The synthesis and application of nanomaterials are often viewed as modern scientific achievements. However, evidence of nanomaterials' use in ancient artifacts challenges this notion. Previous studies have reported the presence of carbon nanotubes (CNTs) and metal nanoparticles in various historical objects, suggesting ancient civilizations possessed an understanding of these materials, even if the scientific principles were unknown. For example, CNTs have been found in Damascus steel, enhancing its strength, and metal nanoparticles were used to improve the luster of Islamic and Renaissance potteries. This study focuses on pottery sherds from Keeladi, Tamil Nadu, India, with a black coating on their inner walls. Radiocarbon dating places the last settlement period between the sixth and third centuries BC. The researchers aimed to determine the composition of this coating and explore the significance of their findings in the context of ancient technologies and nanomaterial synthesis.

Existing literature demonstrates the presence of nanomaterials in various ancient artifacts. Pérez-Areñado et al. (2004) and Alavi et al. (2003) studied the use of metal nanoparticles in luster pottery from the 13th-16th centuries AD and Renaissance luster pottery, respectively. Pradel et al. (2005) contributed insights into the invention of luster in Italy during the 9th and 10th centuries AD. A landmark study by Reibold et al. (2006) revealed the presence of MWCNTs in Damascus steel, explaining its exceptional strength. These previous findings lay the groundwork for the present study, which seeks to push the timeline of nanomaterial use further back in history.

Pottery sherds with a shiny, hard, and durable black inner coating were collected from the Keeladi excavation site. The researchers employed various analytical techniques to characterize this coating. Micro-Raman spectroscopy, using a 532 nm laser, was performed on the inner surface of the sherds to identify the type of carbon present. A portion of the black coating was carefully removed, processed (ground to a fine powder, sonicated, centrifuged, and washed with acetone), and drop-coated onto a gold-coated silicon wafer for further Raman analysis. Transmission electron microscopy (TEM) was used to visualize the structure and morphology of the coating. Samples were prepared using a diamond knife, sonicated in acetone, and dried on carbon-free Pd and Cu holy grids to avoid contamination. Imaging was performed with a FEI Tecnai T20 electron microscope at 200 kV. X-ray photoelectron spectroscopy (XPS) using a PHI VersaProbe III with Al Kα X-ray source was employed to determine the elemental composition and the chemical states of the elements in the coating.

Raman spectroscopy confirmed the carbonaceous nature of the coating, revealing the presence of D and G bands characteristic of hexagonal graphitic carbon and sp² bonded carbon. The presence of a 2D band further indicated the graphitic nature of the material. Deconvolution of the D band revealed additional components (D1, D2, D3), suggesting defects in the graphitic layers. The ratio of D to G band intensity (ID/IG) was calculated as 1.28, and the ratio of 2D to G band intensity (I2D/IG) was 0.6. The crystallite size was estimated to be 15 nm using the formula provided. TEM analysis revealed the presence of SWCNT bundles and MWCNTs. The average diameter of the SWCNTs was 0.6 ± 0.05 nm, remarkably close to the theoretical minimum diameter of 0.4 nm. The spacing between the walls of the MWCNTs was found to be 0.34 nm. XPS analysis confirmed the presence of various elements, including Fe, Al, and Si, possibly as oxides, and revealed the presence of oxygen functional groups, further supporting the Raman findings. The presence of sp² carbon domains was also confirmed by high-resolution C1s XPS spectra.

The discovery of CNTs, particularly SWCNTs with a diameter close to the theoretical limit, in pottery dating back to the sixth century BC is extraordinary. The exceptional stability and adhesion of these nanomaterials for over 2600 years suggest advanced techniques in ancient material processing. The presence of iron, possibly originating from plant sources or the soil, could have acted as a catalyst in the formation of CNTs during the high-temperature firing process of pottery production. The source of carbon remains uncertain but potentially included plant-based materials undergoing torrefaction. The findings raise questions about the ancient inhabitants' understanding of material properties and potential applications of these nanomaterials. Further research is needed to investigate the implications for ancient technology and the potential cytotoxic effects of CNTs and graphene oxide if the pottery was used for food preparation or storage.

This study presents the oldest known discovery of carbon nanostructures, including SWCNTs, MWCNTs, and graphene oxide-like sheets, in pottery from the Keeladi archaeological site. The remarkable stability of these nanomaterials over millennia highlights the advanced materials processing capabilities of ancient civilizations. Future studies should focus on clarifying the synthesis mechanisms, exploring potential applications of these nanomaterials in ancient technologies, and further evaluating the implications of this discovery for archaeology and nanotechnology.

The study's limitations include the potential for contamination during sample preparation, although great care was taken to minimize this risk. Further investigation is needed to fully elucidate the synthesis processes involved in creating the nanomaterials. The exact source of carbon and the specific environmental conditions during the pottery-making process remain to be determined.