Nel blu di pinto di blu: Five Centuries of Blue Roman Glass from the Palatine Hill, A Multi-Analytical Approach to Provenance and Production Technology

The present work is focused on the study and comparison of glass-making processes of the Roman dark-blue glass collection, dated from the 1st century to Late Antiquity, found in the Elagabalus baths in the North East Palatine archaeological site (Rome, Italy), based on major, minor and trace elements characterization. The assemblage consists of 26 shards from different functional objects. The samples were analysed by Optical Microscopy and SEM-EDS to identify homogeneity and glass morphological features. Major and minor element analysis were performed by EMPA whereas trace element composition was done by means of LA-ICP-MS. Chemical results suggest mature sand as silica source, rich in quartz and depleted in heavy minerals. The high Na2O content strongly suggests the use of natron as main flux whereas the high Sr concentration supports the hypothesis of the addition of beach shell fragments as source of lime. The practice of recycling is proposed for some samples in relation to their high Pb content, one of the most used indicators for glass recycling, being lead an impurity in modifiers (decolourants or colourants). This study highlighted a difference in the glass-making processes throughout five centuries of Roman blue glass production. First century glass is mainly Mn-decolourised; from the 2nd to the 4th century, a change to Sb-decolourised Roman glass is observed and finally, during the 4th and 5th century, a high content of heavy elements characterizes the blue glass. The main colourant is CoO, although the presence of Cu in trace amounts can give different shades of blue. On the contrary, two samples were coloured by Cu. A weak trace element correlation with cobalt was identified: specifically, Ni, Zn, Pb, Sn and As, all elements that seem typical of Roman blue glass.