X-ray technique peers beneath archaeology’s surface
BBC: Striking discoveries in archaeology are being made possible by
strong beams of X-rays, say researchers.
A report at the American Physical Society meeting in Dallas, US,
showed how X-ray sources known as synchrotrons can unravel an artefact’s
mysteries.Light given off after an X-ray blast yields a neat list of the
atoms within.The technique can illuminate layers of pigment beneath the
surfaces of artefacts, or even show the traces of tools used thousands
of years ago.
This X-ray fluorescence or XRF works by measuring the after-effects
of X-ray illumination.
As atoms absorb the X-rays, the rays’ energy is redistributed, and
very rarely some is re-emitted as light. Each atom releases a
characteristic colour of light, yielding a full chemical analysis, and
as such the XRF technique is gaining ground as a means to meticulously
analyse artefacts from the past.
Intense sources Small X-ray sources have been used in the past to get
a laundry list of atoms generally present in art, but Robert Thorne of
Cornell University in the US told BBC News that the intense, focused
X-rays from enormous sources known as synchrotrons have more recently
shown their potential.
“These give you extremely intense X-ray beams, and what that allows
you to do is not just collect a spectrum from one point, but you can
‘raster scan’ your sample in front of the beam and collect the full
chemical analysis at each point.”
Compared to handheld sources, he said, “you can get months’ worth of
photons delivered in a second, and that’s critical”.
Professor Thorne and his collaborators were in 2005 the first to use
the technique to analyse inscriptions from Greek and Roman pottery.
The technique has been shown to shed light on layers of glaze beneath
the surface of finished pottery.
It has even shown, in the case of an inscription that had worn
entirely away, that minuscule amount of iron left by the chisel showed a
pristine version of the inscription on what appeared to be smooth stone.
“We did an experiment at Diamond [Light Source in Oxford] last year
on a heavily-worn surface, and we couldn’t quite guess what the letters
were,” he said.
The translation said it was a decree involving three different
individuals. We looked at the pattern of iron we saw from tool wear and
pigments that one letter couldn’t be consistent with the letter that had
been put there - it turns out that letter changed the name of one of the
people, and the story was about three brothers - just down to that one
simple change.
”More recently, the team has turned its attention toward the
Americas. The technique is best used on artefacts whose inscriptions or
decoration has worn away completely - but these, Professor Thorne said,
are much harder to find because collectors and museums have until now
viewed them as less valuable.
“That’s what’s exciting about working on pottery from Mesoamerica,
because there’s a ton of it in American collections, much of which we
can get access to,” he said.
“We’re looking at some Mayan artefacts with some collaborators at
Cornell and they’re interested in the iconography of a particular
subgroup within the Mayans.
“On the pottery a lot of the glaze has flaked off, so what you see is
little black dots on the surface; it’s very hard to tell if those black
dots are glaze or dirt, but with the XRF you can tell. ”Dr Thorne was
guarded about the most recent results from the Mayan studies, which will
be published soon.
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