An investigation into the deterioration of painted Limoges enamel plaques<i>c.</i>1470–530

Abstract

Painted Limoges enamels that date from c. 1470–1530 are susceptible to localized deterioration in the form of crizzling, weeping and spalling of the glass surface. In an investigation of this problem, 58 Limoges enamel plaques from nine museum collections were visually examined and seven enamels from the collection of the Walters Art Gallery were analyzed by energy dispersive X-ray fluorescence (XRF), a non-destructive technique for surface analysis. Spalled glass particles from one of the enamels were analyzed by electron beam microprobe. Visual examination revealed that the blue- and mulberrycolored areas were the most subject to deterioration but other colors were also vulnerable, including purple, green, brown, yellow and black. The white and flesh tones were the only colors that always appeared stable. The elemental analysis indicated that potassium is the chief alkali in the enamel formulation and that the metallic oxides of manganese, iron, copper and cobalt are the coloring agents. Tin is evident as an opacifier and lead is present as a matrix former and possibly opacifier. There was no correlation between the colorants and the occurrence of deterioration, except for the white and flesh tones. The presence of larger concentrations of lead in the well-preserved flesh-tone areas confirms earlier studies which suggest an increase in lead content can improve the stability of some glasses, possibly by reducing the amount of water adsorbed from the atmosphere.