The structure of the glass contains many microscopic cavities that can house metal atoms, which affect the way light passes through the glass. Each metal absorbs light of a different frequency, which gives the glass a characteristic color.It was this principle that gave rise to the stained glass window, one of the glories of medieval cathedrals. When copper was added to molten glass, it became ruby red; blue with cobalt; green with iron; yellow with antimony; and purple with manganese. Leaves of about 25 x 25 cm, in various colours, were produced and then cut to give them the required shape. Then they were assembled to set up the stained glass windows.
The non-uniform thickness of medieval glass gave the stained glass subtle variations in tone, which added beauty to the finished work.
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