Faded shine…. The degradation of brass powder in two nineteenth century paintings

During the examination of two 19th century oil paintings by Swiss artists, the Reading pastor (ca. 1885) by Ferdinand Hodler (1853–1918) and Portrait of a young girl (ca. 1888) by Filippo Franzoni (1857–1911), it was observed that in particular locations, the surface had an unusually granular texture, given by the application of metallic particles and the presence of large translucent green agglomerates. In this paper we investigate the composition and origin of the metal particles and green agglomerates. A combination of bulk analysis (GC–MS and FTIR) and analytical microscopy techniques (X-ray tomographic microscopy, light microscopy, ATR–FTIR–FPA imaging and SEM–EDX) were used. It was concluded that the metal particles are composed of a lower quality brass (copper/zinc alloy) and ground from foil. The current appearance resulted from the reaction of the brass and its corrosion products with the fatty acids in the surrounding paint/varnish matrix. This has lead to the formation of agglomerates of zinc and copper carboxylates, the latter responsible for their green colour.
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Graphical abstract:
The painting entitled Reading pastor (ca. 1885) by Ferdinand Hodler (1853–1918) was one of two paintings studied showing the formation of zinc and copper carboxylates as the alteration of brass pigment. The characterisation was carried out with combined analytical techniques including X-ray tomographic microscopy and ATR–FTIR spectroscopy.