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Neutrons are an important tool in cultural heritage studies, as they are non-destructive and can penetrate deep into solid or, uniquely, liquid materials to reveal what is happening at the atomic or molecular level. They can reveal metal manufacturing methods used on ancient swords, or improve the restoration processes used on centuries-old works of art.
X-ray revelations
Several analytical techniques were then used to study the structure of nanoparticles and evidence of degradation in wood samples following the selection of preventative and curative treatments. The samples were taken from a 2,000 year-old Gallo-Roman barge at the Lugdunum museum (Lyon, France) on loan from ARC-Nucléart (Grenoble, France).
A number of these measurements involved the use of X-rays, taking place at the University of L’Aquila. X-rays are an excellent complementary tool to neutron scattering as while neutrons are exceptionally capable of identifying lighter elements – such as hydrogen – in the molecular structure of materials, X-rays can reveal with great resolution the mid- and heavier-weight elements in a sample.
In this case, X-ray fl uorescence and X-ray powder diffraction were used to analyse the chemical composition of the acid precursors – the iron compounds that emerge from features such as nails and fasteners in the wood. X-ray diffraction measurements were also used to evaluate the structure, composition and crystallinity of the nanoparticles – using a X’PertPRO diffractometer – as well as their penetration inside the wood structure.
An array of microscopy
The selected wood samples were also analysed with a range of microscopy techniques, before and after the washing process as well as after the nanoparticle treatments. The morphological features were investigated by optical stereomicroscopy (SM, Stereozoom S8APO microscope) and Scanning Electron Microscopy (SEM) (SEM-BSE XL30CP). The SEM revealed the diffusion of iron and sulphur around the regions containing nails, that are the origin of the acidifi cation ‘disease’.
The morphology and size of the synthesized calcium hydroxide (CH) and magnesium hydroxide (MH) nanoparticles were investigated using both Transmission Electron Microscopy and Atomic Force Microscopy (with a Cypher Asylum Research microscope using the Partnership for Soft Condensed Matter platform). This revealed that the nanoparticles appear to be made up of hexagonal lamellas (typical of magnesium hydroxide) composed of self-assembling, ever smaller nanoparticles – the so called primary nanoparticles – with a size of 2-3 nm, a hexagonal shape and very thin, with a thickness less than 2 nm.
Advancing the fi eld
Having demonstrated that the new solution has both preventative and curative effects when it comes to preventing the destruction of wooden artefacts, the next steps for this research will involve studying the effects of the solution on a bigger sample. Ideally this would be a piece of wood that is comparable to the thickness of the wood used on a ship.
Understandably, only tiny samples of the Gallo-Roman barge were used in the experiments until we had proven its effectiveness. It is now vital to ensure the right penetration depth and dosage of the nanoparticles is achieved for scaling up the solution for large objects. The team wants to show to those working in restoration that the penetration of the nanoparticles is deep enough to cover the whole thickness of the wood used in the ship. This will ensure the treatment is proven to work in the exact conditions archaeologists and restorers experience on a daily basis.
The small angle neutron scattering instrument D11 at ILL (copyright-Bernhard Lehn)
Experimental Hall (Copyright Briq ecliptique) Rescuing human history
The importance of advancing the techniques used in cultural heritage come into focus when you consider the richness of information that can be obtained about human history from objects such as shipwrecks.
The barge Lyon Saint George 4 used in the study was found in 2003 in the Saône, a river crossing Lyon, during the work on Saint George Park in the town. It was used some 2000 years ago as a barge to transport heavy and bulky loads along the river: loading heavy stones, wood, amphoras for wine, garum (a fermented fi sh sauce used as a condiment in the cuisines of Phoenicia and other ancient people) or olive oil.
The ship was abandoned with six other barges of the same type – dating from the 1st to 3rd century AD – in a location that acted as a sort of ‘cemetery’ of Roman boats: throughout this period of abandonment – until they became buried in the mud – the boats were alternately submerged or in the open air according to the level of the river and the seasons. We know that they were discarded because the boats were found empty without their load, so it was not an accident that caused them to sink. In addition, the large amount of iron salts resulting from the corrosion of the steel of the nails indicated that the barge remained for a long time in the open air and not only at the bottom of the water.
Indications of how ancient civilisations ate, travelled, traded and innovated are contained in the composition, building materials and chemical state of waterlogged wooden objects, so it is exciting to see the research methods in this fi eld continue to advance through pioneering collaborations.
Excavation site in Lyon (Credit ARC-Nucleart)
Views of a part of the gallo-roman barge fl oor (2th a.c) LSG$ from Lugdunum Museum in Lyon after its Poyethylene Glycol Freeze Dryin Treatment (Credit ARC-Nucleart)
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