Coloured glass decay related to metal oxidation states

By studying coloured glass segments which are 200-2000 years old, scientists in Spain have uncovered a link between the degree of decay and the oxidation states of two transition metals. The historical glasses originated from religious buildings, Spanish stained glass windows and an archaeological site at Merida and they were examined by X-ray absorption spectroscopy as described in the Journal of Analytical Atomic Spectrometry.

For glasses containing iron, which is usually present as an impurity of the silica raw material, magnetite-type Fe(II) dominated in non-degraded glass but maghemite-type Fe(III) became more abundant as the surfaces became degraded.

A similar picture emerged for glasses containing copper, where the proportions of Cu(I) and Cu(II) were related to the degree of decay. Non-degraded glass was distinguishable by the dominance of copper(0) colloids. In historical glass containing manganese, Mn(II) was a major component but did not appear to be related to the degree of decay.

The results will be of interest to those involved in the restoration and conservation of old glass artefacts.