Visualizing and Measuring Gold Leaf in Fourteenth
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MacLennan et al. Herit Sci (2019) 7:25 https://doi.org/10.1186/s40494-019-0271-0 RESEARCH ARTICLE Open Access Visualizing and measuring gold leaf in fourteenth- and ffteenth-century Italian gold ground paintings using scanning macro X-ray fuorescence spectroscopy: a new tool for advancing art historical research Douglas MacLennan1*, Laura Llewellyn2, John K. Delaney3, Kathryn A. Dooley3, Catherine Schmidt Patterson1, Yvonne Szafran2 and Karen Trentelman1 Abstract The size, shape, and application techniques of gold leaf in fourteenth- and ffteenth-century gold ground panel paint- ings attributed to artists working in Florence, Siena, and Fabriano was directly visualized using in situ scanning macro X-ray fuorescence spectroscopy (MA-XRF). The resulting gold (Au) Lα (9.712 keV) element distribution maps are of sufcient spatial resolution to enable an accurate and reproducible measurement of the shape and size of individual gold leaves. The resulting leaf measurements are discussed in the context of historical guild regulations, in particular, the 1403 reforms to the statutes of the Florentine Guild of Doctors and Apothecaries (Arte dei medici e speziale), which standardized the dimensions of gold leaf produced in Florence. The dimensions of the gold leaf from Florentine paintings created before and after the 1403 reforms were compared to each other as well as to gold ground paintings created in Siena and Fabriano. The results revealed the gold leaf squares in fourteenth-century Florentine and Sienese panels had side lengths averaging about 8 cm. In contrast, the gold leaf squares used in the ffteenth-century Floren- tine and Sienese paintings were smaller, with side lengths measuring about 7 cm. In addition, the degree of overlap between adjacent gold leaves was also measured. The amount of overlap was found to vary between artists, and the degree of overlap was consistent within the oeuvre of a specifc artist. Taken together, these results suggest that the dimensions of the gold leaves found in panel paintings relate to the place of production on the Italian peninsula and the period in which they were created, while the degree of overlap relates to the individual hand of a gilder or artist/ workshop. Keywords: MA-XRF imaging, Gold leaf, Gilding, Italian painting, Guilds, Giovanni di Paolo, Gentile da Fabriano, Fra Angelico, Masaccio, Taddeo Gaddi Introduction fourteenth- and ffteenth-centuries. Gilded backgrounds Gold leaf—small, thin, square sheets of gold—was a were composed of individual sheets of gold leaf laid down key component of ornately decorated panel paintings over an iron-rich clay adhesive layer (called bole), and created throughout the Italian peninsula during the burnished to achieve a smooth, mirror-like surface sug- gestive of solid gold [1]. Gold was prized for the embel- lishment of works of art for a variety of factors including *Correspondence: [email protected] its innate preciousness, related symbolic connotations 1 Getty Conservation Institute, 1200 Getty Center Drive, Suite 700, of divinity, and aesthetic appeal [2]. Since gold was a Los Angeles, CA 90049, USA precious commodity, the production of gold leaf was Full list of author information is available at the end of the article © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/ publi cdoma in/zero/1.0/) applies to the data made available in this article, unless otherwise stated. MacLennan et al. Herit Sci (2019) 7:25 Page 2 of 9 subject to strict guild regulations. In Florence, painters six artists working primarily in Florence and Siena (see and other professionals associated with the craft of paint- Table 1 for complete object details). Eleven paintings date ing, including many gold beaters, belonged to the Guild to the early ffteenth century and two were produced in of Doctors and Apothecaries (Arte dei medici e speziale). the 1330s. Te fve panels attributed to the Sienese art- In 1403 this guild issued reforms to its existing statutes, ist Giovanni di Paolo are thought to have once belonged which included new specifcations and standardizations to a single polyptych. Similarly, the two panels attributed for the dimensions of fne gold (oro fne), part gold (oro to Fra Angelico likely originated from a single ensemble. di metà), and silver leaf (ariento battuto). Te ordinance MA-XRF scanning was carried out on each of the paint- established that the costly oro fne was to be manufac- ings, and the size of the individual gold leaf and overlaps tured to a size between 1/8th and 1/9th of a Florentine between leaves in each object measured from the result- braccio di panno, equivalent to squares with side lengths ing gold (Au) Lα element distribution maps. Te results, of about 7 cm in modern units. Te less expensive oro di contextualized within Italian guild regulations, provide metà and ariento battuta were to be manufactured to a an indication of the geographic and/or temporal origin larger size, between 1/7th and 1/8th of a Florentine brac- of a painting and help elucidate the gilding technique of cio di panno, corresponding to squares with side lengths individual artists, providing new information regarding measuring about 8 cm [3–5]. the painting technique of this time. Despite the preponderance of gold leaf in paintings of this period, relatively little is known about the size, shape, Experimental and application patterns of individual gold leaves in Gentile da Fabriano’s Madonna and Child Enthroned, works of art due to the difculty in visualizing them using in the Samuel H. Kress Collection at the National Gal- traditional broadband visible light-, infrared-, ultraviolet-, lery of Art (NGA), Washington DC, was studied at the or even X-ray imaging techniques. In instances where the NGA using a custom-built macro XRF imaging spec- gold leaves are unusually thick, broadband X-radiogra- trometer [21]. Te instrument consists of a 50 WRh phy has been used to examine individual leaves [6], and X-ray tube with polycapillary focusing optics (XOS Labs, a limited ‘view’ may also be possible with the naked eye New Jersey) and an SDD detector with a 50 mm2 active in cases where the gold has been thinned as the result of area (XGLabs S.r.l., Milan). Te painting was moved on surface abrasion or other damage [7, 8]. However, in the a motorized 2-D computer-controlled easel (SatScan Sys- vast majority of cases, traditional imaging techniques do tem, LG Motion, UK) in front of the X-ray source and not provide for the direct visualization of gold leaf neces- detector. Te gold distribution map was obtained by inte- sary to characterize any potential variation in the dimen- grating the area under the Au Lα emission line (ftted to a sions of the individual leaves. Gaussian function) [22]. Te utility of scanning macro-X-ray fuorescence (MA- Te remaining twelve panels were scanned at the Getty XRF) spectroscopy for the study of paintings and manuscript Conservation Institute (GCI) using a Bruker M6 Jet- illuminations—primarily to characterize the distribution of stream macro XRF spectrometer (Bruker Nano GmbH, elements from which pigments may be inferred—has been Berlin). Te M6 is outftted with a 30 WRh X-ray tube previously demonstrated with great success [9–11]. Impor- with polycapillary focusing optics. Te scanning param- tantly, the element distribution maps generated by this tech- eters were selected individually for each object, deter- nique can also provide new information that results in abetter mined both by the overall size of the scan and the level understanding of other aspects of an artist’s technique. For of detail that could be achieved in the time during which example, scanning MA-XRF as well as other spatially- the painting was available for study. Te gold element dis- resolved imaging spectroscopies has allowed for the visuali- tribution maps were generated using the Bruker ESPRIT zation of preliminary or abandoned versions of compositions, software. revealed the sequence in which paint layers were applied, For all thirteen scans, a spot size between 500 and and even helped track changes that may have occurred over 1000 µm was employed, ensuring comparability of the time [12–20]. In this work, the use of MA-XRF imaging spec- data sets and providing sufciently high spatial resolution troscopy to visualize individual gold leaves in Italian panel to accurately measure the dimensions of the individual paintings was explored. It proved to have sufcient spatial gold leaves. Specifc instrumental acquisition param- resolution to enable the size of individual gold leaves to be eters for each scan are provided in the appropriate fgure measured with unprecedented accuracy. Tis new informa- captions. tion allows researchers to better understand the place and Te intensity of the Au Lα signal is higher in those areas manner by which these objects were created. where, in some areas (see e.g., Figs. 4c, 5c) it can be seen Te current study was carried out on a group of thir- with the naked eye that two gold leaves overlap. Similar teen Italian gold-ground panel paintings attributed to areas of higher intensity, visible throughout all the maps, MacLennan et al. HeritSci (2019)7:25 et al. Table 1 List of scanned objects and summary results of leaf measurements Artist Title, owner, accession no. Date Region n Mean vertical Mean horizontal Mean leaf Mean horizontal Mean vertical leaf length (cm) length (cm) length (cm) leaf overlaps (cm) overlaps (cm) Simone Martini St Luke 1330 s Siena 14 8.0 0.2 8.1 0.2 8.1 0.2 0.5 0.1 0.5 0.1 J.