The colours of ancient Roman glass


The different types of sand used to manufacture ancient glass contained iron and manganese impurities which gave it colour.  The iron ions gave the glass a light-green tinge, and the manganese produced pink and violet colours. The sands used to make glass also contained sulphurs in the form of sodium sulphate which  gave colours from yellow to dark green.


Ancient glassmakers also deliberately added minerals to colour class. Blue glass was made by adding copper containing compounds such as azurite , chrysocolla and chalcopyrite. Darker blues were produced by added cobalt-rich minerals, such as asbolite. Green glass came from iron compounds. Hues from pink to violet were the result of different manganese oxides . Yellows and umber were produced with iron oxides and carbon. For yellow glass silver oxides wee often added. A brilliant-yellow was achieved by mixing antimony and lead  which resulted in a yellow precipitate of lead pyroantimonate distributed throughout the glass mixture. For white glass  tin was added.


Quite sophisticted chemistry was used by ancient glassmakers to produce red-opaque glass by mixing copper and lead compounds and then keeping  the firing furnace oxygen free (a reducing atmosphere). Even a small amount of air would oxidize the glass and it would turn out blue. In the reducing atmosphere, cuprous oxide  crystallized throughout the glass mixture, giving the glass a rust-red opaque colour. Even though making red-opaque glass involved complicated chemistry, near-eastern glassmakers were making it by the 9th cenutry BC.


Almost colourless glass could be created by the careful selection of fine silver free sand, but manganese and, it seems, above all, antimony, was the most effective decolourants agents.  Strong colours are a characteristic feature of the earliest blown glass but  by the late 1st century AD  as the effects of manganese were exploited,  tastes were beginning to change, and virtually colourless glass came to dominate  by the 3rd century AD.

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