Scientists have discovered the oldest known human-made nano-materials in the "unique black coatings" of ancient pottery shards - dated to 600 BC - unearthed from an archeological site in Keeladi, Tamil Nadu.
The research, published recently in the journal Scientific Reports, revealed that these coatings are made of carbon nanotubes (CNTs) which have enabled the layer to last more than 2600 years, raising questions on the tools used during those periods to achieve high temperatures for making earthenwares.
According to the scientists, including those from Vellore Institute of Technology (VIT) in Tamil Nadu, the coatings are "the oldest nano-structures observed till now."
"Until this discovery, to our knowledge, the most ancient known nanostructures in human-made artifacts are from the eighth or ninth century AD," study co-author Vijayanand Chandrasekaran from Vellore Institute of Technology told news agency Press Trust of India.
Carbon Nanotubes are tubular structures of carbon atoms arranged in an ordered manner, Mr Chandrasekaran said, adding that coatings in ancient artifacts may not usually last this long due to wear and tear caused by changing conditions.
"But the robust mechanical properties of the CNT based coating has helped the layer sustain more than 2600 years," he added.
Carbon nanotubes have superlative properties, including high thermal and electrical conductivity, and very high mechanical strength, explained nano-material scientist MM Shaijumon from IISER Thiruvananthapuram, who was unrelated to the study.
"But the people of this time may not have intentionally added CNTs, instead, during the processing at high temperatures, these would have just formed accidentally," Shaijumon told news agency PTI.
"If there is some processing of the potteries, which probably would have involved some high-temperature treatment, then it will add more justification to the findings," he added.
According to Mr Chandrasekaran, the closest scientific explanation for the finding is that some "vegetal fluid or extract" might have been used in the coatings of these pots which may have led to the formation of CNTs during high-temperature processing.
Rajavelu S, Professor of History at Alagappa University in Tamil Nadu, who was unrelated to the study, told news agency PTI that the people of this time may have added or coated something similar to plant-sap to the inside of the pots, and subject it to the nearly 1100-1400 degree Celsius high-temperature fire treatment as seen in kilns.
"This fire treatment may have led to the formation of the coating which has likely strengthened the pot and made the coating durable," Mr Rajavelu said.
"Normally with high-temperature processing of carbon, they form these type of tubular nano-structures, but until about the 1990s there were no sophisticated instruments available to characterise them. So these structures are already even present in nature and only now we are observing them," explained Mr Shaijumon.
Mr Rajavelu concurred.
He said the ancient people would not have known these as CNTs, but may have just had the need to make their pots have high durability, "and may have needed a certain colour out of their products on applying high-temperature firing."
"They likely knew the technique to make these coatings practically, but may not have known this as a thesis with any kinds of formulae," Mr Rajavelu added.
Commenting on the significance of the research, Sharada Srinivasan, an expert in archaemetallurgy associated with the National Institute of Advanced Studies (NIAS) in Bengaluru, said nanotechnology has made strides since the 90s with the advent of advanced techniques like the scanning tunnelling microscopy.
"But it is increasingly recognised from archaeological studies that past skilled artisans sometimes accidentally or empirically made nano-materials - such as the famous Egyptian Blue - without being aware of the science of working at the nano-scale," Ms Srinivasan told Press Trust of India.
Based on their analysis, Mr Chandrasekaran said the ancient Tamil civilization of this time were aware of, and mastered high-temperature processing, but added that the means and mechanism by which they produced these artifacts with carbon-nanotubes is not widely explored.
"Black and red pottery ware associated with megalithic sites in southern India continues into Keeladi dated back to 6th century BCE. The fine black and red effect was achieved by high temperature firing temperatures at about 1100 degrees in the presence of carbon-rich matter and iron-rich red soils," Ms Srinivasan said.
"They do not look like normal pots, these have glazed finishes, and are made of high-quality clay," Mr Rajavelu added.
He said these earthenwares were likely used by the "sophisticated people of the time," adding that "a lot of the shards" have been found in Keeladi, "some dating back even as early as 900 BCE."
"We have known for a long time that in iron smelting and manufacture, India was a world leader at the time. Even the Sangam ancient Tamil literature has noted about steel manufacture," Mr Rajavelu said.
"The technological skills of the Tamils in high-temperature manipulation of carbonaceous matter to make ultra-high carbon crucible steel known as wootz by about the mid to late centuries BCE was also reported by us, while carbon nanotubes were reported in medieval patterned 'Damascus' blades forged from such steel," Ms Srinivasan explained.
She believes the findings expand the broader knowledge of the history of science and technology in India, and point to potential future applications of such nano-materials as durable coatings.
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