when burned, factors over which the glassmaker had little control. This variability explains the problems that glassmakers had in trying to produce glass of a consistent quality. Large amounts of ash would have to be prepared and mixed together to give the homogeneity needed to give a predictable glass composition. A typical yield of ash from beech is only about 1%, so using Theophilus' recipe of two parts of sand to one part of ash means it would take 63 kg of beech wood to produce one kilo of glass. It has been estimated that, including fuel, 150–200 kg of wood would be needed per kilo of glass. Fritting Then the prepared ash and sand were heated together, but not melted, at a relatively low temperature (up to about 900 °C or 1650 °F) in a process known as fritting. Theophilus specifies 'for the space of a day and night.' This process, which could be monitored by changes in colour as temperature increased, caused a decrease in volume, prior to charging crucibles for the final melting stage, thus minimising the number of times the furnace would need to be opened, and also, consolidating the light powdery ash that might blow about in the furnace causing contamination. Melting The final stage was to melt the fritted material in crucibles in a covered furnace to give molten glass. The furnace needed to operate at as high a temperature as possible since quick melting and the need for less flux improved the quality of glass. The change from natron to potash required an increase in melting temperature of about 200 °C to around 1350 °C, necessitating a fundamental change in furnace technology and the development of high-temperature ceramics. At this higher temperature, normal clay would react chemically with the glass. Working Once melted, the glass would be blown into vessels or into cylinders which then were opened into sheets for window glass. The final stage is to anneal the finished glass to avoid damage from shrinkage stresses. Furnace design Besides the descriptions of Theophilus and Agricola, the only depiction of an early forest glasshouse is from Bohemia in approximately 1380 (The Mandeville Miniature). This shows a furnace where all the high temperature processes of glassmaking were performed in the one structure containing several ovens whose varying temperatures might be controlled to the necessary extent by constant attention. The raw materials are mixed at