Marika Spring, Catherine Higgitt and David Saunders
Technical Bulletin Volume 26, 2005
The results of examination of paintings in the National Gallery in which the blue cobalt glass pigment smalt has degraded are presented. The appearance on the paint surface is described; the smalt does not only lose its colour but also causes yellowing of the oil so that the paint appears brownish-yellow or greenish grey. Other paint defects are discussed, such as drying cracks and patchy blanching which can make the paint appear lighter at the surface, suggesting disintegration of the binding medium or efflorescence. Paint samples were examined using light microscopy, energy dispersive X-ray analysis, Fourier transform infrared microscopy and gas chromatography–mass spectrometry, and the results compared with those from artificially aged paint samples. Leaching of the alkaline component of the glass (potassium) as smalt deteriorates was confirmed, as has been found in earlier studies. Quantitative analysis of the smalt particles by SEM-EDX determined the extent of loss of potassium. This would be expected to cause concurrent alteration and degradation of the oil binding medium. Analyses by FTIR microscopy confirm that potassium soaps of fatty acids from the oil medium are formed. These are located both within the paint and in crusts on the surface. Other salts similar to those found on degraded vessel glass, such as calcium oxalate, were also identified in the surface crust. It is suggested that these changes caused by deterioration could disrupt the physical integrity of the paint film, and could explain the appearance of discoloured smalt in paintings.
degradation, FTIR microscopy, glass, paintings, potassium soaps, quantitative analysis, SEM-EDX, smalt
To cite this article we suggest using
Spring, M., Higgitt, C., Saunders, D. 'Investigation of Pigment-Medium Interaction Processes in Oil Paint containing Degraded Smalt'. National Gallery Technical Bulletin Vol 26, pp 56–70.
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