Purple and Colorless Glass

Mold-blown juglet
1st century A.D.
Ht., 8.5 cm

A manganese-rich mineral, such as pyrolusite, will produce a purple glass in a furnace with an oxidizing atmosphere, but a colorless glass in one with a reducing atmosphere.

The decolorization process within colorless glass is a subtle optical effect in which the aquablue due to the iron, and pink due to the manganese, cancel each other out. If there is a slight imbalance in the chemistry, an interaction involving any sulfur in the mix will take place and create a slight yellow tinge in the glass. Streaking can occur with poor mixing and/or incomplete fusion of the ingredients, or if the atmosphere in the furnace is non-uniform.