New spectroscopic data on “marmi mischi” baroque glass decorations from “pallium altaris” of Messina (Sicily, Italy)

Abstract

The ―Marmi Mischi‖ (mixed glass mosaic tesserae) are artificial calcium-rich glass decorations, typically used in southern Italy and Sicily during the Baroque period (1600-1700) to decorate walls and altars of religious edifices. In this study, we provide new archaeometric information on the raw materials, pigments and opacifying agents used to produce ―Marmi Mischi‖ of ―Pallium altaris‖ from the Messina Regional Museum. To reach this objective, all the studied glass decorations were analysed by means of non-destructive portable X-ray fluorescence (pXRF). Some of the decorations were further subjected to Environmental scanning electron microscopy (ESEM) and X-ray powder diffraction (XRPD) in the laboratory. Two types of glasses were recognised. The first one – bluish green in colour – is a basic or slightly modified ―carcara glass‖, characterized by high concentrations of P. The second type – dark blue, turquoise, light turquoise and red in colour – is a ―carcara glass‖ modified by craftsmen, containing high amounts of colouring agents (Cu, Pb, Sn, Sb). The presence of schreibersite in some of the analysed samples indicates that forming occurred in extremely reducing conditions and proves that the glasses were produced in CaO furnaces called ―carcara‖ and used to produce lime. Moreover, our results suggest that: i) the dark blue, turquoise and bluish green nuances are as a result of Fe2+, with minor contribution by Cu and Co; ii) the light turquoise nuance is related to Fe2+ and Cu2+, with minor contribution by Co; and iii) the red colour is as a result of the reduction of copper to the metallic state by Sn and Pb. Ca-antimonate (Ca2Sb2O7) was identified as an opacifying agent for the blue glassy decorations, whereas metallic Cu was identified for the red. The overall results of the chemical and physical characterization of the ―Pallium altaris‖ glassy decorations from the Messina Regional Museum suggest that the used raw material was ―carcara glass,‖ to which various chemical elements were suitably added to obtain the desired nuances.