Forest Glass - Glassmaking

Glassmaking

It is important to distinguish between glass making from raw materials and glass working, which is the production of finished articles by melting pieces of raw glass or cullet which may have been made elsewhere or by recycling old glass. Glass consists of four principal components:

1. A former – to provide the network of atoms forming the matrix of the glass. This is Silica (SiO₂), which in ancient times was added as crushed quartz, and from Roman times onwards in the form of sand.
2. An alkali flux – to lower the temperature at which the silica melts, making it achievable using currently available working temperatures. In ancient times, the ash of sodium-rich plants growing in arid areas around the eastern Mediterranean provided soda (Na₂O) as flux. In Roman times the mineral natron was used, a naturally occurring mixture of alkaline sodium salts, sourced from the Wadi El Natrun area of Egypt. Post-Roman Islamic glass-makers reverted to using sodium-rich plant ash, while in Northern Europe, a method using ash from wood was developed to provide potash (K₂O) as flux. Calcium oxide (lime, CaO) can also act as a flux.
3. A stabiliser - to stop the glass dissolving in water and increase corrosion resistance. The most effective is lime (CaO) but alumina (Al₂O₃) and magnesia (MgO) can achieve this to some effect. These minerals may already be present in varying quantities in sand.
4. A colourant or opacifier - These can be naturally present in the glass due to impurities in the raw materials or can be deliberately added to the melted glass as minerals or as slag from metalworking processes. The most important contributions are from iron, copper, cobalt, manganese, tin, antimony and lead. Opacity can be due to bubbles in the glass or the inclusion of opacifying agents such as tin and antimony. The resulting colour and opacity from a given composition can also be controlled by the temperature and redox conditions inside the furnace.