Chemistry of cultural glasses: the early medieval glasses of Monselice's hill (Padova, Italy)

To investigate the mechanisms of deterioration of historical glasses, under natural evolution, some early medieval glasses from the archaeological site of the Monselice’s hill are analysed. By an archaeological approach, developed at the Department of Scienze dell’Antichità, University of Padova, the glasses were dated between the VI and the beginning of the VII century and they were ascribed to the same artist or school. By a geological approach, developed at the Department of Mineralogia e Petrologia, University of Padova, it was found that some pieces of glasses, from the same archaeological site, were silicates, rich in sodium and calcium, with iron and manganese. The composition was analogous the one of glasses produced during Roman empire, using “natron” (Na2CO3•NaHCO3•2H2O) as melting agent and glasses produced during medieval age, in the Mediterranean countries, using dust of plants like “Salsola Kali” as melting agent. It was also found that there was a surface layer, with a special structure “lamellare-mammellonare”, easy to remove. The surface layer was found poor in alkali and alkaline-earth elements. By surface and microscopic analyses (optical microscopy, SEM-EDS, microRaman, XPS, SIMS and Mössbauer) it was found that all the samples have a composition rich in silica, sodium and calcium except one that, unexpectedly, is rich in potassium and poorer in sodium. This sample, as composition, seems just like medieval glasses produced around Germany, using dust of plants like ferns as melting agent. In all the samples the surface layers have less alkaline elements and the depletion is still strong up to ten m of depth. The extreme consequence of this depletion is the formation, in some samples, of an alteration layer, easy to remove, that the XPS analyses tell us it is made of silica very hydrated. In the surface layers there is a little enrichment of calcium. The calcium makes also some birefringent crystal aggregates that, in same samples, are around one millimetre large. These aggregates have a circular shape, with a nucleation centre in them. By microRaman spectroscopy it was found that the crystal aggregates are made by vateritic and calcitic calcium carbonate. By Mössbauer spectroscopy it was found that the flat glasses, coloured by yellow, richer in iron, are mainly made of iron (III), instead the pale green ones, poorer in iron, are mainly made of iron (II).