a pit nearby and carried down in pans to be fritted in one of the ovens, optimum temperature up to 1100 °C. The frit is melted at high temperature up to 1400 °C in crucibles in a second oven, and when ready, the glass is being blown into objects. These are placed in the annealing oven to cool. The whole structure is enclosed in a wooden building, and it is likely that wood was stored and dried above the furnace. Remains of a similar structure from the late fifteenth century have been found in Eichsfeld in Germany. Another design found archaeologically from the seventeenth century is the 'butterfly furnace'. These furnaces were made from stone and the crucibles from imported, highly-refractory clay. They differ in style from the Islamic furnaces of the east, and those of southern Europe, the 'beehive' style where the annealing chamber is above the main oven rather than on the same level. The furnace firing cycle would be optimised for fuel consumption, output, and humanpower, and, as the technology improved, larger glasshouses operated on an almost continuous basis. It has been estimated that a large glasshouse typically, might use 67 tonnes of wood a week operating for 40 weeks a year. Location of glasshouses The vast amounts of wood needed to produce glass in this way dictated that glasshouses be located in forest areas and that the woodland be managed carefully by coppicing and pollarding to maximise the wood resource and to optimise the size of wood pieces used. Even so, periodically the glasshouse would have to relocate, as the woodland was depleted. The glass industry had to compete for wood supplies with other industries such as mining, and domestic demand. In sixteenth-century England, an embargo was placed on the use of wood for fuel for glassmaking. Glasshouses often were located in forests owned by the church. One of the main uses of forest glass was for ecclesiastical stained glass windows. See also Heart of Stone (German fairy tale) - a glass works in the Black Forest is a key element of this German folk tale References Bibliography Agricola, G. 1556, De Re Metallica, Book XII, Basel, (translated by H.C. and L.H. Hoover) Dover reprint 1950. Cable, M., 1998, The operation of wood-fired glass-melting furnaces. In: P. McCray and D. Kingery (eds.), The *Prehistory and History of Glassmaking Technology, 315–330. Crossley, D.,1998, The English glassmaker and