Forest Glass - Furnace Design

Furnace Design

Besides the descriptions of Theophilus and Agricola, the only depiction of an early forest glasshouse is from Bohemia in about 1380 (The Mandeville Miniature) This shows a furnace where all the high temperature processes of glass-making 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 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 15th C have been found in Eichsfeld in Germany. Another design found archaeologically from the 17th 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 manpower, and as the technology improved larger glass houses operated on an almost continuous basis. It has been estimated that a large glass-house might typically use 67 tonnes of wood a week operating for 40 weeks a year.