Lost transparency! Weathering phenomena on the archaeological window glass collection of the Cistercian Abbey of the Dunes - Koksijde (Belgium)

As far as Belgium and archaeological window glass is concerned, the most important site is the Dunes Abbey, a former Cistercian abbey near the Flemish coastline. The collection contains approximately 15,000 fragments dating from the 13th to the 16th century. This glass was exposed to atmospheric weathering while in situ for several hundred years, buried for up to 400 years, excavated by different individual excavators in different eras and for over half a century stored in uncontrolled conditions. Moreover, different conservation treatments have been applied to the glass. Due to this, the collection was in a friable condition and we assume half of it has already been completely lost. The remaining collection retains fragments whose condition ranges between almost perfectly preserved material to being completely weathered to the point that no original glass survives. In this research, an important asset is recognizing what has already been lost and maximizing what is still available. During recent conservation and stock making campaigns, the different weathering phenomena were separated into 9 groups based on empirical criteria and detailed registration. As a first step to further investigation of the weathering processes, quantitative SEM-EDX analyses are used to give better insight into the chemical composition of these groups. The aim is to bridge the gap between interpretative archaeologically and archaeological science and to develop a common terminology to evaluate the complexity of weathering phenomena in archaeological window glass collections which can be used as a tool for the assembling and interpretation of these collections.