Introduction

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.

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

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REFERENCES

1) Brill, R.H., 1965: "The Chemistry of the Lycurgus Cup," Proceedings of the VIIth International Congress on Glass, paper 223 ((Brussels: I.C.G.).

2) Brill, R.H., 1988: in Excavations at Jalame, pp. 257-294 and Table 9-6 (ed., G.D. Weinberg: University of Missouri Press, Columbia).

3) Brill, R.H. and Cahill, N.D., 1988: "A Red Opaque Glass from Sardis and Some Thoughts on Red Opaques in General," Journal of Glass Studies 30, 16-27.

4) Brill, R.H. and Schreurs, J.W.H., 1984: "Iron and Sulphur Related Colors in Ancient Glass," Archaeometry 26, 199-209.

5) Fleming, S.J., 1999: Roman Glass: Reflections on Cultural Change, Appendix A (University of Pennsylvania Museum, Philadelphia).

6) Henderson, J., 1985: "The Raw Materials of Early Glass Production," Oxford Journal of Archaeology 4, 267-291.

7) Weyl, W.A., 1951: Coloured Glasses, 121-131 and 420-432 (Sheffield: The Society of Glass Technology).

8) Whitehouse, D. (ed.), 1990: "The Portland Vase," Journal of Glass Studies 32, 14-188.