Achaeometrical Research in Hungary II., 1988
ANALYSES - GLASS - Edit MESTER - István SZABÓ: Research of medieval glass vessels and glasshauses in Visegrád and Diósjenő
K 2 0, less the percentage amount of the trivalent oxides previously mentioned, while "RO" represents MgO, CaO and some divalent oxides such as MnO, CoO, CuO and ZnO. This simplified approach permits comparisons between the composition of glass samples in terms of these reduced variables in a ternary diagram (Fig. 2). It may be seen that the point representing the compositions of the specimens, such as Sample No. 3, has the lowest "Si0 2 " and the highest "R 2 0" content, contrary to the first group of samples. SEM analyses of specimens with fissured surfaces (Fig. 3) fit with these results of the chemical analysis. The structure of the surface layer changed more in case of Sample No. 3. The XRD studies showed the ß-wollastonite phase in powdered and heat treated glasses, both in Samples No. 1 and No. 3. These similarities in crystallisation ability are originating from the CaO contents. The transformation temperatures and the peak temperatures of crystallisation ranged between 540-570 °C and 900-920 °C, respectively. The weight loss of crusted samples (No. 3.) was 14 times higher than the values measured in specimens of the first group, resulting in a change of glass structure and quality. After these examinations the following conclusions may be drawn. To date, the two sample groups are in quite different conditions. The first group is made of fine and usually clear glass, covered only with thin iridescent layer. The second group is made of thicker and heterogeneous glass which by now became weathered. The high alkalic alkaline earth and other metallic oxide contents are the cause of this rapid structural transformation. Different types of glass, therefore, are not equally resistant against environmental influences. As has been shown by the chemical composition of glass sherds, glass-workers used polluted materials during the manufacturing of the glass in the less resistant second group. Their glasshouses were less developed, they probably had neither access to convenient technologies, nor clean materials. Cheap household and industrial glass vessels were made in such glasshouses. Similar glass-works were excavated near Diósjenő in Hungary and in neighbouring countries as well (HEJDOVÁ, 1991: 18-21.). Individual local products as well as the imitations of imported goods were probably made in these glasshouses. The first sample group represents typical Venetian quality, types relevant to this statement as well. Further examinations are required to obtain more detailed analytical results that confirm these findings. Our main goal in the future is to analyse glass fragments recovered from glasshouses. 4. Medieval glasshouses at Visegrád Bene-ground and near Diósjenő in Nógrád county Péter Gróf and Dániel Gróh excavated a medieval glasshouse at Visegrád - Beneground in 1984. The glasshouse was situated on the outskirts of the city on the riverside. The surface of the system of holes excavated was covered by a burnt layer. The excavators found post-holes which belonged to a wooden construction. In the centre of the findspot was an oval-shaped stone-surface. The functions of stone clusters were determined on the basis of the glass finds (e.g. glass-drops, glass-fragments, scrap-glass, glass-melt, and melting-pots). These artefacts were related to a medieval glasshouse, although the furnaces were not reconstructed (Figs. 5-8). This glasshouse operated between the second half of the 14 th century to the first half of the 15 th century. A medieval document concerning this glasshouse was also found. In 1491, the archbishop of Esztergom bought 5000 panes of glass from a glazier named John of Visegrád (VOIT, 1958: 283-315). An oven and a bone-carving workshop were also excavated in the proximity of this glasshouse. 102
