The blues of Romuliana

The paper presents a set of glass fragments excavated at several different locations within and outside the late Roman fortified imperial residence Felix Romuliana (Gamzigrad, Serbia). This small group of eighteen fragments and mosaic glass tesserae are distinguished by their cobalt blue colour. The majority of the finds are mosaic tesserae (six pcs) and sheets of glass (five pcs), which could be related to architectural decoration (sectilia panels). Others are pieces left behind from secondary glass working (four pcs). There are also two fragments tentatively identified as window pane pieces, and only one find is a vessel sherd. The materials are dated to the 4th century. Significantly, some of the production debris and the two ?window pane? fragments were found inside the destruction of a glass furnace. The analyses of the chemical glass composition of the finds confirmed that the blue colourant in all samples is cobalt, and antimony is also present at notable levels (except for one sample), likely to produce opacification of the glass. Regarding the origin of the raw glass, the data on almost all pieces suggests a Syro-Palestinian provenance, and a single sample could be related to Egyptian primary glass production. Importantly, the concentrations of the oxides added to the base glasses in order to modify the colour are positively correlated in certain samples, hinting at the makeup of the cobalt bearing ingredient and at a likely existence of particular production practices of the late Roman period.