Glass

Transparent surfaces provides views, light and solar warmth. However, like the rest of a building envelope, they must protect against rain, cold, heat and noise. Few materials can satisfy these different demands at the same time. There have been many alternatives throughout history: shell, horn, parchment, alabaster, oiled textiles, gypsum (selenite) and thin sheets of marble. Eskimos have used the skin of intestines. In Siberia mica is cut into sheets for windows, known as Russian glass.

None of these seriously rival glass, and the only alternatives commonly in use are rice paper, used in Japan for letting light pass from room to room internally; and more recently, plastic alternatives, such as the polycarbonate sheet much used in greenhouses.

Ordinary clear glass lets about 85 to 90% of daylight pass through. There are many other types of glass on the market: diffuse, coloured, metal-coated, reinforced, heat reflecting, etc. Glass has also been developed to perform other functions, such as insulation blocks (foam glass) and as fibres in glass wool, the latter having a very large proportion of the insulation market nowadays.

HISTORY

The Phoenicians were probably the first to produce glass, about 7000 years ago.The oldest known piece of glass is a blue amulet from Egypt. Glass painting began in the eighteenth Pharaonic dynasty (1580-1350 BC), but it is difficult to say if glass windows were produced during this period.

A broken window measuring 70 by100 cm and 1.7 mmthick, opaqueand probably cast in a mould, was excavated from the ruins of Pompeii. It was originally mounted in a bronze frame in a public bath.

Plate glass technology spread slowly through Europe. Glass craftsmen kept their knowledge secret, and only the Church, with a few exceptions, was allowed to share the secrets. Early glass was blue-green or brown, partly because ferrous sand was used as a raw material. Later it was discovered that adding magnesium oxide (glassblowers' soap), neutralized the effect.

During the eighteenth century glass became affordable for use in all houses. It was still expensive, and far into the nineteenth century it was normal to put many small pieces together to make one larger pane. From 1840 methods of plate glass production improved and glass became cheaper.The methods of production were still basically manual - glass spheres were blown then divided.

In Belgium in 1907 the first glass was produced by machine. In 1959, float glass was developed, for the first time giving a completely homogeneous surface without irregularities.

Different proportions of raw materials can be used to make glass, but it usually consists of 15% soda ash (sodium carbonate), 15% limestone and dolomite and 70% silicon dioxide in the form of quartz sand. The formula for the process is:

Na2 CO3 + CaCO3 + SiO2 = Na2O x CaO x 6SiO2 + CO2 (6)

This glass, based on soda ash, is the most common. Replacing the soda ash with potash (K2CO3) gives a slightly harder glass. Lead glass is achieved by replacing limestone in the potash glass with lead (Pb).

For glass that needs high translucency for ultraviolet light an important constituent is phosphorous pentoxide (P2O5).

Fluorine compound agents decrease the viscosity and melting point of glass mixtures, which can reduce the use of energy. Antimony trioxide (Sb3O2) can be added to improve malleability, and arsenic trioxide (As2O3) acts as an oxidizing agent to remove air bubbles from the molten glass. Both are added in a proportion of about 1%. Stabilizers that increase the chemical resistance are often used: CaO, MgO, Al2O3, PbO, BaO, ZnO and TiO2.

Coloured glass contains substances that include metal oxides of tin, gold, iron, chrome, copper, cobalt, nickel and cadmium. Traditionally, coloured glass has been used for decoration. In modern coloured glass the colouring is very sparse and it can be difficult to differentiate from normal glass. Decorative qualities are here less important than the ability of the coloured glass sheet to absorb and/or reflect light and warmth. The aim is to reduce the overheating of rooms or, conversely, reduce heat loss. Products that achieve this are usually known as energy glass, and have a high energy saving potential.

There are two types: 'absorption glass', which is coloured or laminated with coloured film, and 'reflective glass', which has a metal or metallic oxide applied to it in the form of vapour. Early types of energy glass reduced the amount of light entering the building by up to 70%; these days, they are much more translucent, but the area of glass in a room may still need to be increased to achieve adequate levels of light.

Production of glass for windows

To produce good glass, good quality raw materials with no impurities must be used. The ingredients are ground to a fine powder, mixed and melted.

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