In 2013, the Ferens Art Gallery in Hull acquired Pietro Lorenzetti's Sienese gold-ground panel Christ between Saints Paul and Peter, a newly discovered example of the Italian artist's work. For four years, the painting underwent intensive conservation treatment and scientific study at The National Gallery in London, revealing vibrant colours and minute details previously obscured by layers of discoloured varnish and earlier conservation efforts. Another of Lorenzetti's works, Virgin and Child Enthroned and Donor, Angels, hangs in the Philadelphia Museum of Art. In work recently published in Studies in Conservation, researchers from the National Gallery and the Natural History Museum in London worked with scientists from Diamond's I18 beamline to analyse samples of mordant gilding taken from both paintings, where previous studies had shown the presence of a mordant tinted with orpiment (a bright yellow mineral pigment containing arsenic) used to adhere overlying layers of silver and gold leaf. Synchrotron microfocus X-ray techniques (SR µ-XANES, µ-XRF and µ-XRD) were used to reveal the chemical migration and nature of altered phases in the orpiment-containing mordant layer. The results add to our understanding of the painting techniques used during this exciting period, and will inform ongoing conservation and restoration efforts.
Italian painter Pietro Lorenzetti was active from the early to the mid-fourteenth century. He and his younger brother Ambrogio foreshadowed the art of the Renaissance, experimenting with three-dimensional and spatial arrangements. Previous research carried out by The National Gallery and Philadelphia Museum of Art suggests that two of Lorenzetti's artworks - Virgin and Child Enthroned and Donor, Angels and Christ between Saints Paul and Peter - may have been part of a multi-panelled altarpiece (polyptych), possibly created for the church of Santa Maria dei Servi in Siena. Comparison of X-radiographs of the two panels identified corresponding structural features suggesting that the panels were cut from consecutive flitches of the same log.
says Dr Helen Howard of the National Gallery, London, says:
At this particular time, artists were using a lot of new methods and experimenting in new ways to try to obtain particular effects, particularly with the use of metal leaf. A contemporary painting by Giotto in the Scrovegni chapel, for example, uses four different types of metal leaf: gold, silver, part-gold (where gold and silver leaves are beaten together to form a single leaf) and gold over tin. Artists could also use varnishes and coloured glazes over the various metal leafs. In addition, the colour of the underlying bole or mordant could affect the overall appearance and mordant gilding might also have a slightly raised effect. The end result is very complex, and understanding what materials the artist used - and how they have degraded - is important for restoring and conserving the artwork.
During the conservation of Christ between Saints Paul and Peter, the research team took tiny samples of mordant, one from the golden highlights on Christ's drapery and another from the golden threads depicted in the borders of Saint Peter's robes. Studies showed that Lorenzetti had used a complex orpiment-tinted mordant to adhere two separate layers of silver and gold leaf, and that these materials had degraded over time.
The unusual composition of the samples prompted the team to request that the Philadelphia Museum of Art take comparative samples from gilded areas of Virgin and Child Enthroned and Donor, Angels, where a similar stratigraphy and deterioration was detected.
The researchers worked with scientists on Diamond's I18 microfocus beamline, using synchrotron X-ray techniques to gain a deeper understanding of the layered paint samples.
Dr Howard explains:
The beamline at Diamond was crucial because we could focus down to about 2 microns and look across all the layers to see where the deterioration products are. We used µ-XRF (micro X-ray fluorescence) to map the products of deterioration, and µ-XRD (micro X-ray diffraction) and µ-XANES (micro X-ray Absorption Near Edge Structure). The µ-XANES was particularly important, because it shows us the amorphous products, as well as the crystalline. It's one of the only ways we have to do that, so it's very, very important.
The results show that a thin layer of silver leaf has either reacted with surrounding sulfur-bearing compounds in the orpiment or atmospheric hydrogen sulfide. The darkening of the mordant was most likely caused by finely dispersed grey-coloured acanthite (silver sulfide) particles, although it also contains some silver oxide. There is evidence of arsenolite, indicating photo-degradation of the orpiment, and that elements are migrating through the sample, with chemical reactions occurring between the silver, arsenic and sulfur.
Tina Geraki, beamline scientist at I18 commented:
For me what was most interesting about this project, scientific and conservation interests aside, was how a range of expertise was brought together. Dr Howard had a question that needed solving and in her quest for suitable techniques she paired up with Dr Schofield, one of the most experienced users of I18, although from a very different scientific field. What the team managed to accomplish in these four days of beamtime was remarkable, as they combined a variety of techniques, but the road ahead was even longer. The analysis and interpretation of data took quite involved work by the whole team, and I am very pleased to see the outcome bringing everything together.
The presence of arsenic, in the original orpiment and its degradation products, is vital information for conservators, who must take appropriate precautions when handling the painting to avoid toxicity. This more detailed understanding of the complex nature of the artwork will inform appropriate conservation strategies and treatment for these important works of art.
To find out more about the I18 beamline or discuss potential applications, please contact Principal Beamline Scientist Konstantin Ignatyev:
Howard H et al. Degradation of Fourteenth-century Mordant Gilding Layers: Synchrotron-based Microfocus XRF, XRD, and XANES Analyses of Two Paintings by Pietro Lorenzetti. Studies in Conservation (2023): 1-16. DOI:10.1080/00393630.2023.2201094.
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