Investigating a Byzantine technology: experimental replicas of Ca-phosphate opacified glass

Abstract

Abstract The Byzantine glass opacification technique based on the use of animal bones is here investigated by means of experimental replicas, aiming to shed light on the production technologies of Ca-phosphate opacified glass. To minimize the variables depending on the species or individuals, a single piece of bovine bone was selected. The current replicas were produced by mixing the piece of bone with silica-soda-lime glass having a chemical composition that is quite comparable to that of ancient natron glass. The bovine bone was used both in an untreated state and after its preliminary firing at different temperatures (350, 450, 600 and 800\u2009°C) and analysed by means of ICP-OES and XRPD. Two sets of experiments were carried out in order to obtain an opaque glass that was texturally and mineralogically similar to ancient samples. A first set of replicas was produced by maintaining a constant melting time (5\u2009hours), temperature (700\u2009°C), and cooling rate (quenching) and by using differently pre-treated bone samples (untreated and fired at the above temperatures); a second set of experiments was conducted by using a single bone sample (fired at 800\u2009°C) and by varying melting times (18 and 36\u2009hours) and temperatures (700 and 1100\u2009°C) as well as cooling rates (quenching and slow cooling). The replicas were all characterised by means of SEM-EDS and micro-Raman spectroscopy. The results demonstrate that the production of Ca-phosphate opacified glass requires a preliminary firing of the bone, a melting temperature of approximately 700\u2009°C and long melting times (up to 36\u2009hours). The use of unfired bone did not give satisfactory results, nor did short (5\u2009hours) or very long melting times (>\u200936\u2009hours), because the micro-textures obtained are not similar to those identified in ancient samples. The use of a higher melting temperature (1100\u2009°C) can also be excluded, independent of the melting times and cooling rates, as it produced a transparent glass.