Organische Überzüge auf metallischen Museumsobjekten
The innovative application of the advanced analytical techniques GC-MS, Py-GC-MS, and FTIR-microscopy for the investigation of organic coatings on metal museum objects
In the frame of the project running between 2005-2010 the Conservation Science Department and the Collection of Sculpture and Decorative Arts of the Kunsthistorisches Museum collaborated with several institutions involved in the field of the preservation of Cultural Heritage, in particular with the University of Applied Arts Vienna, the Metropolitan Museum, New York, and the Institute of Chemical Technology, Prague. This multidisciplinary cooperation of conservation scientists, restorers, and art historians was necessary to fulfil the goals of the project. Its first part was devoted to the investigation of organic coatings – “patinas” – from Renaissance and Baroque bronze statues belonging to the Collection of the Sculpture and Decorative Arts (Kunstkammer) of the KHM, while the second part was dealing with the investigation of the organic multilayer coatings on metal sarcophagi from the Imperial Crypt Vienna (Kapuzinergruft).
Before the research on authentic patinas from bronzes could be performed it was crucial to clarify numerous uncertainties in studying natural materials by using gas chromatography-mass spectrometry (GC-MS) as analytical technique. Therefore, a series of model coatings empirically prepared according to medieval recipes in the Metropolitan Museum were studied. The data obtained from this pilot research enabled the identification of the nature of various natural resins and oils. The organic patinas used on the bronze sculptures are actually oil or oil-resinous varnishes based on drying oils, like linseed oil or walnut oil, cooked together with various diterpene and triterpene resins. Within the project nearly 70 sculptures – most of them of Italian provenance and dated to the 16th century – were examined. The chemical composition of their patinas appears, in general, to be quite similar. The final tint of the translucent patinas – from reddish-brownish to greenish or golden coloured – seems to depend as much on how they were applied as it does to their composition. The colour is developed by baking the varnished metal at a temperature over 100 oC. In addition, the assumption of associating a certain artist with a particular varnish recipe could not be confirmed. In fact, the Renaissance and Baroque sculptors were quite versatile, using individualised methods for each single object.
Furthermore, the technical examination of the surface coatings from three tin-alloy sarcophagi from the Imperial Crypt – the principal burial site of the Habsburg dynasty since 1633 – was performed. Since unfavourable climatic conditions in the Crypt were always a drawback till the recent past, the metal sarcophagi were subject to corrosion processes. Consequently, the serious state of the surface damage evoked several restoration campaigns, when sarcophagi were treated with different types of coatings to protect the original metal surface. However, after these interventions a new problem came up, as many damages occurred right within the parts where the secondary coatings had been applied. Knowledge of the chemical composition of the multi-layered coating systems as well as their influence on the metal surfaces became, therefore, the crucial questions to be answered. Hence, a set of samples from three tin alloy sarcophagi was studied to detect the number and stratigraphy of the secondary surface coatings as well as their composition.
All three sarcophagi are made of a tin alloy with a very high portion of tin and are rather corroded on their metal surface. The deterioration was caused, as anticipated, by the corrosion of metallic tin into tin oxides and not by the transformation of tin (tin pest). The microscopic investigation showed that the coatings were composed of several layers containing either pure organic binder or binder with an inorganic filler composed of tin powder. As organic binders GC-MS mostly detected epoxy resin together with drying oils and additional traces of pine resin, shellac, and beeswax. The investigations support our hypothesis that despite of the recent improved indoor environment the corrosion processes have not been completely inhibited. The reason for the progressive corrosion are most probably fine cracks, both in the coatings and on the metal surface, caused by the different expansibility of the metal and the thick coating layers. Through these cracks even a negligible amount of humidity still penetrates to the metal leading to further corrosion in the long term and, therefore, influencing future restoration campaigns.
V. Pitthard, M. Griesser, S. Stanek, H. Hanzer, C. Kryza-Gersch, “Comprehensive Investigations of the Organic Patina on Renaissance and Baroque Indoor Bronze Sculptures from the Collection of the Kunsthistorisches Museum, Vienna”, in: J.H. Townsend, L. Toniolo, F. Cappitelli (Eds), “Conservation Science 2007. Papers from the conference held in Milan, Italy, 10.-11.5. 2007”; London (2008) 49-55.
V. Pitthard, R. Stone, C. Kryza-Gersch, S. Stanek, M. Griesser, H. Hanzer, “Organic Patinas on Renaissance and Baroque Bronzes – Interpretation of Compositions of the Original Patination by Using a Set of Simulated Varnished Bronze Coupons”, Journal of Cultural Heritage 12 (2011) 44-53.
R. Stone, “Organic Patinas on Small Bronzes of the Italian Renaissance”, Metropolitan Museum Journal 45 (2010) 107-124.
T. Bayerova, V. Pitthard, M. Griesser, M. Griesser-Stermscheg, “Metal Sarcophagi of the Habsburg Imperial Crypt, Church of the Capuchin Friars, in Vienna, Austria: Analysis of Surface Coatings”, Studies in Conservation 57 (2012) 19-26.
DI Dr. Martina GRIESSER
Dr. Václav PITTHARD, DI Sabine STANEK, Mag. Helene HANZER (Kunstkammer), Dr. Claudia KRYZA-GERSCH (Kunstkammer), in Kooperation mit der Universität für angewandte Kunst, Institut für Konservierung und Restaurierung
FWF Projekt Nr. L187-N11