Glassmaking furnace
Chigi ms., 15th century A.D.


Deliberate coloration of glass requires the addition of an appropriate mineral, e.g. malachite for green, pyrolusite for purple, etc. The crushed mineral and crushed frit [see Glassmaking] are mixed together and then heated through to the point of fusion.(Often, these mixtures have a lower fusion point than naturally colored glass.)

The glass's color, and its intensity, are dependent upon the furnace atmosphere during firing. The usual, oxygen-rich environment (i.e. an oxidizing state) will produce one color while an oxygen-starved environment (i.e. a reducing state) will produce another. For example, the iron impurities that usually color glass aquablue or light green will produce an amber shade when fused in a reducing environment.