Iridescence in Ancient Glass: A Morphological and Chemical Investigation

Visible damage on the surface of historic glass, iridescence, is developed by the influence of environmental factors during time. Iridescence is usually considered as an age indicator in historic glass. Samples from the ancient Sasanian historic site called “Tomb‐e Pargan” in the Boushehr Province of Iran were subjected to microscopic analyzing with ESEM , ICP ‐ OES , and polarization microscopy to characterize structural damages and its impact on the surface deposition. Chemical–mineralogical studies were also performed. The current investigation suggests that the iridescent layer is developed as the result of alkali ion depletion on the glass surface following the formation of a shelly layer. Due to the porosity of ancient glass surface, ions migrate and deposit continuously on the surface in the form of crust. The phenomenon causes fine lamellar structure on the surface. Samples from Tomb‐e Pargan characteristically contain potassium and magnesium.