Civil Engineering Reference
In-Depth Information
Figure 6.5: The production of crown glass: (a) the glass is blown up into a bubble; (b) an iron
rod is fixed to the glass bubble; (c) the blowpipe is removed; (d) the glass bubble opens up after
being warmed and rotated; (e) when completely open, the bubble becomes a flat, circular pane
of glass; (f) the iron rod is removed. The pane of glass has a thick edge and centre, but is
otherwise clear.
Smelting
As early as the Middle Ages, glass-works used 'pot kilns'. The method is compa-
rable to ordinary cooling. The pot is warmed up 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 reaches 650°C. The usual
working temperature in the production of windows is about 1000°-1200°C. The
capacity of a pot kiln is about half a ton per day. They are still used in smaller
glass-blowing workshops for glass goods, but not in the production of windows.
In more industrial smelting methods, closed tanks with an inbuilt oil burner or
electrical element are used. The tank is made of fireproof stone and has a capac-
ity of 200-300 tons per day. The working temperature etc. is the same as that of
the pot kiln. A tank kiln will be worked at full capacity continuously and may
only last two to three years. The glass produced can be shaped using a series of
different techniques.
