Medieval and Renaissance glass technology in Tuscany. Part 4: the XIVth sites of Santa Cristina (Gambassi-Firenze) and Poggio Imperiale (Siena)

A scientific classification of the finds discovered at Santa Cristina, a XIVth century site of Valdelsa devoted to the glass manufacture, and the relative production cycle is proposed on the basis of a physico-chemical investigation, carried out especially by optical and SEM-EDS microscopy, Mössbaüer and ICP spectroscopy, XRD diffractometry and TG-DTA thermogravimetry. It was verified that the glasses can be divided according to their mean composition in two groups: one sodic-calcic in nature (three glass fragments and two glass masses) similar to that found for the coeval Germagnana glasses, the other sodic-potassic in nature (four glass masses) with a significantly different composition. Furthermore, a relevant number of vitreous samples, found inside a water tank at Poggio Imperiale (Poggibonsi-Siena) certainly filled in 1313, has been completely characterized. These pale yellow-fumé glasses have a sodic-calcic composition. They have been obtained by melting together ashes from coastal plants with non-particularly pure sand and have been decolorized by the intentional addition of manganese (IV) dioxide. They do not reach the purity grade of the coeval Santa Cristina sodic-calcic glasses. A comparison of the obtained data with those already published for the coeval Medieval Germagnana site or the Renaissance Gambassi site offers a satisfactory view of the glass production technology in Valdelsa and of its evolution from the XIVth to the XVIth century.