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With its intrinsic preciousness, symbolic, and aesthetic connotations, gold leaf was essential to the decoration of artworks from antiquity to the Renaissance. Despite this importance, the physical characteristics and chemical composition of gold leaf in works of art have not, to date, been extensively studied. In this work, case studies from a number of works of art demonstrate the ability of scanning macro X-ray fluorescence (MA-XRF) spectroscopy for in situ, non-invasive examination of the physical characteristics and the elemental composition of historic gold leaf in works of art in different media. Macro-XRF scanning has opened up new avenues of research by providing insight into how these micron-thin sheets of gold were manipulated and applied in individual objects [1,2]. Besides elemental composition, data provided by MA-XRF on gilded objects includes the visualization of the shape, size, and application techniques of individual gold leaves. For example, measurements of the dimensions of individual gold leaves obtained directly from XRF map data of thirteen 14th and 15th century Italian panel paintings reveals differences in the dimensions of gold leaf across different administrative regions throughout the Italian peninsula during this period. This work suggests a deterministic system of leaf production, which varied between the city-states, controlled by the different city guilds. In addition to leaf dimensions, a comparison of the degree to which gold leaves were overlapped during the gilding process reveals important clues about the artistic hand of individual artists. Overlap measurements appear consistent between artworks ascribed to the same artists, even when that artist is gilding with gold leaf of different dimensions, but differ between artworks painted by different artists. Taken together, these measurements advance art historical scholarship by providing a material understanding of artistic practice. In addition to elucidating several facets of the original artistic creative process, XRF maps have also helped identify subsequent interventions, providing new evidence of possible historic conservation or restoration efforts.
Our knowledge of how gold leaf was manufactured is based on historical treatises rather than material analysis. The ability to confidently detect variations in minor and trace elements in historical gold leaf may provide new tools to better understand dating, location of production, and trades. The suitability and inherent limitations of MA-XRF for the semi-quantitative analysis of the gold leaves, based on a feasibility study using a set of modern gold leaf samples will also be discussed. A ground truth for the concentrations of these, and other trace, elements was validated using inductively coupled plasma-mass spectrometry (ICP-MS). Linear regressions for gold, silver, and copper provide a model for relating XRF intensity to concentration that can be tested on the XRF map data obtained from the historic artworks.
[1] D. MacLennan, L. Llewellyn, J.K. Delaney, C. Schmidt Patterson, Y. Szafran, K. Trentelman, “Visualizing and measuring gold leaf in 14th and 15th century Italian gold ground paintings using scanning macro X-ray fluorescence spectroscopy (MA-XRF),” Heritage Science, Submitted (2019).
[2] D. MacLennan, L. Llewellyn, “Visualizing and measuring gold leaf: Case Study: Gentile da Fabriano,” The Burlington Magazine, Submitted (2019).
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