Red pot metal glass is often undesirably dark in colour and very expensive. The method developed to produce red glass was called flashing. In this procedure, a semi-molten gather of coloured glass was dipped into a pot of clear glass. As the bubble became enlarged, the red glass formed a thin coating on the inside. The formed glass was cut, flattened and annealed as any other blown sheet.
There were a number of advantages to this technique. It allowed a variety in the depth of red – and other deep colours - ranging from very dark and almost opaque, and sometimes merely tinted. The other advantage was that the colour of double-layered glass could be engraved, abraded, or etched to show colourless glass underneath. Other base colours are also used in making flashed glass, for example red flashed onto a pale green base.
There still exist a number of glass factories, notably in Germany, USA, England, France, Poland and Russia which continue to produce high quality glass by traditional methods primarily for the restoration of ancient windows.
Edited from Wikipedia and other sources
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