As early as the Middle Ages, glass-works used 'pot kilns'. The method is comparable to ordinary cooking. The pot is heated by a fire or gas flame. Dry glass mix is poured into the pot and heated to 1400-1500 °C. Recycled glass only needs 1200 °C. When the mass has become even and clear, the temperature is lowered, and the substance removed in small portions and cast into a mould. In theory, the glass is soft and can be worked until the temperature falls to around 650 °C. The usual working temperature in the production of windows is about 10001200 °C. The capacity of a pot kiln is about half a ton per day. They are still used in smaller glassblowing workshops for glass artefacts, but not in the production of windows.
In industrial methods, closed tanks with an inbuilt oil burner or electrical element are used. Typical tanks are made of fireproof stone and have a capacity of 200 to 300 tons per day. The working temperature is the same as that of the pot kiln. A tank kiln will run at full capacity continuously and may only last two to three years. The glass produced can be shaped using a series of different techniques.
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