For tens of thousands of years, Aboriginal Australians have created some of the most amazing art in the world. Today, their work perpetuates long lines of ancient traditions, stories from the past, and connections to today’s cultural landscapes, which is why researchers want to better understand and preserve the cultural heritage found there.
In particular, knowing the chemical composition of the pigments and binders used by Australian Aboriginal artists could enable archaeologists and art restorers to identify these materials in important cultural heritage objects. Today, researchers are turning to X-ray science to help reveal the composition of materials used in Australia’s Aboriginal cultural heritage, beginning with the analysis of centuries-old samples of plant secretions, or exudates.
Australian Aborigines continue to use plant exudates, such as resins and gums, to create bark and rock paintings and for practical applications, such as setting stone points on handles. But the exact composition of these plant materials is not well understood.
So scientists at six universities and laboratories around the world turned to high-energy X-rays at the Stanford Synchrotron Radiation Light Source (SSRL) at the Department of Energy’s SLAC National Accelerator Laboratory and the SOLEIL synchrotron in France. The team directed X-rays at 10 well-preserved plant exudate samples from the native Australian genus Eucalyptus, Callitris, and xanthorrhea Y Acacia. The samples were collected over a century ago and were kept at various institutions in South Australia.
The results of their study were clearer and deeper than expected.
“We got the groundbreaking data we were hoping for,” said Uwe Bergmann, a physicist at the University of Wisconsin-Madison and a former SLAC scientist developing new X-ray methods. The molecular structure of a well-preserved collection of native Australian plant samples can allow us to discover its existence in other important objects of cultural heritage. »
The researchers published their results today in the Proceedings of the National Academy of Sciences.
look below the surface
Over time, the surface of plant exudates can change as the age of the material. Although these changes are only a few nanometers thick, they can still block the view below.
“We had to see most of the material below that upper layer, otherwise we wouldn’t have any new information about plant exudates,” said SSRL Senior Scientist Dimosthenis Sokaras.
Conventionally, molecules containing carbon and oxygen are studied with lower-energy, so-called “soft” X-rays, which would not be able to penetrate through the debris layer. For this study, the researchers sent high-energy X-ray photons, called “hard” X-rays, into the sample. The photons slipped past the nebulous upper layers and into the elemental arrays in the sample below. Hard X-rays don’t get stuck on the surface as soft X-rays do, Sokaras said.
Once inside, the high-energy photons scattered from elements in the plant exudate and were captured by a wide array of perfectly aligned silicon crystals in the SSRL. The crystals only filtered out scattered X-rays of a specific wavelength and funneled them into a small detector, much like a kitchen sink funnels water into your sink.
Next, the team compared the wavelength difference between the incident and scattered photons with the carbon and oxygen energy levels of a plant ooze, providing detailed molecular information about the unique Australian samples.
A path for the future
Understanding the chemistry of each plant exudate will provide insights into approaches to identifying and conserving Australian Aboriginal art and tools, said Rafaella Georgiou, a physicist at Synchrotron SOLEIL.
“Now we can go ahead and study other culturally significant organic materials using this powerful X-ray technique,” he said.
The researchers hope that people working on cultural heritage analysis will see this powerful synchrotron radiation technique as a valuable method for determining the chemistry of their samples.
“We want to reach out to this scientific community and say, ‘Listen, if you want to learn more about your cultural heritage samples, you can go to synchrotrons like SSRL,'” Bergmann said.
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