Nano-Computed Tomography as a Tool for the Morphological Characterisation and Validation of Paint Mock-Ups

The development of new conservation treatments is ideally carried out on model systems, so-called mock-ups, thus avoiding direct testing on cultural heritage objects. Additionally, mockups allow for statistically significant test results as they provide unlimited, reproducible test material. They are prepared with similar or same classes of materials as identified in the objects they try to mimic. In the case of paint mock-ups, used for the development and assessment of consolidation methods, the creation of model systems with similar morphology, including porosity, is optimal, even if technically challenging. To produce paint mock-ups, representative for a painted area of the painting "Beach Landscape with Trees and Boats" (1905-06) by Edvard Munch, microsamples of the target area were analysed with a combination of analytical methods. FTIR and Py-GC-MS were used for the identification of pigment and binder and nano computed X-ray tomography (nano-CT) for the characterisation of the porosity. The pigment and binder were identified as synthetic ultramarine blue and casein, respectively. The pigment-binder ratio was estimated based on the FTIR spectra. According to the results, three mock-up types were prepared with three different types of synthetic ultramarine blue pigments: two commercially available and one historical synthetic ultramarine blue pigment from the MUNCH’s collection of historical art materials by E. Munch. To compare their porosity, tomography on samples of the original and the three mock-up types was performed using a laboratory-based nano-CT setup. The results showed that all tested synthetic ultramarine blue pigments have distinct particle size distributions. Thus, none of the model systems exactly could reproduce the porosity found in the original. However, the combination of results on chemical composition, approximate pigment-binder ratio, porosity, and pigment particle size distribution allowed the selection of the model system which properties were closest to those of the original.