History of glass
Glass remained a luxury material, and the disasters that overtook Late Bronze Age civilizations seem to have brought glass-making to a halt. It picked up again in its former sites, Syria and Cyprus, in the 9th century BC, when the techniques for making colorless glass were discovered. The first glassmaking "manual" dates back to ca. 650 BC. Instructions on how to make glass are contained in cuneiform tablets discovered in the library of the Assyrian king Ashurbanipal. In Egypt glass-making did not revive until it was reintroduced in Ptolemaic Alexandria. Core-formed vessels and beads were still widely produced, but other techniques came to the fore with experimentation and technological advancements. During the Hellenistic period many new techniques of glass production were introduced and glass began to be used to make larger pieces, notably table wares. Techniques developed during this period include 'slumping' viscous (but not fully molten) glass over a mould in order to form a dish and 'millefiori' (meaning 'thousand flowers') technique, where canes of multicolored glass were sliced and the slices arranged together and fused in a mould to create a mosaic-like effect. It was also during this period that colorless or decolored glass began to be prized and methods for achieving this effect were investigated more fully.
Anglo-Saxon glass
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.