The Discovery of Glass

Glass has always been present on Earth, formed whenever certain silicon-containing rocks melt due to phenomena such as volcanic eruptions, lightning strikes or the impact of meteorites, and then cool rapidly, solidifying in the process.

Archaeologists studying the neolithic period (c. 9000 BC) have demonstrated that cutting tools made of obsidian, a dark-coloured glass of volcanic origin, may have been used widely. However, the manufacture of glass objects, initially confined to glass beads, is not thought to have commenced until at least 4000 BC, with finds in Egypt and Eastern Mesopotamia tentatively dated to this period.

Glass vessels were first produced in about 1500 BC, when Egyptian craftsmen developed a method for producing glass pots by dipping a core mould of compacted sand into molten glass. As the glass cooled and solidified, it took the shape of the core which could easily be removed later. The next major technological breakthrough in glassmaking was the discovery of glassblowing during the reign of Augustus (27 BC - 14 AD), attributed to Syrian craftsmen working in the region of Sidon, near the site of Babylon.

For centuries, the pre-eminent glass producing area in the world was the island of Murano. Here, the high quality glass which formed the basis of the Venetian trading empire was guarded with such jealousy that the traditional punishment for a glassblower revealing the secrets of his trade was death.

The Manufacture of Glass

sand or silica (SiO₂)

sodium carbonate(Na₂CO₃)

limestone (CaCO₃)

magnesium carbonate (MgCO₃)

additives to improve the glass quality and to colour the glass

At high temperatures, carbon dioxide is expelled from the sodium carbonate, producing sodium oxide(Na₂O), a substance which reduces the minimum temperature at which the silica will fuse from 1700°C to about 800°C. By the same mechanism, calcium oxide (CaO) and magnesium oxide (MgO) are produced from their respective carbonates, and their presence causes the glass mixture to become insoluble in water.

Accordingly, these materials are crushed, mixed and heated for a long period of time at high temperature to ensure that all air bubbles, which might otherwise cause flaws in the final product, are eliminated. The mixture is then quickly cooled to room temperature.

There are four major families of glass, each a variation on the basic soda-lime glass and named after its additives.

1. Lead-alkali glass (also called lead glass)

lead oxide (PbO) is used in place of calcium oxide.

more expensive than soda-lime glass

excellent electrical insulating properties

poor resistance to high temperatures and sudden changes of temperature.

used in the ceramic rings on pylons for its electrical insulation properties

appreciable resistance to high temperature or sudden changes in temperature

medium resistance to chemical attack

moderately expensive to make.

used for light bulbs, photochromic glasses, sealed-beam headlights, laboratory ware, and some bake-ware products.

alumina (Al₂O₃) is added to the glass batch to improve the properties of the glass.

good resistance to high temperature or sudden changes in temperature

difficult to make.

used in plasma display screens and optical fibres

made only from silicon dioxide (SiO₂) in the noncrystalline state.

expensive and difficult to make due to its maximal resistance to high temperature (900°C for extended periods, 1200°C for short periods).

used in special applications such as optical waveguides, crucibles for growing crystals.

Unless the raw materials are very pure, the glass produced by modern methods is green. In order to change the colour of the glass, red colourants (the complementary colour to green)must be added to allow decolourisation. The precise colour needed depends on theoxidation state of the colourant, the composition of the glass and the thermal treatment.