The basic components of glass are silica (sand), soda (or potash) and lime. The silica, when melted by heat, forms the glass; the soda acts as a flux to allow the glass to melt at a lower temperature; and the lime is a stabiliser.
The combination can be modified. Lead oxide used as the flux makes the glass less brittle and more stable but harder to work when hot. Various oxides and silicates add colour. Tin oxide makes the glass opaque.
By 2000BC, Egyptian craftsman were skilled at making glass articles. They developed four techniques:
- Glass jars were moulded around cores of mud or straw. This technique was common by the 18th Dynasty (1567-1320 BC). This technique can produce objects formed from bands of coloured glass.
- Glass was cooled and then carved and polished to the desired shape. The glass was often coloured to resemble a semi-precious stone. This technique was known for many centuries but became widespread in the eighth century BC.
- Molten glass was poured into a mould. Sometimes a ground paste of glass was poured into a mould and then melted.
- Small pieces of glass were laid together into a mould and fused together by firing.
Around the first century BC, probably in Syria, glass blowing was discovered. The new technique effectively created a new material – thin, transparent and flexible. It was found that glass vessels could be produced easily and quickly by blowing glass into moulds.
This moulded glass became common throughout the Roman Empire. Glassmaking reached a highpoint not exceeded until at least the 15th or 16th century. Glass in strong colours was popular with techniques such as mould fusion and mosaic being used for luxury pieces. Overlaid , then carved cameo glass, were developed. By the time of the eruption of Mount Vesuvius in 72 AD, many houses had glass windowpanes mounted in wooden or metal frames.
From the end of the second century AD, there was a trend away from coloured glass to cut or engraved colourless glass.
The technique of glass making spread to all parts of the Roman Empire and as far as China along the Silk Road from Syria. (Although relatively little use was made of it in China except as a glaze for pottery.)
With the fragmentation of the Roman Empire, separate styles developed in different areas. And as the Empire declined, there was an accelerating decline in in production standards.
Did you know?
The famous Portland Vase in the British Museum is a Roman amphoriskos made of royal-blue translucent glass cased in a layer of white glass that was cut away to leave a frieze of classical figures. It was not until 1873 that John Northwood succeeded in making the first modern reproduction of this vase.