Organic patinas on Renaissance and Baroque bronzes–Interpretation of compositions of the original patination by using a set of simulated varnished bronze coupons

Within the frame of a research project dealing with the identification of ‘organic patinas’ on small bronzes from the collection of the Kunsthistorisches Museum, Vienna, a set of model varnishes on bronze coupons prepared at the Metropolitan Museum of Art, New York, was subjected to GC-MS analyses. The study of model varnishes clarified various uncertainties in the identification and interpretation of natural materials previously detected in the authentic varnishes of Renaissance and Baroque bronze sculptures. For instance, it became apparent that the alterations in the composition of the oil-resinous coatings noted in the authentic varnishes were caused not only by the ageing processes over time but also by the specific method of the preparation and application of the coating by which the varnishes were originally applied on the surface of the bronzes, e.g. that the varnishes were often directly baked on the surface of the statuettes at temperatures exceeding 100 ̊C. Relating the GC-MS results to the compositions of the model coatings helped to better understand some of the difficulties in the detection and identification of original oil-resinous varnish components. In particular, the study allowed the low abundance of the mastic resin in the varnishes to be explained and specific markers distinguishing individual resins within a Pinaceae family have been proposed.