Imaging microspectroscopic, secondary ion mass spectrometric and electron microscopic studies on discoloured and partially discoloured smalt in cross-sections of 16th century paintings

Paint cross-sections of five 16th century paintings with areas of discolour ed smalt oil paint were investigated. Semi-quantitative SEM-EDX analysis re vealed that potassium was relatively low in all discoloured smalts, while c obalt remained at an approximately steady level. Reflection light microscop y demonstrated the presence of partially discoloured smalt particles with a remaining blue core. Imaging SIMS demonstrated that cobalt has a uniform d istribution in the glass particles. Potassium however shows relatively high levels in the oil paint matrix around the discoloured glass particles and an uneven distribution in smalt particles with a blue core. These blue core s always have a higher K level compared to the discoloured rim of the parti cle. The loss of potassium from smalt in oil paint is interpreted as a leac hing process which lowers the basicity of the glass below a critical level for colour maintenance. The colour change of the smalt is thus an indicator of a change in alkalinity inside the glass. The critical level appears to be near a K:Co of about 1:1 in 16th century potash glass on the basis of se mi-quantitative SEM-EDX data. The migrated K is thought to be accommodated on the many fatty acid groups of the mature oil network ionomer in the aged oil paint. Ca was observed in a number of blue remaining smalts which sugg est a role in preservation of the blue glass or retardation of the leaching process. It is estimated that the colour change is an early phenomenon pos sibly related to the early stages of oxidation and hydrolysis of the cross- linking oil paint. The differences in potassium levels in the remaining blu e part of the smalt particles point to variability in the quality of the ba se potash glass in the 16th century. Analysis of partially discoloured smal t from two panels of a triptych by Dirck Barendsz (1534-1592) demonstrated that he had access to two chemically different smalts.