Arsenic and Artistry: Uncovering Secrets in Pietro Lorenzetti’s Paintings

Synchrotron studies highlight the toxic legacy of ground-breaking 14th century art techniques

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.

Investigating Lorenzetti’s Masterpieces

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.

Microscopic Insights into Paint Samples

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.

Read more on Diamond Light Source website

Image: Pietro Lorenzetti, Christ between Saints Paul and Peter, about 1320. Detail showing the mordant gilding on St Peter’s robes. © Ferens Art Gallery, Hull Museums.

Credit: The National Gallery, London.