Leaching studies of potash–lime–silica glass with medieval composition by IRRAS

Abstract

Potash–lime–silica glasses were extensively used during medieval times for several art objects and stained glass windows which are considered as a fundamental part of the decorative plan of churches and cathedrals in Northern Europe. The high potash content of this particular type of glass is the reason for its low chemical durability and hence accelerated deterioration, which entails the formation of leached layers and corrosion products, especially when exposed to atmospheric agents and to air pollutants.In order to preserve the value and to avoid the degradation of these artistic glasses, it is important to understand the chemical processes occurring on the glass surfaces by studying the role of glass composition and corrosion agents. For this purpose the leaching progress of two types of potash–lime–silica glasses (glass M1 with a silica content lower than 50% and high amounts of K2O and CaO, and glass M3 with 60% SiO2, 25% CaO and 15% K2O) was studied by using Infrared Reflection Absorption Spectroscopy (IRRAS) and the dependency of the corrosion rate on the nature of the acids (sulphuric, nitric and hydrochloric acids) was tested. The corrosion rate during the early stages of corrosion is different, depending on the type of acid used during the experiments: The results showed that glass M1 presents a high corrosion rate when leached in sulphuric acid in comparison to nitric and hydrochloric acids; in contrast to this result glass M3 is more resistant to the attack of sulphuric acid compared to the other acidic solutions and has a much higher chemical stability in comparison to glass M1.Furthermore the applicability of a self-built unit for IRRAS measurements was tested monitoring the progression of the leaching processes, in order to continue with further in-situ aging studies on the weathering mechanisms occurring on the glass surfaces.