Organic Patinas on Small Historical Bronzes: From Mock-Ups to Actual Artworks

This paper deals with the study of organic coatings (patinas) on historical bronzes, specifically those applied on small-size statues in Renaissance workshops. These coatings, often transparent and translucent, contain a mixture of organic and inorganic components and may be still preserved in hidden parts of statues in indoor displays. However, the complexity of the original varnishes, their degradation and alteration over time, and the coexistence of materials added for conservation and maintenance purposes are challenging for their characterization. The often well-preserved surface of varnished bronzes and their small size make it mandatory to make the most of using noninvasive techniques for their investigation. To this end, to simulate the actual historical coatings, we prepared a set of mock-ups following ancient recipes and using materials that were available in the Renaissance. We used the samples to assess to what extent it is possible to disclose the formulation (binders, colourants, and other additives) and the thickness of a Renaissance patina with noninvasive methods. Microprofilometry (MP), optical coherence tomography (OCT), and eddy current (EC) gauge were tested on the samples and the results were combined with reflectance Fourier transform infrared (FT-IR) spectroscopy. The analyses performed on the mock-ups set the ground for investigating a Renaissance bronze featuring reddish semi-transparent varnish layers. The achievements are discussed in this paper, along with the limitations of the use of a noninvasive approach.