dispersion, used in high-quality glassware and to make lenses and prisms. It was originally made from crushed flints to give pure, colorless silica; later, sand was used, with increasing amounts of lead (II) oxide. Borosilicate glass is used in applications where high thermal stresses must be withstood. Pyrex is a borosilicate glass used in chemical and industrial apparatus and in ovenware. It is inert, a good insulator, and heat-resistant (having a low coefficient of expansion and a high softening temperature).
The manufacture of the various kinds of glass begins by mixing the raw materials – sand, limestone, sodium nitrate or carbonate, etc. – and melting them in large crucibles in a furnace. The molten glass, having been refined (free from bubbles) by standing, is formed to the shape required and then annealed. Some safety glass is not annealed, but rapidly cooled to induce superficial compressive stresses which yield greater strength. Plate glass is made by passing a continuous sheet of soft glass between rollers, grinding and polishing it on both sides, and cutting it up so as to eliminate flaws. A newer method (the float glass process) involves pouring the molten glass into molten metal, such as tin, and to allow it to cool slowly; the surface touching the metal is perfectly flat and needs no polishing. Special glass products include foam glass, made by sintering a mixture of glass and an agent that gives off a gas on heating, used for insulation; photosensitive glass, which darkens reversibly in bright light; and fiberglass.
Glass in the form of filaments is widely used for heat insulation (as glass wool), for fabrics, and with a plastic resin to make a construction and repair material called GRP (glass-reinforced plastic). GRP, a rigid composite material, is commonly referred to as fiberglass, and is a popular medium for car bodies, boats, aircraft parts, and containers. It resists heat, corrosion, rot, and most chemicals and can be, weight for weight, stronger than steel.
Short lengths of fiberglass are made by blasting air through molten glass. Most forms of fiberglass are made by forcing molten glass through fine metal nozzles or spinnerets. The resulting continuous filaments are usually bundled together to form strands. These may then be chopped, twisted, or woven according to the product required.
Safety glass is a form of glass that is less hazardous than ordinary glass when broken. One form of safety glass consists of two sheets of 3mm (1/8in) plate glass bonded to a thinner central sheet of transparent plastic. If an impact breaks the glass, the plastic holds the fragments in place. Bullet-proof glass consists of several layers of glass and plastic. Wired glass has an embedded wire mesh to hold the fragments in place. Toughened glass is glass that has been treated to make it stronger than ordinary glass. When shattered, the glass forms small blunt fragments, which are much less dangerous than the sharp splinters from ordinary glass.
Stained glass is colored glass used for decorative effect in windows, most commonly in churches. The oldest surviving examples of complete stained glass windows are in Augsburg Cathedral, Germany, and date back from about 1100.
In its purest form, stained glass is made by adding metal oxide coloring agents during the manufacture of glass. Shapes cut from the resulting sheets are then arranged to form patterns or images. These shapes are joined and supported by flexible strips of lead that form dark, emphatic contours. Details are painted onto the glass surfaces in liquid enamel and fused on by heat. Other techniques include the application of colored surface coatings onto clear glass. Intricate designs may be scraped into the surface. In modern windows, the pieces of glass are sometimes joined using an adhesive instead of lead strips.