A study of smalt and its conservation problems in two sixteenth-century wall paintings in Rome

AbstractThe durability of glass depends both on its chemical composition and on the environment in which it is conserved. The glassy particles of smalt in samples from two sixteenth-century wall paintings in Rome were investigated to study the technology of glass melting and colouration, and the origin of the pigment discolouration. A potash–lime silica and an unusual soda–potash–lime silica were identified through analysis. These compositions were compared with recipes recorded in Italian medieval and Renaissance treatises that help understand the technology used in the preparation of smalt. The mechanisms of weathering are discussed in terms of glass durability in relation to its chemical composition and to aggressive environmental conditions. In the case of the two wall paintings studied here, smalt discolouration was found to be due to several factors: leaching of alkali, leaching of cobalt, formation of micro-cracks and glass corrosion.