Green mosaic tesserae from “Grotta del Buontalenti” Giardino di Boboli (Florence)

The Grotta of Buontalenti (GB) was built at the end of the 1500s during the Medicean period. One of the rooms that constitutes the Grotta is decorated with mosaics composed of tesserae of stained glass. The GB underwent several restorations, which took place between the end of the 1700s and 1980s, which are poorly described and documented, leading to some uncertainties about the extent and phase(s) of restorations of the mosaics. The original green tesserae of the GB were analysed for their mineralogical and chemical composition. Cu-based green glasses are known since very ancient time on the contrary modern procedures in glass making have introduced the use of other chromophore elements to obtain green such as Cr and less frequently Ni. The general features of the green glass tesserae reflect at least two different operations. The first one may be referred to the glass colored by Cu (used as chromophore since before the Medicean period) so the green Cu-bearing tesserae are possibly original or reflect subsequently restoration in which the glasses were prepared following ancient procedure. Cr colored glass and in particular the sinter, are instead relatively recent (about 50 years) also supported by the presence of cement in their mortar. The presence of Ni observed in Cu-colored glass can be accidental or intentional to produce drab green. In the case the addition of Ni was intentional the Cu-Ni colored sample has to be included in relatively recent tesserae.