Amethyst glass vase, colour obtained using manganese, by James Powell and Sons (Whitefriars) Limited, Wealdstone, Harrow, Greater London, England, 1935

This is an example of glass which has been coloured by the addition of chemical elements and oxides to the glass material itself, rather than through colourants being added during the manufacture of the object or in a coating.

This particular vase has been coloured purple by the addition of manganese. Manganese is one of the oldest glass additives, and was often used to neutralise a greenish tint caused by iron content in the glass. In larger amounts, manganese colours the glass an amethyst purple, and the earliest example of purple manganese glass dates back to the early Egyptian period.

Glass is coloured by the presence of metallic oxides, either as impurities in the raw materials or as deliberate additions. Generally red colours are more difficult to produce. Blue is made with cobalt or copper and green is produced by the addition of iron or chromium. Uranium oxide or silver gives yellow, and orange, which is difficult to produce, is made by adding selenium. Pale pink and shades of violet and purple are made with manganese and red with copper, gold and cadmium selenide or zinc sulphide. Black results if any colouring agent is used to excess.