Measuring 15th century gold samples from the Baptistery Gates of Florence
15 Dec 2023
- Alison Oliver



A long-term collaboration using our muon facility continues to reveal fascinating insights into the manufacture of one of the famous depictions of the Old Testament.




South Gate of Baptistery.jpgISIS Neutron and Muon Source has had a long-term collaboration with the National Research Council (CNR)​ and the National Institute for Nuclear Physics (INFN)​ of Italy, which has led to many different projects​ over the years. In 2017, the collaboration with INFN led to the launch of the CHNet-TANDEM project, which involved the implementation and development of non-destructive and non-invasive large-scale facilities techniques for Heritage Science. In particular, the project aimed at the improvement of the Muonic atom X-ray spectroscopy technique (µ-XES), performed at RIKEN port 4 in Target Station 1. The method uses negative muons to unveil the elemental composition of materials. In the first stage, preliminary experiments were conducted on samples from the Museum of Archaeology in Florence, Italy – two small pieces of votive boats from 7th-8th century BC – to understand how they were originally manufactured.

M_supervisors3.JPGMassimiliano Clemenza, Technical Manager of the Radioactivity and Mass Spectrometry Laboratory at the Università degli Studi di Milano Bicocca supervises ISIS PhD student Matteo Cataldo, who is jointly financed by Milano Bicocca and ISIS. Matteo's research focuses on the characterization of gilded objects, centring upon the data analysis of the gold layers of the Baptistery of St John in Florence. The octagonal baptistry stands in both the Piazza del Duomo and the Piazza San Giovanni, across from Florence Cathedral and the Campanile di Giotto. The Gates have been praised by generations of artists and art historians for their compelling portrayal of scenes from the Old Testament. Over time, the seventeen-foot-tall, three-ton bronze doors became an icon of the Renaissance and one of the most famous works of art in the world. The workmanship of panels demonstrates that the Florentine artists had mastered linear perspective and the classical idiom by the early 15th century.

The materials examined at ISIS were from the South doors, designed by Andrea Pisano, which originally took six years to complete. The gate consists of 28 quatrefoil panels, called “formella", with twenty depicting scenes from the life of St John the Baptist.  They were originally installed in 1336 and transferred in 1452 to their long-term location at the Baptistery where they remained until after five hundred years of exposure and damage, when the original panels were moved to the Museo dell'Opera del Duomo, the museum of the Duomo's art and sculpture for restoration work.


In 2019, when the Gates were being taken down, there was a narrow window of opportunity to perform experiments, which took place with muons and on our instruments INES and IMAT. For the experiment, the formella of the Annunciation to Zachary travelled to ISIS where muon instrument scientist Adrian Hillier led the muons investigations together with Francesco Grazzi, the principal investigator. The aim of the muon experiment was to assess the thickness of the gold layer covering the figures depicted on the formella. Results reported a similar thickness of the gilding for the Angel and Zachary figures, of around 16 microns, meaning that the figures could have been gilded together. Whereas for the altar that divides the two figures, the analysis revealed strange distributions in terms of depth, which was surmised as being due to chiselling of the surface of the gold, with thickness that ranged from a few microns up to as many as 50. ​

This original analysis led to interesting results that required better data interpretation. That is why Matteo, in his current experiment, is measuring standard samples of 3, 5 and 8 micron gold layers. With the known thickness of the gold layer, by modelling the sample in simulation software and comparing the data of the analysis, Matteo can then try to assess this thickness. This is highly useful because by doing this, he can validate the simulations with real experiments. Matteo is now focussing on gold layers to continue Adrian's work from the past and close in on the analysis for the samples for their thickness to be measured. He is also working on other standards as they are interesting to measure and much of his PhD is centred on Monte Carlo simulations, which is a tool to use for particle physics.

“This is an interesting project due to the application of muons, because this method is very useful to investigate layered materials. With muons, we can penetrate depending on the material density and size," explained Matteo. “With gold, the instrument RIKEN is excellent but with other common techniques such as X-ray or X-ref, it is not so easy to investigate. Thanks to muons and the simulations, we can try to assess the size and thickness of the materials.

Contact: Oliver, Alison (STFC,RAL,ISIS)