salts, sourced from the Wadi El Natrun area of Egypt. Post-Roman Islamic glassmakers reverted to using sodium-rich plant ash, while in Northern Europe, a method using ash from wood was developed to provide potash (K2CO3) as flux. Calcium oxide (lime, CaO) can also act as a flux. 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. 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 also may be controlled by the temperature and redox conditions inside the furnace. Chemistry In post-Roman times political problems in the Wadi El Natrun area disrupted the supply of natron so alternatives had to be developed. Eastern glassmakers reverted to using sodium-rich plant ash and for a while supplied southern Europe, using existing Roman trade routes, however, the Venetian glassmakers, who had inherited the Roman glassmaking skills, monopolised the trade in plant ash and banned craftsmen from working outside the city. The rest of Europe, north of the Alps, had to find another way of producing glass. The former and stabiliser components of glass occur in all regions as sand or quartz and as lime of various forms. The northern Europeans experimented with using ash from wood, ferns, and bracken as a source of the alkali flux. At its height the Roman glass industry was producing high quality, thin, colourless, and clear glass of consistent composition. The earlier surviving Forest glass vessels are characterised by a wide variety of compositions and lower quality, often being greenish to brownish in colour, thick-walled, and with inclusions and bubbles in the fabric. This suggests that using wood-ash was not just a case of changing the raw material, but necessitated a whole new technology with attendant development problems. Whereas Roman and earlier glass (of Si/Na/Ca composition) was of a marked uniformity over a wide