New Insights into the Cleaning of Paintings

Extended Abstract—Oxalate-Rich Surface
Layers on Paintings: Implications
for Interpretation and Cleaning
Ken Sutherland, Beth Price, Andrew Lins, and Irma Passeri
Ken Sutherland, Beth Price, and Andrew Lins, Conservation
Department, Philadelphia Museum of
Art, Box 7646, Philadelphia, Pennsylvania 19101-
7646, USA. Irma Passeri, Conservation Department,
Yale University Art Gallery, Box 208271,
New Haven, Connecticut 06520- 8271, USA.
Correspondence: ksutherland@philamuseum
.org. Manuscript received 19 November 2010;
accepted 24 August 2012.
Although oxalate salts have been the subject of extensive research as alteration
products on calcareous substrates (e.g. stone and fresco), there has been relatively
little discussion of their occurrence on other objects, such as easel paintings. An
understanding of these materials is important since they can be responsible for significant
changes in the surface appearance of artworks and the solubility of the matrices in which
the oxalates are formed. The altered, oxalate- rich surface layers can present substantial
challenges for the visual interpretation of the painted surfaces, the accurate scientific
characterization of the original materials, and the development of appropriate and effective
cleaning methods.
Oxalate- containing layers or deposits have been reported on a variety of noncalcareous
substrates, including glass (Drewello and Weissmann, 1997; Messal et al., 1999),
bronze (Alunno- Rossetti and Marabelli, 1976; Nassau et al., 1987; Mazzeo et al., 1989;
Olmi et al., 1991; Matteini et al., 1991; Selwyn et al., 1996), human remains (mummy
skin; Cotte et al., 2005), and polychrome wood (Arbizzani and Casellato, 2002) and easel
paintings (Salvadó et al., 2002; Bracco and Ciappi, 2002:329; Matteini et al., 2002:396–
397; Sutherland et al., 2005; Spring et al., 2005; Higgitt and White, 2005; Sutherland et
al., 2006; van Loon, 2008:184–186). The oxalate salts of calcium, whewellite (calcium
oxalate monohydrate) and weddellite (calcium oxalate dihydrate), are those most commonly
encountered on painted surfaces, although copper oxalates have also been identified
in paint layers containing copper pigments (Higgitt and White, 2005:93; Castro et al.,
2008). In most cases these compounds are found in deteriorated organic surface layers.
Samples from a number of paintings in the collection of the Philadelphia Museum
of Art, including three Italian paintings on wood panel dating from the fourteenth and
sixteenth centuries, were characterized using Fourier transform infrared microspectroscopy,
X- ray diffraction, gas chromatography mass spectrometry, and scanning electron
microscopy with energy dispersive spectroscopy. The surface layers studied were remnants
of what appeared to be colored glazes in the case of two paintings by Pontormo
(circa 1494–1556/1557) and Jacopino del Conte (1510–1598) and an early varnish on a
painting by Ugolino de Nerio (fourteenth century). Among the components detected in
all three cases were significant levels of calcium oxalates.
A degraded brown layer on Pontormo’s Portrait of Alessandro de’ Medici (1534–
1535; see Figure 1) was studied in detail, and results from the analysis of cross- section and
scraping samples suggested that it consisted of remnants of an early or original pigmented
glaze layer in which particulate surface dirt had become embedded. Fourier transform