Saturday, May 19, 2007

Neutron diffraction offers non-destructive artifact analysis

I was doing some research on an image of a "killed" Greek helmet that I photographed at the Walters Art Museum several years ago and came across this article about a relatively new non-destructive technique for analyzing artifacts to see if they are a true antiquity or have been "tampered with".

"Neutron diffraction, an established diagnostic tool for materials analysis and non-destructive testing of engineering components, can also be used to characterise archaeological artefacts and museum objects. The phase and microstructural information obtained – without damaging an object of value – can help answer questions of authenticity, as recent investigations of 16th-century silver/copper coins and an obviously repaired 7th-century BC Greek bronze helmet show.

Neutron diffraction is a rather new diagnostic tool for studying archaeological materials. Neutrons easily penetrate through thick coatings or corrosion layers and provide information from the bulk rather than from surface areas; sampling techniques such as coring or even powdering for analysis some portion of a museum object can therefore be avoided. The large neutron beams generally used illuminate a considerable volume portion of the object and, as a result, average and representative structural information is obtained – the problems associated with the si0ngle-spot analysis of many conventional archaeometric techniques are therefore avoided. Neutron diffraction provides information on the mineral and metal phase compositions or corrosion products in objects, on the crystal structures of the constituent phases and on the microstructures. In the material sciences it is widely used for volume
texture analysis, i.e. determination of the orientations of the crystallites in polycrystalline material. Many
processes such as primary crystallisation or plastic deformations impose a characteristic texture on the
material which means that, for example, details of the production method may be imprinted in the
microstructure. Mapping grain orientation distributions – a technique called texture analysis –
reveals the creation and deformation history of an object. The crystallite distribution can be displayed
via ‘pole figures’, 2D projections of the spatial orientation distribution function that are obtained by
recording diffraction patterns for a multitude of sample orientations. The structure and texture
information can therefore provide clues on the type of material and the manufacturing techniques used
by the ancient craftsmen. " - Genuine or fake? Neutron diffraction for non-destructive testing of museum objects , Isis 2003 Science Highlights.

As for the helmet they analyzed, they discovered the nose piece had been replaced and, like the helmet I photographed at the Walters, had been ritually "killed".

"It was the custom for victorious Greek cities to dedicate tropaia, ‘trophies’ of armour from the defeated, in the sanctuary of one of the gods. When the trophy collapsed from age or when the sanctuary became too full the armour was buried, but first it was ‘killed’ as part of the process of offering it to the gods: the cheekpieces were bent
back and the noseguard turned up to render the helmet useless in this world. The finder of the helmet – probably in the 19th century and in order to sell it – straightened out the cheekpieces, which cracked at the edges and left a clear fold-line running across each of them. It is also clear that the noseguard had come off altogether, probably during
similar cosmetic straightening by the finder, for there is a clear overlapping join at the bridge of the nose."

I wonder if the Romans had any special rituals for disposing of war trophies?