Renaissance Venetian filigree glass: A successful invention investigated through the analyses of archaeological samples

Filigrana (filigree glass) is a sophisticated decorative technique, which makes use of colourless canes with thin, white, or sometimes coloured, opaque glass threads twisted in different ways, encased in transparent (colourless or coloured) glass and incorporated into blown objects. The first official document related to the filigrana manufacture is a patent application dated October 1527 of two Muranese glassmakers, stating that they had invented an entirely new technique “a facete con retortoli a fil” (bands with twisted threads). The success was immediate and the export to other parts of Europe spread rapidly. Since the second part of the 16th century the façon de Venise glass produced in several places outside of Venice included also filigree works. In Murano the filigree production continues today. Despite the importance of Renaissance Venetian filigree, only a few analyses of these objects were published. In this paper for the first time their technology is investigated based on quantitative chemical analyses by SEM-EDS and µEDXRF of sixteen (four of which decorated also with blue glass) of archaeological sherds excavated in the Venetian lagoon dated to the 16th - and early 17th century. The chemical composition of the transparent glass and of the white opaque glass (the average composition and the composition of the glassy phase and crystals) were determined and discussed. The composition of the colourless and blue transparent glass corresponds to the traditional soda-lime-silica Venetian vitrum blanchum glass. The white particles are tin oxide crystals added as lead–tin calx to a soda-lime-silica glass, 27 which in general differs slightly in composition from the glass of the object. Analyzes of the glassy phase between the tin oxide crystals, carried out for the first time, show that it systematically contains tin, in variable (and considerable) amounts from one sample to another.