Surfaces and interfaces in multilayered paint systems – analytical challenges in art conservation sciences

The surface of an artwork is the interface to its environment and its viewer. The choice of a specific behaviour and appearance by the artist is an essential part of the making and – of how we perceive the expression of the artist. Colour hue and intensity, reflectivity, surface structure, surface finishing and protection, etc. are elemental properties that are achieved with an often complex, multilayered and multicomponent system build-up of paintings or painted and varnished artwork. Chemical interaction between these components and layers, which age over time, influenced by environmental conditions, can lead to reactions that alter the visual impression and/or integrity of an artwork. Conservation science is thus confronted with alterations at different interfaces and levels such as for example the pigment–environment (e.g. Cato et al. 2017), pigment-binder (Fig. 1, Ferreira et al. 2015) and the varnish-environment (e.g. Soulier et al. 2012) interface.
Contrary to the development of new materials, understanding and reconstructing the past is generally hampered by alterations through time, poor or lost documentation, the limited availability of sample material that is scarce and, in the case of e.g. paintings or musical instruments, is barely accessible due to the high value of the objects. Furthermore, the experimental nature of the artists leads to the situation where the analyst is confronted with non-standardised materials and unconventional working techniques, such as for example the “tempera” painting technique (e.g Zumbühl et al. 2014; Ferreira et al. 2015). The variety of painted substrates (paper, polymer, textile, leather, wood, metal, glass, rock, mortar…) also implies that we have to deal with a wide range of surface properties and interface behaviour. Generalised approaches thus end in casework when it comes to interpretation of analytical data. The lack of historic reference material may lead to experimental re-production according to ancient recipes, in the case of Guignet green unveiling temperature dependant surface behaviour of pigment particles and as such variable end products (Zumbühl et al. 2009).
Our current strategy is minimally invasive, combines the complementary techniques of infrared imaging (ATR-FTIR-FPA), Raman spectroscopy and scanning electron microscopy (VP-SEM, SE/BSE/EDS), applied sequentially to micro-samples. This combination not only covers a wide range of materials, but also delivers morphological and 2D stratigraphic information from minimal amounts of sample material. Specific developments such as the methodology to derivatise interfering compounds prior to FTIR measurement have further helped to significantly increase the selectivity of infrared spectroscopy and reach a new level of information in reconstructing historic coatings (Zumbühl et al. 2017).


Figure 1. a) ‚Portrait of a young girl’ by Filippo Franzoni (ca. 1888, oil on canvas). b) Close-up view of the surface showing ‚gold’ particles turning green. c-e) crosssection of metal particle showing alteration products from pigment-binder interaction. (Ferreira et al. 2015)


Literature References
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Ferreira E. et al. (2015) Herit Sci 3(1): 1-11.
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Ferreira E. et al. (2016) In: Beltinger K. & Nadolny J. (Eds) KUNSTmaterial 4, Archetype Pub., Zürich, 205-227.
Soulier et al. (2012). Z Kunsttechnol Konserv 108-117.
Zumbühl S. et al. (2009) Stud Conserv 54, 149-159.
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Zumbühl S. et al. (2014) Appl Spectrosc 68(4): 458-465.
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Zumbühl, S. et al. (2017) Microchem J 134, 317-326.
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https://www.hkb.bfh.ch/en/campus/art-technological-laboratory/