Authenticating French Gilt Bronzes through X-Ray Fluorescence (XRF)

Eighteenth-century Parisian gilt bronzes were widely counterfeited in the 19th and 20th centuries. Authentication had been extremely challenging until the improved use of x-ray fluorescence (XRF), which reveals the alloy composition of the artwork. The goal of this project was to develop reference standards and build a collaborative database that tracks alloy compositions found through advanced machine-learning techniques to support multiple museums in their efforts to authenticate and accurately date French gilt bronzes.

• With a consortium of other museums and universities, we developed a new set of certified bronze and brass reference standards known as the CHARM (Cultural Heritage Alloy Reference Material) set, specifically tailored to the study of historic copper alloys using XRF. This has been critical to making our analytical results comparable with those of our collaborators.
• A detailed XRF calibration protocol has been defined that utilizes the new CHARM standards along with a freely available open-source software for spectral analysis (PyMca).
• The first iteration of the alloy database has been published along with the machine-learning protocol used for authentication and dating.
• The data compiled to date have yielded considerable insight into the working methods and materials of French bronze founders and has proved to be of considerable utility for the evaluation and authentication of gilt bronzes in Getty’s collection.

For decades, researchers and scientists have tried to use bronze alloy composition to aid in authentication and attribution of works of art. XRF is an appealing, non-destructive technique for measuring alloy composition, but there have long been questions about the accuracy and precision of the method.

In 2007, the National Gallery of Art in Washington, DC gathered representatives from seven museums to address how to accurately share comparable, quantitative XRF data between institutions. That meeting resulted in a study on inter-laboratory reproducibility of results, which concluded that there was a great need for improvement.

In 2010, a subset of researchers began the process of designing and commissioning a new set of reference materials, optimized for quantitative XRF analysis of heritage copper alloys, leading to the development of the copper CHARM set.

Around 2014, work began on developing new software solutions for the interpretation of XRF spectra taken from bronze and other copper alloy artworks. This led to the dissemination of the so-called CHARMed PyMCA protocol which can be used by museums and universities around the world to ensure accurate and precise measurements.

The majority of the data about Getty’s collection dating has been acquired using portable, handheld XRF instruments. Analysis of copper alloys, as currently practiced at Getty, returns compositional data for 13 elements, including the major alloying elements, copper, zinc, tin and lead. Over 1,000 compositional analyses have been performed on objects with known provenance, which are used as ground truth for authentication and dating.

The XRF studies at Getty have focused on the period from 1675 to the present. Thus far, the data gathered include the results of approximately 1,300 XRF analyses of discrete components belonging to approximately 250 different objects.

Partnerships with other institutions have been critical to advancing research using XRF technologies and to establishing and using the CHARM set. This includes but is not limited to the Metropolitan Museum of Art, the Rijksmuseum, the Frick Collection, the Wallace Collection, the Philadelphia Museum of Art, the Centre for Research and Restoration of the Museums of France, the British Museum, the Kunstgewerbe Musuem Dresden, and the Stiftung Preussische Schlösser und Gärten.