Unusual Pigments Found in a Painting by Giotto (C
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Berrie et al. Herit Sci (2016) 4:1 DOI 10.1186/s40494-016-0070-9 RESEARCH ARTICLE Open Access Unusual pigments found in a painting by Giotto (c. 1266‑1337) reveal diversity of materials used by medieval artists Barbara H. Berrie1*, Marco Leona2 and Richard McLaughlin3 Abstract Background: The important trecento Florentine artist Giotto (c. 1266-1337) is renowned for his naturalistic and real- istic works in tempera and fresco. His innovative paintng style involved painting expressive, emotive faces and use of pictorial devices for depicting space. This report focuses on the analysis of the materials and methods used in a panel in the collection of the National Gallery of Art, Madonna and Child (1310/1315). Results: Giotto used inky washes under thin layers of egg tempera paint. Yellow iron earth and lead tin yellow are present in the paint used to depict the lining of the Virgin’s mantle. SEM-EDX of one of the yellow pigments confirmed it is lead tin yellow type II, PbSn1-xSixO3. The ratio of colorant to the glassy phase indicates this material was produced for use as a pigment rather than as a glass. Ultramarine was not used in this painting, azurite is the blue pigment. The azurite used here does not contain elemental impurities, however malachite and the rare green–blue mineral mixite, BiCu6(OH)6(AsO4)3(H2O)3, are found in the blue paint. Keywords: Giotto di Bordone, Mixite, Lead tin yellow type II, Raman, EBSD, Copper arsenates Background which might have been part of the same altarpiece [1]. The important trecento Florentine artist Giotto di Bor- Giotto used techniques similar to those described for done (c. 1265-1337) is renowned for his naturalistic and painting on wood in 1400 by the Florentine artist Cen- realistic works in tempera and fresco. His innovative nino Cennini which have been observed in works by his style was revered in his own time and involved painting contempories [2]. The artist used an inky wash to lay in expressive, emotive faces and employing novel pictorial the shaded parts of the flesh and the mantle. Then, work- devices to communicate a sense of space to the viewer. ing in egg tempera he applied overlapping strokes of thin The panel Madonna and Child attributed to Giotto in the paint to model the forms. For the most part the pigments collection of the National Gallery of Art, Washington, he used are part of the typical early fourteenth century DC, is dated c. 1310/1315 (Fig. 1). It evokes the tender- palette, minus the expensive blue pigment lapis lazuli ness of the human bond between the Christ Child and which is often found in early fourteenth century panels his Mother. The panel is believed to have been part of a and frescos. However, microanalysis has shown in addi- polyptych, but its original location is unknown. tion to typical colorants the unexpected presence of a During a recent conservation treatment a technical green–blue mineral, mixite, a copper-bismuth arsenate, examination was undertaken to obtain information on which has not been reported in any paintings before now. the materials and methods used in creating this panel to In this paper, we focus on the use of microscopy and establish the connection between it and other paintings microanalysis to obtain a more complete characteriza- tion of the yellow pigment, a variant of glassy lead tin yel- low type II, the quality of the azurite, and the discovery *Correspondence: b‑[email protected] 1 Scientific Research Department, National Gallery of Art, Washington, of the presence of the green–blue copper mineral, mix- DC, USA ite. The novel finding of mixite is a reminder that painters Full list of author information is available at the end of the article and other artists and craftsmen used a diverse range of © 2016 Berrie et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/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://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Berrie et al. Herit Sci (2016) 4:1 Page 2 of 9 detector. The lines from different planes of atoms form Kikuchi patterns which can be related to crystal struc- tures without need for a specific orientation of the crys- tals. The Kikuchi patterns can be compared to patterns from standard and vouchered samples. Pattern formation is a result of electron diffraction at the sample surface down to 20–30 nm. For pattern acquisition, a sample is tilted at high angle, typically at 70°, reducing spatial reso- lution of the analysis, however excellent patterns can be obtained from crystallites as small as 10 nm, therefore EBSD can be very useful for identification of pigment particles. The cross-section samples were extraordinarily sensi- tive to water used during the initial grinding and polish- ing process of preparing the cross sections—much more than typical, perhaps due to the ground having a low con- centration of gypsum in glue. This sensitivity meant that the surface could not be polished flat. Argon ion milling was used to try to smooth the surface, but this was not successful. The difficulty of the sample work up meant that the analyses had to be conducted on rough surfaces, nevertheless we were able to obtain interesting results and novel discoveries. From surface examination of the painting the lining of the Virgin’s mantle looks as if it had been painted using a glaze of azurite over a layer of yellow paint. However, Fig. 1 Giotto, Madonna and Child c. 1310/1315, Samuel H. Kress Collection, National Gallery of Art, Washington, D.C. (1939.1.256). The cross sections show that in the painting there are pas- location of the samples described here is indicated by a black dot sages where the yellow is on top of the blue and others where the colors are mixed (Fig. 2). XRF confirmed the general palette of inorganic pig- materials, and instrumental analysis is valuable for their ments by inference from elemental compositon as lead identification, complementing and augmenting infor- white, yellow iron earths, green earth, azurite, lead tin mation given in treatises. The results may also help the yellow, and vermilion. Spectra from the blue drapery investigation of geographical sources, and hence trade, of contained peaks for lead, mercury, and a very small peak pigments used by trecento artists. for the Kβ line of arsenic. This was tentatively thought to be due to orpiment, a pigment that has been found in Results Giotto’s works, but a close surface examination did not This work relied on close visual stereomicroscopy of reveal any of the typical yellow tablet-shaped particles of the surface of the painting and air-path x-ray fluores- orpiment. cence analysis (XRF) analysis. A small number of cross Lead tin yellow type II section samples and paint scrapings were obtained for more detailed characterization of the pigments. All the Two yellow pigments were found: iron earth and lead samples described here were taken from the bottom tin yellow type II, as expected for a fourteenth century edge of the panel at or close to the site in the Madon- painting. Lead–tin yellow type II is a term used to cover a na’s robe indicated by a black dot in Fig. 1. The samples range of pigments that are opaque yellow glasses or glassy were analyzed using scanning electron microscopy with materials [3]. The formula is given as PbSn1-xSixO3 where energy dispersive x-ray analysis (SEM-EDX), polarized x ≥ 0. The crystallographic group of the endmember light microscopy (PLM), microRaman spectroscopy and PbSnO3 is cubic. The backcatter electon (BSE) image of a electron backscatter diffraction (EBSD). EBSD was cru- single particle at higher magnification is shown in Fig. 3a. cial to identificaton of the phases. Electrons can be dif- This image and EDX spectra obtained at diffierent spots fracted by atoms vibrating in lattice planes of crystalline within the pigment particle (Fig. 3b) show that the part- compounds. These diffracted electrons form cones which cle is composed of a Ca–Pb-Si matrix holding densely due to the low angle of scattering appear as lines on a packed crystallites of lead tin oxide, lead silicate and lead Berrie et al. Herit Sci (2016) 4:1 Page 3 of 9 Fig. 2 Cross sections from the yellow-green lining of the Virgin’s mantle at the bottom edge of the panel. Two samples (A and B) both from the very bottom edge of the panel at the black dot in Fig. 1. A was modified by stack focussing using a plug-in for ImageJ. The scale bars are 25 µm. The layers in A are from the bottom: a gesso ground; b a dark inky wash; c a white wash that is more easily visualized in Fig. 5a; d an uneven layer of blue and green particles, azurite (labeled 1), malachite, mixite (labeled 2); e yellow iron oxide; f lead white; g lead tin yellow. The sample in B, taken adjacent to that in A, is simpler: a a layer of gesso is covered by b paint with a mixtures of lead tin yellow and copper-based green and blue pigments, azurite (1) and mixite (2) tin silicate—the grey levels giving a clue to the diversity of the composition of the particles.