Ancient Roman glass fragment changed from green to gold over time

Iridescent patches discovered on a fragment of Roman glass are caused by photonic crystals that grew naturally over time. The finding provides new insight into self-assembly and pH-driven nanofabrication processes, according to a team of Italian and US-based heritage and materials scientists.

The glass fragment was recovered from the clay-rich soil in the outskirts of Aquileia, an ancient city in northeast Italy, and is around 2000 years old. Over that period sections of the glass have changed from their original green to display iridescence – changing colour when viewed from different angles – while a large portion has taken on a metallic gold appearance. The effect is a form of structural colour caused by nanoscale assemblies that have been formed by surface weathering over hundreds of years. The highly ordered structures formed due to silica dissolution and reprecipitation under mild alkaline conditions.