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•A rare wood ash glass tessera from Malbork Madonna was studied in detail.•It was found to be original mediaeval, from the area north of Alps, coloured by Co.•Opacification was ...achieved by Ca-phosphate grains, a Mediterranean technology.•Two phases detected in tessera for the first time: leucite and pseudowollastonite.•Pseudowollastonite indicates production temperature above ca. 1125 °C.
While the natron and plant ash glass tesserae may be found on places of importance across the former Roman and Byzantine empires, wood ash glass tesserae are scarce. This is the first time a wood ash glass tessera is studied in detail. It was part of a magnificent 8-metres tall statue of Madonna in Malbork, Poland, created at the end of the 14th century and destroyed at the end of World War Two. It was found to be coloured by cobalt with possible impact of copper, and opacified by Ca-phosphate. Processes previously described in sodium-rich glasses were observed also in the studied potassium-rich wood ash glass tessera, such as diffusion of the respective alkali metal into the Ca-phosphate grains. The elemental composition of the tessera indicates that it is original – mediaeval, from the area north of Alps. Two phases were identified for the first time, to authors’ best knowledge, in any glass tessera – leucite (tetragonal KAlSi2O6) and pseudowollastonite (monoclinic CaSiO3). As pseudowollastonite is a high-temperature phase, it may serve as an indicator of production temperature, which was further supported by the study of polymerisation index of model glasses. This study contributes to the knowledge of old technologies and showed that the know-how for opacification was imported from the Mediterranean, while the raw materials employed for the base glass preparation were from the area north of Alps.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The Bohemian historical glasses have been neglected and have not been in focus of Archaeometric studies so far. Potassium-calcium wood ash glasses, produced in Bohemia and Moravia, from the beginning ...of the 14th century to the first half of the 18th century, are chemically different from the glasses produced in the same period in Western Europe. There are no written sources for glass batch recipes for Gothic (14th–1st half of 16th c.) and Renaissance (16th–17th c.) glass, while there are only few for the Baroque (end of 17th–18th c.) glass recipes. Systematically investigating the chemical composition and typology of archaeological glasses, we have chosen to reconstruct the glass recipes of potassium-calcium glasses from the three periods. In this study, the glass recipes (the ratio of the raw materials) were calculated based on the chemical composition of the historical glasses studied by X-Ray Fluorescence (XRF) and Scanning Electron Microscopy/Energy Dispersive System (SEM/EDS). The composition of the authentic natural raw materials was studied by XRF and X-Ray Diffraction (XRD): sand or quartz pebbles, beech ash and potash, limestone, NaCl, and As
2
O
3
. Model glasses confirmed our presumption of gradual development in the Bohemian glass batch recipes, which used very simple raw materials ratios.
Analyzing the chemical composition of archaeological glasses can provide an insight into their provenance and raw materials used in their making. However, to the authors’ knowledge, the historical ...production process itself and melting characteristics of the glasses have not yet been extensively investigated. The main focus of this paper is to describe the melting process of three main types of Bohemian historical glasses: Gothic (14th–1st half of 16th c.); Renaissance (16th–17th c.); and Baroque (end of 17th–18th c.). The model glasses were prepared from natural raw materials and processes that take place during melting were investigated using optical microscopy, SEM-EDS, XRD, and DTA-TG methods. Furthermore, the viscosity of model glasses and thermal dilatation was measured and used to calculate the reference viscosity points. The results illustrate the complexity of historical glass melting, as well as the technological progress between different periods.
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CEKLJ, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
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