Glass

C. Corvaia

Introduction

Glass manufacture is believed to have originated around 5000 years ago in Mesopotamia or Egypt. The glass beads, figurines and vessels produced were similar in appearance to ceramic objects. They were made using the techniques of core forming, mosaic, fusion, mould casting and cold cutting. Glass blowing was developed approximately 2000 years ago, most probably in Syria. The techniques for hand working glass have continued essentially unchanged since that time.

Ancient glass was formed from a mixture of silica (sand, crushed quartz or flint), alkali (soda or potash from plant ash) and lime (probably included as an impurity in the sand). Oxides of metals such as copper, iron, cobalt and manganese were added to produce coloured glass. Some glasses which were not purposely coloured had yellow or green tinges. This was due to iron oxide impurities in the sand. A colourless glass was first produced about 1900 years ago by adding manganese dioxide to the usual glass components.

Until about 1000 years ago soda from the ashes of marine plants were universally used to modify the silica network. After this time the glass makers of Northern Europe replaced soda with potash obtained from beech wood ash. The resultant glass was known as ‘forest glass’ and was mainly green or brown in colour.

By the 15th century Venice was established as the most prominent glass manufacturing centre in Europe. ‘Cristallo’, a fine clear glass was developed by adding lime to the soda-silica mixture. In Bohemia in the early 16th century, a clear solid crystal glass was produced when lime was added to the potash-silica mixture. This glass was ideal for cutting and engraving. Lead glass, also known as ‘glass of flint’, was developed in Britain in the 17th century. This glass, with a 30 per cent lead oxide content, is characterised by its clarity and brightness.

Mould casting was revived in North America in the early 19th century. The use of press moulding allowed intricate patterns to be produced cheaply and quickly. Early pressed glass objects were made predominantly from colourless lead glass. In 1864 a ‘lime’ glass was developed which was most suitable for pressing. It resembled lead glass but cost less. This was important as the production of glass containers became mechanised in the late 19th century. Hand blown bottles were still produced as late as the 1930s but only for certain types such as pharmaceutical bottles and cosmetic ware.

Most of the glassware used during the 18th and 19th centuries in Australia and other parts of the then British Empire was imported from Britain. In comparison with the common ‘black’ glass bottles, there is a scarcity of the colourless and pale green flint glass objects on early Australian sites. This is due to manufacturing limitations and differences in excise duty that existed between flint glass and black glass in Britain from 1746 to 1845 (Boow 1991). Older, cheaper methods of manufacture, such as those using wooden moulds and push-ups, were continued for black glass bottles until about 1870.