Conservation treatments on altered glass. A case study: the glass mosaic of the Porta della Mandorla in Florence

This paper presents the diagnostic analyses conducted on the vitreous tesserae of the mosaic above the Porta della Mandorla, on the north facade of the Cathedral of Santa Maria del Fiore, in Florence, and the studies about the restoration work are presented. The restoration works were preceded by an analysis of the state of conservation of the vitreous tesserae and by an extensive study, performed in the laboratory and in situ, on some products proposed for the conservation of the altered tiles. The products, belonging to different chemical categories, were studied in the laboratory (applied to testing glass subject to thermal and Xenon test) and in situ. This study is still in progress and some preliminary results are reported.