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New research by a team at the UK’s ISIS neutron and muon research facility has revealed how fundamental components of the over six hundred year old Milan Cathedral were constructed and also demonstrated a method to independently validate new on-site monitoring techniques that could help protect ancient buildings in the future.

Iron tie rods visible between pillars of Milan Cathedral. (Credit: M. Bellanova, Politecnico di Milano)

Researchers from Università Milano-Bicocca and Politecnico di Milano worked alongside a team from the UK’s ISIS Neutron and Muon Source in Oxfordshire and a team from the Budapest Neutron Centre to non-destructively study this artefact, an ancient metal tie-rod.

Tie rods are long metallic rods that reinforce buildings. Each tie rod in Milan Cathedral is between 9-18 metres long and is suspended between 16-30 metres above the ground in the nave of the cathedral. After the broken tie rod was discovered in 2012 several studies were carried out on the sample. The remaining suspended tie rods were also inspected on-site.

Various different neutron techniques were used in this study of the all with the aim of researching the metal without needing to damage the sample itself. Neutron Resonant Capture Analysis allowed single elements of the rod to be detected, which was concluded to be made from iron with zinc impurities.

The technique of neutron diffractometry was used to provide details on the inner regions of the rod, returning crystalline phases and a map of strains in the metal.

As one of the biggest and widest examples of gothic architecture in the world Milan Cathedral, or Duomo di Milano, is a prominent feature on the skyline of the city. The cathedral has a very long and complex construction history, which started in 1386 and lasted more than five centuries. Today the cathedral draws huge crowds, attracting around 5 million visitors every year.

Towering more than 100 metres above ground the structure of the cathedral supports a number of heavy architectural features including 325,000 tonnes of statues. In 2012 the replacement of a broken tie rod in the cathedral presented a rare opportunity to study a piece of the buildings history, sparking a flurry of research.

One of the Italian researchers, Dr Daniela Di Martino of Università Milano-Bicocca, explained why this testing of the tie rod is so important. “A deep characterization of this metallic reinforcement is greatly relevant; it’s not only an opportunity for us to study a sample of medieval technology but a chance for us to investigate a structural element that’s remained in full operation for more than 600 years”.

A portion of the broken tie rod, subject of this research. (Credit: M. Bellanova, Politecnico di Milano)

In this study two neutron instruments – ENGIN-X and INES were used to characterise the broken tie rod.

“INES is frequently used for archaeometric analyses we’ve equipped the instrument with a specially developed frame conceived for handling big and delicate samples. In this experiment we used a dedicated sample holder, able to hold up very heavy specimens up to 30kg” explains Dr Antonella Scherillo, INES instrument scientist at ISIS Neutron and Muon Source.

Neutrons are the perfect tool to study ancient artefacts for a number of reasons. Firstly, they’re highly penetrating which allows researchers to understand the mean bulk composition of a sample - not just its surface composition. Secondly, neutron analysis is non-destructive and samples don’t need cleaning, allowing ancient artefacts to remain intact.

“The instruments at ISIS Neutron and Muon Source have studied many ancient artefacts over the years. A lot of people think ENGIN-X is only used to study engineering components, but the instrument has examined Napoleonic copper bolts, medieval spearheads, Egyptian grave goods and more“, says ENGIN-X instrument scientist Dr Joe Kelleher.

Further analyses were also run at Budapest Neutron Centre (BNC, Hungary). The behaviour of cracks and voids suggested at least two parts of iron were assembled to fabricate the tie rod over 600 years ago.

Dr Daniela Di Martino, lead author of the paper, describes the impact of their findings:

“Our neutron study derived very useful information, without preparation, or sampling in a non-invasive way. Thus, neutron techniques could always be applied to archaeometallurgical samples prior to other investigations. No other conventional non-destructive technique will allow the same characterization, for the benefit of the cultural heritage research on archaeometallurgy and this study could also be really important to independently validate new on site monitoring techniques.”

Media contact

Melissa Warren
Press Officer STFC

Notes to Editors

The full research publication can be found here: D. Di Martino et al “A neutron diffraction and imaging study of ancient iron tie rods” 2018 JINST 13 C05009 DOI:10.1088/1748-0221/13/05/C05009

Find out more about our instruments ENGIN-X and IMAT at ISIS Neutron and Muon Source.