Glass Facts

The word glass is thought to derive from the Old German word glasa used to designate “shining” or “shimmering” objects. Glass is a solid, largely non-crystalline, brittle, inorganic material that has no defined melting temperature. Glass possesses no more than a minimal conductive capacity for electrical energy and heat. When heated, glass can be cast, blown, pressed, drawn and rolled into various shapes.

Ingredients:
Glass is comprised of so-called glass formers, flux agents and stabilizers. Glass forming materials solidify without crystallization when they are cooled from a molten state, i.e. they effectively remain in an amorphous state. The most important glass formers include silica (also referred to as silicon dioxide), boric oxide and phosphorous pentoxide. Flux materials have the effect of lowering the temperature at which glass will melt (e.g. to temperatures of 1,500° C or lower). The most important flux materials include carbonates, nitrates and sulphates derived from alkaline metals. Stabilizers are used to chemically increase the viscosity of (or stiffen) glass. The most commonly used stabilizers include alkaline earth metals such as lead and zinc, usually in the form of carbonates or oxides.

Of special significance when it comes to the processing and shaping of glass are the properties strength, surface tension and inclination to crystallize. These properties are largely determined by the type and amounts of the melted raw materials, including: quartz sand, soda, sodium sulphate, limestone, dolomite, feldspar, potash, borax, potassium nitrate, alkaline stone, lead oxide, barite, zinc oxide, arsenic and sodium chloride.

Manufacturing:
Smaller amounts of raw materials are melted in crucibles or pots, larger amounts in pot furnaces or in continuous duty tank furnaces. In modern glassworks, the raw materials are automatically weighed, mixed and conveyed to the respective heating units. The process that takes place in the furnace at temperatures of more than 1,400 °C consists of several reaction phases. The product at the end of the preliminary melting phase is an inhomogeneous, strongly streaked and bubbly molten mass.

The preliminary melting is then followed a stage in which the molten mass is brought to yet higher temperatures so as to release the gas bubbles. When the molten glass is then cooled to a temperature of around 1,200° C, its viscosity or stiffness increases to a point at which it is amenable to processing or shaping. Flat or sheet glass is produced by a controlled drawing out of the molten glass. Mirror glass is produced by pouring and rolling the molten glass and then grading and polishing it.

Optical glass for the production of lenses and mirrors is required to be completely homogenous, i.e. free of flaws, bubbles, streaks and tension. Borosilicate glass and quartz glass are used in the manufacturing laboratory instruments and heat-resistant commercial products. This glass distinguishes itself in terms of its high chemical and thermal resistance.

More information on the subject of glass and glass production is available at:
http://en.wikipedia.org/wiki/Glass
http://en.wikipedia.org/wiki/Category:Glass