Le tecniche di produzione dei vetri opacizzati con fosfato di calcio: una ricerca multidisciplinare

The present multi-disciplinary study focuses on the opacification technique of glass, based on Ca-phosphate, added to the batch by means of animal bones. This opacifier was identified in late-Antique glass tesserae from various Mediterranean sites, among which Padua in Italy and Tyana in Turkey. The present study is composed of two parts, carried out with a multi-disciplinary and multi-methodological approach: the first one concerns the archaeometric characterisation of late-antique glass tesserae from Padua and Tyana, in order to advance hypotheses on their production technologies; the second one aims at verifying these hypotheses, in terms of firing temperatures and times, and possible pre-treatments of bones, by means of experimental replicas. Results of archaeometric study on late-Antique tesserae show that Ca-phosphate grains are mineralogically ascribed to hydroxyapatite, characterised by an evident reaction rim, composed of sodium calcium phosphate, named as β-rhenanite; wollastonite crystals are also formed in the proximity of calcium phosphate grains. Notwithstanding the geographical distance between Padua and Tyana (more than 2000 km), comparable results were found in all the tesserae from both sites, suggesting similar and highly standardised production technologies of late-Antique tesserae with calcium phosphate as opacifier, which can be summarised as follows: bone powder was added to a transparent (coloured) glass as opacifier. The heating at about 700°C caused both the softening of the glass and the reactions between glass and hydroxyapatite particles with formation of β-rhenanite and wollastonite. Results of experimental replicas demonstrate that the production of Ca-phosphate opacified glass requires a preliminary firing of bone, a temperature of about 700°C and long firing times (up to 36 hours). The use of unfired bone did not give satisfying results, as well as short firing times (5 hours), because the micro-textures obtained are not comparable to those identified in ancient samples. The use of higher firing temperatures (1100°C) can be also excluded, independently from firing times, as it produced a clear transparent glass.