Dr. Monica Ganio Profile

Dr. Monica Ganio

at Getty Conservation Institute

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Publications (2)

Proceedings Article | 22 July 2019 Presentation

Analysis of the physical characteristics and chemical composition of gold leaf in works of art by scanning macro x-ray fluorescence (MA-XRF) spectroscopy (Conference Presentation)

Douglas MacLennan, Arlen Heginbotham, Monica Ganio, John Delaney, Lynn Lee, Laura Llewellyn, Karen Trentelman

Proceedings Volume 11058, 110580Y (2019) https://doi.org/10.1117/12.2526230

KEYWORDS: Gold, Chemical analysis, X-ray fluorescence spectroscopy, Spectroscopy, Fluorescence spectroscopy, Visualization, Data modeling, Manufacturing, Materials analysis, Trace element detection

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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).

Proceedings Article | 22 July 2019 Presentation

Unbending light: new computational methods for the correction of 3D effects in scanning XRF (Conference Presentation)

Monica Ganio, Stephen Parsons, Seth Parker, Marie Svoboda, Brent Seales, Catherine Schmidt Patterson

Proceedings Volume 11058, 110580S (2019) https://doi.org/10.1117/12.2525038

KEYWORDS: 3D scanning, Calibration, Luminescence, Visualization, Beam shaping, Data corrections, Analytical research, Fluorescence spectroscopy, Spectroscopy, X-ray optics

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Scanning macro‐X‐ray fluorescence (XRF) spectroscopy on works of art provides researchers with rich data sets containing information about material composition and technique of material use in a compelling visual format in the form of element‐specific distribution maps. The accuracy of these maps, however, is influenced by the topography of the object, which ideally is two dimensional, relatively flat and able to be placed parallel to the data collection x-ray optics. In reality, few works of art are truly flat. Small nuances in the visualized elemental intensity may be introduced into element distribution maps by the presence of topography, whether the curve of a centuries-old panel painting, the natural warping of works on paper or parchment, or, in the most extreme cases, in actual three dimensional objects. The inability to confidently ascribe a change in signal intensity to actual elemental composition versus topographically-induced variance, therefore, presents a challenge, particularly when attempting to identify markers of artists’ techniques, compare several objects, or overlay/register images from scanning XRF with those from other imaging modalities. To address this challenge, this paper introduces a new methodology for post-processing scanning XRF data sets to correct for elemental intensity variations as a function of topography. The method augments the acquired XRF data based on a three-dimensional reconstruction of an object and a set of elemental intensity/distance response functions. These response functions act as a calibrated guide for modifying the intensity map based on depth variation. The geometry-based parameters of local surface shape (curvature), distance of the XRF detector from the surface, region of intersection of the incident fluorescence beam with the surface, and the orientation of the incident beam with respect to the surface normal, are each accounted for in the calibration phase as a large set of pre-acquired examples. This provides a mechanism for capturing and understanding the anticipated variations in the macro-XRF data, interpolating the examples in order to smoothly estimate variations, and applying those variations as corrections to macro-XRF data collected on non-planar surfaces. The acquisition and representation of the macro-XRF variation as a function of the geometry is explained, with an emphasis on understanding the parameters that induce the most severe errors in the XRF estimates. The representational framework for collecting, storing, and summarizing calibration data over a large number of scans is discussed, followed by several proof of concept examples, including data from one of the masterpieces of the J. Paul Getty Museum collection: Mummy portrait of a woman (JPGM #81.AP.42), also known as Isidora. This 1st century Romano-Egyptian funeral portrait on wood was originally included in mummy wrappings, and is therefore curved to match the natural curves of the embalmed subject. An XRF scan of Isidora was recently undertaken as part of a long-standing project – Ancient Panel Paintings: Examination, Analysis, and Research (APPEAR) – that seeks to increase our knowledge on the materials and manufacture of paintings of this type. The natural curvature of this panel painting, together with the rich texture typical of the encaustic technique, makes Isidora the perfect candidate to test the proposed methodology.

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