Anglo-Saxon Glass - Glass Manufacture - Glass Making

Glass Making

Glass consists of four principal components; a former, alkali flux, stabiliser and colourants/opacifiers.

• Former: Silica, which in the Roman and Anglo-Saxon period was added in the form of sand.

• Alkali flux: Soda or potash. The Roman sources, in addition to chemical analysis of glass dating to the Roman and Anglo-Saxon periods, suggest that mineral natron was used as the flux. Pliny also states that the preferred source of natron for glass making was from Egypt. At the end of the 1st millennium AD a new source of flux was being used, wood ash, which is a source of potash.

• Stabiliser: Lime or magnesia. In Anglo-Saxon glass the main stabiliser was lime. This may have been deliberately added to the glass batch, but may simply have been a component of the sand that was used. Roman writer Pliny stated that sand from the River Belus was particularly suitable for glass-making, this is a calcareous sand, rich in marine shells, and therefore lime.

• Colourants/Opacifiers: These can be naturally present in the glass due to impurities in the raw materials, e.g. in the case of green/blue-green glass which results from the presence of iron in the sand. Other colourants are likely to be deliberate additions to the glass melt of small quantities of mineral-rich material or in some cases slags from metalworking processes. The elements in ancient glass that affect its appearance are mainly iron, manganese, cobalt, copper, tin and antimony. The presence or absence of lead is also important, while it doesn’t produce a colour itself (except in the form lead-tin oxide or lead-antimony oxide) it can change the hue of other colourants. In addition when added to opaque glasses it ensures that the colourants form in a controlled way and are uniformly distributed. Opacity in glass can be due to a number of factors; intensity of colour, bubbles in the glass or the inclusion of opacifying agents, such as tin (SnO₂ & PbSnO₃) and antimony (Ca₂Sb₂O₇ & CaSb₂O₆ & Pb₂Sb₂O₇).

The main type of glass found in the Anglo-Saxon period is a soda-lime-silica glass, continuing the Roman tradition of producing glass. There is very little evidence for glass making from the raw materials in Roman Britain and even less evidence in Anglo-Saxon Britain. It would have been nearly impossible to transport the natron from the Middle East to Britain. It is therefore far more likely, that as in the Roman period, glass was being produced near the raw materials and then lumps of raw glass transported. Another source of glass was cullet, recycled broken or crushed glass. Recycling was carried out throughout the Roman period, and large deposits of broken glass at Winchester and Hamwic suggest glass was also being recycled in the Anglo-Saxon period. It is possible that by recycling glass in this way it was possible to keep up to the demand for glass products without new raw glass having to be introduced into the system. Additional cullet may also have been collected from the ruins of abandoned Romano-British sites. In the late 7-8th century with the construction of many ecclesiastical establishments and large windows, the demand for glass grew. At this time political problems in the Delta-Wadi Natrun region caused a shortage of natron in the Middle East where the raw glass was produced. This possibly led to glass makers experimenting with new fluxes, which finally led to the introduction of wood ash glasses using potash as the main alkali flux, which was more readily available.

Table above: Compositions of glass from the 1st Millennium AD. Adapted from data published in Sanderson, Hunter and Warren’s analysis of 1st Millennium AD glass (top) and data published by Freestone’s study of the provenance of ancient glass through compositional analysis (bottom). HIMT stands for high iron, manganese and titanium glass, nm for not measured and where there were only traces or the value was below the analytical equipment's detection limits <0.1 is used.