conditions, but also may give a yellow colour. Manganese has its own purple colour which may balance out the iron colour to make colourless glass. For instance, glass made from beech wood grown on meagre lime-rich soil (e.g. Kleinlutzel, Jura) is high in manganese and thus, nearly colourless while that in a clay-rich area (e.g. Court-Chalvet, Jura) is olive green. Thus, a variety of colours may be produced and experimentation allowed the glassmakers to progress from the early muddy green-yellow-brown colours toward clear-coloured and colourless glass. Local conditions allowed some areas to produce finer glass at an earlier stage. In Bohemia at the end of the sixteenth century the decolourising powers of manganese were used to produce a clear glass suitable for engraving. The amount of carbon left in the wood ash also may affect the colour of the glass by modifying the furnace atmosphere. The glass in York Minster has been shown to be 90% naturally coloured, without added colorants. Other clear colours were produce by deliberate addition of metal oxides, often the byproducts of local metalworking; copper oxide to give green or turquoise, cobalt for strong blue. Red was particularly difficult to produce, using particles of copper under delicately-controlled redox conditions. There is little evidence of antimony- or tin-based opacifiers being used, or the use of lead to modify other colours. Operation of the glasshouse There are only two historical descriptions of European glassmaking in medieval times. In 1120 Theophilus Presbyter, writing in Germany, gave detailed recipes and instructions and in 1530 Georgius Agricola wrote about current glassmaking. Other useful information comes from archaeological finds and experimental and theoretical reconstructions. Sourcing and collection of raw materials The sand likely was collected from river beds, where it was relatively clean and of more uniform particle size. The felling, transporting, drying, and storage of wood both for ash production and as fuel for the furnaces was labour-intensive and required a high level of organization. Preparation of ash Theophilus recommends the use of beech logs, which analysis has shown has a high proportion of CaO when grown on calcareous soil. Whatever wood is used, the amount of potash and CaO it provides, as well as other components that might affect colour and opacity, varies considerably with the age and part of the tree, soil chemistry, climate, the time of year when the tree was cut and the dryness of the wood