Paint cross-sections of five 16th century paintings with areas of discoloured smalt oil paint were investigated. Semi-quantitative SEM-EDX analysis revealed that potassium was relatively low in all discoloured smalts, while cobalt remained at an approximately steady level. Reflection light microscopy demonstrated the presence of partially discoloured smalt particles with a remaining blue core. Imaging SIMS demonstrated that cobalt has a uniform distribution 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 cores always have a higher K level compared to the discoloured rim of the particles. The loss of potassium from smalt in oil paint is interpreted as a leaching 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 semi-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 suggest 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 possibly related to the early stages of oxidation and hydrolysis of the cross-linking oil paint. The differences in potassium levels in the remaining blue part of the smalt articles point to variability in the quality of the base potash glass in the 16th century. Analysis of partially discoloured smalt from two panels of a triptych by Dirck Barendsz (1534-1592) demonstrated that he had access to two chemically different smalts.

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Journal Chimia
Boon, J. J, Keune, K, van der Weerd, J, Geldof, M, & van Asperen de Boer, J. R. J. (2001). Imaging microspectroscopic, secondary ion mass spectrometric and electron microscopic studies on discoloured and partially discoloured smalt in crosssections of 16th century paintings. Chimia, 55, 952–960.