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