GLASS AND CERAMICS Mirrored with molten lead: Convex mirror glass through the ages Gerhard Eggert* State Academy of Art and Design Stuttgart, Germany gerhard.eggert@abk-stuttgart.de https://www.abk-stuttgart.de/ Andrea Fischer State Academy of Art and Design Stuttgart, Germany andrea.fischer@abk-stuttgart.de *Author for correspondence Keywords amalgams, basic sodium lead carbonate, conservation, drossing, glass mirrors, lead, mirroring Abstract Convex glass mirrors were produced in the Ro- man, mediaeval and Renaissance periods with molten lead rather than tin. The melt was poured into hot-blown spheres and then spread over the inner surface by swivelling or rotating. According to source texts, lead alloys came into use in the 16th century, with Kunckel being the first to de- scribe lead-bismuth amalgams. Lead mirroring of flat glass was only achieved for the first time in Birmingham in the 19th century. Elemental analy- sis can determine the alloys used, and stable lead isotopes hint at the provenance; the presence of radioactive lead-210 points to recent production. In the samples studied, in addition to usual lead corrosion products, a basic sodium lead carbonate was found due to corrosion induced by soda glass. These types of mirrors should be stored in an at- mosphere that is not too dry (to avoid crizzling) and free from carbonyl pollutants. Some conser- vation materials and solvents (acetate esters) can be sources of acetic acid that have hitherto been overlooked. ‘[Y]et nothing doth so plainly represent Images as Lead foil‘d upon Glass.’ Giambattista della Porta, Natural Magick, Bk. 17, ch. XXII INTRODUCTION According to the traditional holistic description of the scope of the ICOM-CC Glass and Ceramics Working Group, it ‘deals with the technology, deterioration, conservation and restoration of glass and ceramics […]’. In light of this, this contribution aims to develop a similar approach to a nearly forgotten group of objects despite its two millennia of history ranging from the ancient Roman Empire to modern India: convex glass cut from hollow spheres mirrored inside with molten lead. For this purpose, this study combines information from historical source texts, archaeological finds whose scientific investigation included examining the corrosion, reports of current manufacture in India, the authors’ own reproduction experiments and considerations on conservation. TECHNICAL ART HISTORY Polished stones or metals were used as mirrors in antiquity. According to Pliny the Elder (Naturalis Historia, Book 36, 193), glass mirrors were invented in Sidon. Indeed, small convex (therefore attenuating) glass mirrors from the Roman, mediaeval and Renaissance periods have been documented in objects and pictures. People were familiar with them. Parmigianino (1503–1540 CE) used one for his Self-portrait in a Convex Mirror (Figure 1), which depicts the young artist in the middle of a room distorted by the use of a convex mirror. The hand in the foreground is greatly elongated, illustrating the optical differences between reflections in flat versus convex mirrors. Occurrences of convex lead-coated glass Some leaded convex mirrors from the Eastern and Western Roman Empire have been found dating from the 2nd to the 4th century CE (Saldern 2004, 517), but they seem to be quite rare. In the example in Figure 2, the only indication of the original existence of leading is some white corrosion containing lead (detected by portable X-ray fluorescence (pXRF) spectroscopy, see below). Relevant handbooks and encyclopaedias provide very little information, if any; for a recent more detailed discussion see