Thermogravimetric and Differential Scanning Calorimetric Analysis of Archaeological Pottery Samples from Keeladi, Tamil Nadu, India

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Arun Luiz, Akhil Krishnan, V. P. Yatheesh Kumar This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8132400/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Ancient pottery samples from Keeladi, Tamil Nadu, dated to the 6 th century BCE, were analyzed using Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). The main objective of this study is to investigate the thermal behavior of the samples and to evaluate the physical and chemical transformations that take place during heating. The findings highlight a clear technological progression from simple open-firing practices to more controlled, high-temperature kiln technology. The samples vary from lower fired earthenware fired below 700°C to high-temperature ceramics fired between 900-1050 °C, demonstrating a technological evolution toward dense, high-performance ceramics in Keeladi’s pottery tradition. These results indicate a more pronounced vitrification process, with qualities comparable to those seen in modern high-performance ceramics. The red colour in the pottery is attributed to hematite mineralogy, while the dual-redox nature of Black-and-Red Ware corresponds to limited-oxygen reduction in the core during the production stage is an example for well managed variable atmosphere firing. Altogether, the results identify Keeladi as an advanced pottery production center with notable kiln control, raising an important question about how this 2500-year-old community achieved such high firing temperatures. Keeladi TGA-DSC Firing temperature Kiln Black-and-Red Ware Figures Figure 1 Figure 2 Figure 3 1. Introduction Pottery can be considered one of the earliest technological inventions by humankind. The durability of pottery allows it to survive in the archaeological record for thousands of years [ 1 ]. Spectroscopic analysis of pottery provides information about raw materials, kiln conditions, and the technological sophistication of ancient societies. Organic additives (like dung or plant fibers) and temper materials (such as sand or grog) are usually added by artisans to clay for better workability and to boost high-temperature bearing potential [ 2 ]. The understanding of maximum firing temperature, soaking time, and firing atmosphere allows us to estimate the technological status of ancient societies [ 3 ]. Thus, Keeladi, a prominent archaeological site in the Sivagangai District of Tamil Nadu, can be considered a transformative archaeological discovery in South India. This bridges the Iron Age (12th -6th century BCE) with the Early Historic Period (6th -4th century BCE) [ 4 ]. The findings from the site offer strong evidence of early urbanization and advanced ceramic production in South India. This gives an insight into the possibility of advanced ceramic production processes comparable to those of other Early Historic civilizations, such as the Indus Valley [ 2 , 4 ] The thermal behaviour of the clay matrix depends on aluminosilicate minerals and organic tempering materials, which directly influence the physical and chromatic properties of the ceramics [ 2 ]. The dehydroxylation of kaolinite and the combustion of organic residues result in weight losses and structural reorganization within the ceramic body, leading to vitrification and pore reduction [ 5 ]. Such thermal and mineral transformations, in turn, reflect the firing atmosphere, heating rate, and compositional heterogeneity of the clayey raw material [ 6 ]. Therefore, the evaluation of thermal responses, coupled with mineralogical data, constitutes a reliable basis for classifying the Keeladi pottery and determining its firing range. Thermoanalytical methods, particularly thermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses, were employed in this study to investigate the thermal effects occurring in Keeladi ceramics during progressive heating [ 7 ]. The TGA and DSC analyses of Keeladi pottery provide a direct reconstruction of firing strategies, oxidation states, and thermal transformations, offering more scientific insights into the ancient technological intelligence that shaped one of South India’s earliest urban traditions [ 8 ]. 2. Materials and methods In this study, five pottery samples were collected from the Keeladi archaeological site (dated to the Sangam period) in southern India (Fig. 1 ) and investigated through a Thermogravimetric Analyser (TGA) and Differential Scanning Calorimetry Analyser (DSC) in the temperature range of 20-1200°C under an air atmosphere, with a heating rate of 10°C min⁻¹ using SDT Q600 V8.3 Build 101 instrument available in the Central Instrumentation Facility (CIF), at CSIR - CECRI, Karaikudi, Tamil Nadu, India. The controlled heating in a TGA helps detect and quantify mass losses, as well as identify processes such as dehydration, dehydroxylation, and decarbonation. Additionally, the DSC curves reveal the enthalpic changes in the material through variations in heat flow during heating, which are either exothermic or endothermic [ 8 ]. The analysed samples consist of Red Slipped Ware (KLD-1), Red Ware (KLD-2, KLD-4, KLD-5), and Black and Red Ware (KLD-3) (Fig. 2 ). Before analysis, the samples were finely powdered using an agate mortar and sieved through a 2 mm mesh. 3. Results and discussion 3.1 Overview of Thermal Behaviour Thermogravimetric (TGA) and Differential Scanning Calorimetry (DSC) analyses were carried out up to 1200°C using five pottery samples from Keeladi (Fig. 3 ), which include Red Slipped Ware (KLD-1), Red Ware (KLD-2, KLD-4, KLD-5), and Black and Red Ware (KLD-3). The combined analysis exhibits multi-stage thermal weight loss and heat flow patterns, effectively revealing their mineral, organic, and thermal history through temperature-dependent decomposition [ 9 , 10 ]. These curves illustrate the combined effects of physically adsorbed water loss, combustion of organic matter, breakdown of carbonates, and potential high-temperature silicate transformations. Altogether, the TGA-DSC analysis provides comprehensive insights into the material characteristics of the ancient pottery [ 5 – 7 , 9 ]. 3.2 Low‑Temperature Region (25–300°C ) In the initial stage, all the samples show an initial minor weight loss ranging from 0.9–3.9%, which is attributed to the removal of physically adsorbed moisture, and volatile compounds often originate from ancient food residues, adhesives, resins, or natural waxes that were once part of the pottery's period of use or post-burial environment [ 7 , 10 – 12 ]. Samples such as Red Ware (KLD-2) and Black-and-Red Ware (KLD-3) exhibit higher early weight loss (3.3–3.9%). It suggests greater porosity and water retention, characterised by less-fired or raw clay fabrics. In contrast, the Red Slipped Ware (KLD-1) shows very low initial loss (~ 0.9%) with a weak endothermic DSC peak at 49°C (2.6 mW), indicating limited adsorbed moisture and a well oxidized slip [ 13 ]. 3.3 Medium‑Temperature Transformation (300–600°C) The dehydroxylation, combustion of organic matter occurred in the temperature range of 300–600°C. The endothermic peaks at 553–557°C in all samples correspond to kaolinite dehydroxylation (Al₂Si₂O₅(OH)₄ → Al₂Si₂O₇ + 2H₂O). The most intense dehydroxylation event was recorded for the Black-and-Red Ware (KLD-3) with 55.3 mW. This was followed by the Red Wares, KLD-5 with 54.9 mW and KLD-2 with 52.1 mW [ 14 , 15 ]. Both samples show evidence of a high kaolinite content and organic/volatile loss in those samples. The Red Slipped Ware (KLD-1) and Red Ware (KLD-4) showed moderate endothermic peaks, indicating more advanced clay transformations during firing. The Red Ware (KLD-2) experienced up to 4.8% mass loss during dehydroxylation, confirming incomplete firing and classifying it as low-fired earthenware [ 5 ]. Meanwhile, KLD-1 and KLD-4 retained higher residual masses (~ 93–94% at 600°C), consistent with well-developed metakaolinite and improved firing efficiency. These variations correspond to differing clay refinement levels and firing intensities—from low‑temperature earthenware to highly processed, dense ceramics [ 5 , 15 , 16 ]. 3.4 High‑Temperature Phase Formation (600–1200°C) When the temperature exceeds 600°C, sample‑specific differences were evident: Red Slipped Ware (KLD‑1) develops minor weight gain (600–800°C) possibly due to the oxidation of ferrous iron (Fe²⁺ → Fe³⁺) to form Fe₂O₃, the hematite pigment for the red slip. The breakdown of carbonate occurs between 800°C and 1000°C (-4.7%) is consistent with decarbonation of CaCO₃ into CaO + CO₂ [ 10 , 15 ]. DSC features at ~ 1060–1147°C are consistent with mullite crystallization and partial vitrification. Overall weight loss (~ 10%) and DSC data of KLD-1support the idea of original firing at temperatures likely in the ≈ 500–900°C range under oxidizing conditions, and consistent with a red-slipped finish [ 10 , 15 ]. Red Ware (KLD‑2) shows primary dehydroxylation near 557°C (DSC ≈ 52.1 mW). The TGA curve shows mass loss and TGA inflection at ≈ 937°C (residual ≈ 92.7%) and an end point near 1194°C (residual ≈ 88.3%). The absence of previously formed mullite suggests that firing was limited to lower temperatures during production. Strong endothermic peaks at 1065–1070°C during analysis indicate new mullite formation in the laboratory re-heating. This confirms that the sample is a lower fired (incomplete mullitization) earthenware with original firing temperature likely in the ≈ 400–700°C range [ 10 , 15 ]. Black‑and‑Red Ware (KLD‑3 ) shows multiple higher-temperature DSC peaks at ≈ 688°C, ≈ 782°C (≈ 69.9 mW), and ≈ 844°C (≈ 67.9 mW). These correspond to organic combustion, iron oxide oxidation, and carbonate decomposition during progressive laboratory heating. The black core and red surface of pottery indicate firing under multiple atmospheres (reduction in the core, oxidation at the surface) during original firing. The last peak at 1098°C marks the formation of mullite. Based on the profiles and comparative studies the original firing temperature of the ware is estimated to be in the range of the ≈ 800–900°C This reveals the complex redox control involved in the firing of this ware [ 15 ]. Red Ware (KLD‑4) shows dehydroxylation near ≈ 555°C (DSC ≈ 49.6 mW) and advanced high-temperature events at ≈ 1048°C, ≈ 1106°C, and ≈ 1147°C. The latter two peaks indicate progressive mullite formation, glass-phase development, and advanced sintering. The small total weight loss (~ 8%) and strong high-temperature DSC activity indicate an optimum firing range closer to ≈ 800–1000°C in original production process [ 15 ]. Red Ware (KLD‑5) shows dehydroxylation near ≈ 553°C (DSC ≈ 54.9 mW). The sample shows similar behaviour to KLD-4, with peaks at 1037°C and 1065°C (≈ 41.6–46.6 mW), indicating strong vitrification and mullite/glassy phase development. The total weight loss of around 11% and high thermal stability confirm an advanced production process. It was likely fired around ≈ 900–1050°C range, which is typical of dense fine-ware [ 15 ]. 3.5 Analysis of Firing Behaviour and Ceramic Technological Development Table 1 indicates the progressive firing technological development of Keeladi pottery. The findings highlight a clear technological progression from simple, open-firing practices to a more controlled, high-temperature kiln technology. The Red Ware (KLD-2) and Black-and-Red Ware (KLD-3) were fired at low temperatures with incomplete vitrification, showing simple earthenware production. The dual red-black coloration of KLD-3 indicates controlled redox atmospheres during firing. The samples (KLD-4 and KLD-5) exhibit strong mullite formation, vitrification, and minimal weight loss, evidencing kiln-based, high-temperature firing approaching 1000°C. The Red Slipped Ware (KLD-1) represents an intermediate refinement stage, showing oxidizing conditions and fine slip control. KLD-5 shows a more pronounced vitrification away from modern high-performance ceramics. Red-based mineralogy explains hematite red colouring, while the dual-redox nature of Black-and-Red Ware is in accordance iron oxidation and localized reduction phases [ 15 , 17 ]. Table 1 Firing Behaviour and Ceramic Technological Development of Keeladi Pottery Sample Firing Temp (°C) Features Technological Interpretation KLD-1 (Red Slipped Ware) ~ 500–900 Moderate dehydroxylation; carbonate decomposition Medium-fired earthenware, Kiln firing KLD-2 (Red Ware) ~ 400–700 low structural maturity; incomplete mullitization Low-fired earthenware, Open firing KLD-3 (Black-and-Red Ware) ~ 500–900 Oxidized slip, dense surface, limited moisture Controlled dual-atmosphere firing, Kiln firing KLD-4 (Red Ware) ~ 800–1000 Mullite formation, glass-phase development, advanced sintering Mature fine pottery Advanced kiln firing, KLD-5 (Red Ware) ~ 900–1050 Intense mullite/glassy phase; vitrification; low porosity Dense fine-ware Advanced kiln control 3.6 Archaeological and Cultural Significance KLD-1: Medium-Fired Pottery Tradition The KLD-1 sample represents the technological maturity of traditional pottery production. It thus confirms a well-established craft tradition, able to manage a controlled firing in the medium-temperature range of 500–900°C [ 2 , 4 ]. Thermal stability and preservation of carbonates indicate a modest yet intended technological setup. This all aligns with local pottery productions at the community level, typically in the early urbanization of Keeladi. Overall, the firing precision in the KLD-1 sample marks a notable technological advance that would be characteristic of the later Iron Age or Early Historic periods, a time when ceramic production became regionally standardized and kiln operations were managed with stable and repeatable firing conditions [ 2 , 4 ]. KLD-2: Low-Fired Earthenware Technology The KLD-2 sample represents a traditional earthenware produced under a low-temperature firing profile ranging from 400°C to 700°C. This represents pottery produced in rural or small-scale production units. The high moisture absorption and dehydroxylation behaviour indicate a fabrication process oriented toward workability and thermal adaptability [ 4 , 15 , 17 ]. These characteristics suggest that the ceramics were primarily intended for everyday domestic use such as cooking or water storage where the natural porosity of the material aided in cooling and allowed for vapour exchange. This pottery reflects an earlier or more conservative phase of ceramic production, during which kiln temperature control was limited [ 4 , 12 , 14 ]. This indicates the technological constraints and resource optimization within the community firing tradition. From a cultural context perspective, this Keeladi sample aligns with utilitarian ceramic industry production that preceded or coexisted with advanced kiln technologies [ 4 ]. KLD-3: Well Managed Variable Atmosphere Firing The KLD-3 sample represents a variable-atmosphere firing stage in ceramic production [ 18 ]. The remarkable clay purity and complex decomposition characteristics suggest that the clay was derived from naturally occurring kaolinite-rich deposits. The presence of a black core and red surface suggests that the pottery was fired in a kiln atmosphere that alternated between reduction (within the core) and oxidation (at the surface), reflecting deliberate control of redox conditions during firing [ 18 , 19 ]. Such pottery represents expertise in ceramic production. Archaeologically, this material reflects a technologically sophisticated and intentionally fired product. It serves as evidence of careful raw material selection, skilled shaping methods, and innovative kiln techniques developed to achieve both aesthetic and functional qualities. KLD-4: Advanced High-Temperature Ceramic Production The KLD-4 sample represents an exceptional advancement in ceramic technology that includes applied kiln control, compositional refinement, and technical sophistication [ 18 ]. The firing range of 800–1000°C and the required complete formation of ceramic phases represent a turning point in pottery production, as it corresponds to specialised workshops and organised systems of manufacture [ 2 , 4 , 14 ]. This sample provides compelling evidence of advanced socio-technical systems capable of producing dense, high-performance ceramics for functional and possibly high-status purposes [ 2 , 19 , 20 ]. The technical sophistication shown in KLD-4 corresponds to a point of cultural complexity in which craftsmanship and a division of labour were consolidated to fuel a market of standardized ceramics for communal or economic use, suggesting a fully matured and industrialized pottery. This high-performance pottery supports the view that Keeladi was a mature urban centre with integrated craft traditions. [ 2 , 4 , 19 , 20 ]. KLD-5: Dense Advanced Fired Ceramic . The KLD-5 sample represents the highest level of technological sophistication among the analysed samples. The vitrification and densification are estimated to be in the range of 900–1050°C. The strong mullite and glassy phase development indicates careful control of the firing temperature [ 18 , 20 ]. This does not seem to be accidental, but the artisan definitely had the technological knowledge to maintain such high temperatures consistently. Archaeologically, these types of ceramics were typically from advanced production centers or workshops [ 2 , 4 , 22 ]. The technical quality of KLD-5 implies a society with established resources and technical expertise for bulk production [ 4 ]. 4. Conclusions The integrated TGA-DSC analyses of the Keeladi pottery provide valuable insights into the technological advancements in firing methods, raw-material preparation, and ceramic production quality. The thermal results show consistent kaolinite dehydroxylation near 553–557°C for all samples. However, their high-temperature behaviour is different. KLD-2 (low-fired) and KLD-3 appear to be variably fired in their original production. In contrast, samples KLD-4 and KLD-5 show high-temperature sintering and vitrification. The study identified that the region's pottery tradition includes lower-fired earthenware below 700°C and well-made ceramics in the range of 900–1050°C. Our Thermoanalytical data provides the first thermogravimetric indications of high-temperature potteries in Keeladi. This confirms a clear technological progression from simple, open-firing practices to a more controlled, high-temperature kiln technology. The red colour in the pottery is attributed to hematite mineralogy, while the dual-redox nature of Black-and-Red Ware corresponds to limited-oxygen reduction in the core during the production stage. Overall, the results of this study strongly support the archaeological view that Keeladi was a highly urbanized region characterized by sophisticated, high-quality ceramic production units. Declarations To enhance readability and improve grammar, AI-assisted tools were used in this article. Acknowledgements The authors thank the State Department of Archaeology, Government of Tamil Nadu, Egmore, Chennai, for providing the samples. They also thank Mr M. Ramesh and Mr R. Ajay Kumar, Archaeological Officers, for their support and assistance. The first author gratefully acknowledges the guidance and encouragement of Dr R. Sivanantham, Joint Director, Tamil Nadu State Department of Archaeology. Author Contributions Gokul Vijay: Conceptualization, Methodology, Investigation, Formal analysis, Data curation, Writing – original draft. T. Arun Luiz: Conceptualization, Supervision, Writing – review & editing. Akhil Krishnan: Software, Data curation, Writing. V. P. Yatheesh Kumar: Writing – review & editing. All authors have read and approved the final manuscript. Corresponding author Correspondence to T. Arun Luiz Data Availability All data are included within the manuscript. 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Arun Luiz","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA1ElEQVRIiWNgGAWjYLCCBwwSDAzsDWA2YwMIEQQJQC08PAdI08LAwCORANNCAPDPPmP2ILHNwt5e8vnjzwUMNrIbDjC3PcCnReJcjrlBYpsEM490jpn0DIY04w0HGNsN8FpzhsdMAqiFDaiFjZmH4XAiUEubBD4d8lAtPDySxx9/5mH4T1iLAVSLBI8Eg4E0D8MBwloMz7CVSSSckzDgOQP0C49BsvHMwwS0yJ1h3ibxoazOnr0d5LAKO9m+4+3P8GpBdycQM5OgfhSMglEwCkYBdgAA7OY+2yZN0fIAAAAASUVORK5CYII=","orcid":"","institution":"Sri Sivasubramaniya Nadar College of Engineering (Autonomous)","correspondingAuthor":true,"prefix":"","firstName":"T.","middleName":"Arun","lastName":"Luiz","suffix":""},{"id":581590851,"identity":"5ae40c18-a981-4f0b-9517-5104c2a62324","order_by":2,"name":"Akhil Krishnan","email":"","orcid":"","institution":"Sri Sivasubramaniya Nadar College of Engineering (Autonomous)","correspondingAuthor":false,"prefix":"","firstName":"Akhil","middleName":"","lastName":"Krishnan","suffix":""},{"id":581590852,"identity":"6b3f8373-68d8-4553-9505-b08c49439be7","order_by":3,"name":"V. P. Yatheesh Kumar","email":"","orcid":"","institution":"State Department of Archaeology Tamilnadu","correspondingAuthor":false,"prefix":"","firstName":"V.","middleName":"P. Yatheesh","lastName":"Kumar","suffix":""}],"badges":[],"createdAt":"2025-11-17 07:53:34","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8132400/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8132400/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":101752150,"identity":"9d4a48d3-2525-451e-99a1-658b7501368f","added_by":"auto","created_at":"2026-02-03 10:25:43","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1108014,"visible":true,"origin":"","legend":"\u003cp\u003eGeographic Map of Keeladi Archaeological Site\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8132400/v1/aa97743ad701a25af98f6a87.png"},{"id":101880587,"identity":"2277e42a-d919-461a-b6e7-538c9857b3cf","added_by":"auto","created_at":"2026-02-04 15:04:06","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":548769,"visible":true,"origin":"","legend":"\u003cp\u003eImages of archaeological pottery samples from Keeladi\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8132400/v1/d97628a7cd114e444dc81632.png"},{"id":101751597,"identity":"8e16329a-2086-486b-88f1-bfa74bc66f0c","added_by":"auto","created_at":"2026-02-03 10:21:36","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":157537,"visible":true,"origin":"","legend":"\u003cp\u003eTGA and DSC curves of Pottery samples: KLD-1, KLD-2, KLD-3, KLD-4, KLD-5\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8132400/v1/e60a640161844a3e6444bf5c.png"},{"id":106517813,"identity":"504160fd-cdec-4879-b8eb-e16eeaac70b4","added_by":"auto","created_at":"2026-04-09 12:13:03","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2730969,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8132400/v1/9f4ec6f3-0e84-486c-b1b7-7ad3ef0410d2.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Thermogravimetric and Differential Scanning Calorimetric Analysis of Archaeological Pottery Samples from Keeladi, Tamil Nadu, India","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003ePottery can be considered one of the earliest technological inventions by humankind. The durability of pottery allows it to survive in the archaeological record for thousands of years [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Spectroscopic analysis of pottery provides information about raw materials, kiln conditions, and the technological sophistication of ancient societies. Organic additives (like dung or plant fibers) and temper materials (such as sand or grog) are usually added by artisans to clay for better workability and to boost high-temperature bearing potential [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The understanding of maximum firing temperature, soaking time, and firing atmosphere allows us to estimate the technological status of ancient societies [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThus, Keeladi, a prominent archaeological site in the Sivagangai District of Tamil Nadu, can be considered a transformative archaeological discovery in South India. This bridges the Iron Age (12th -6th century BCE) with the Early Historic Period (6th -4th century BCE) [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. The findings from the site offer strong evidence of early urbanization and advanced ceramic production in South India. This gives an insight into the possibility of advanced ceramic production processes comparable to those of other Early Historic civilizations, such as the Indus Valley [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/p\u003e \u003cp\u003eThe thermal behaviour of the clay matrix depends on aluminosilicate minerals and organic tempering materials, which directly influence the physical and chromatic properties of the ceramics [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The dehydroxylation of kaolinite and the combustion of organic residues result in weight losses and structural reorganization within the ceramic body, leading to vitrification and pore reduction [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Such thermal and mineral transformations, in turn, reflect the firing atmosphere, heating rate, and compositional heterogeneity of the clayey raw material [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. Therefore, the evaluation of thermal responses, coupled with mineralogical data, constitutes a reliable basis for classifying the Keeladi pottery and determining its firing range. Thermoanalytical methods, particularly thermogravimetric (TGA) and differential scanning calorimetric (DSC) analyses, were employed in this study to investigate the thermal effects occurring in Keeladi ceramics during progressive heating [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. The TGA and DSC analyses of Keeladi pottery provide a direct reconstruction of firing strategies, oxidation states, and thermal transformations, offering more scientific insights into the ancient technological intelligence that shaped one of South India\u0026rsquo;s earliest urban traditions [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cp\u003eIn this study, five pottery samples were collected from the Keeladi archaeological site (dated to the Sangam period) in southern India (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) and investigated through a Thermogravimetric Analyser (TGA) and Differential Scanning Calorimetry Analyser (DSC) in the temperature range of 20-1200\u0026deg;C under an air atmosphere, with a heating rate of 10\u0026deg;C min⁻\u0026sup1; using SDT Q600 V8.3 Build 101 instrument available in the Central Instrumentation Facility (CIF), at CSIR - CECRI, Karaikudi, Tamil Nadu, India. The controlled heating in a TGA helps detect and quantify mass losses, as well as identify processes such as dehydration, dehydroxylation, and decarbonation. Additionally, the DSC curves reveal the enthalpic changes in the material through variations in heat flow during heating, which are either exothermic or endothermic [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe analysed samples consist of Red Slipped Ware (KLD-1), Red Ware (KLD-2, KLD-4, KLD-5), and Black and Red Ware (KLD-3) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Before analysis, the samples were finely powdered using an agate mortar and sieved through a 2 mm mesh.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Overview of Thermal Behaviour\u003c/h2\u003e \u003cp\u003eThermogravimetric (TGA) and Differential Scanning Calorimetry (DSC) analyses were carried out up to 1200\u0026deg;C using five pottery samples from Keeladi (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), which include Red Slipped Ware (KLD-1), Red Ware (KLD-2, KLD-4, KLD-5), and Black and Red Ware (KLD-3). The combined analysis exhibits multi-stage thermal weight loss and heat flow patterns, effectively revealing their mineral, organic, and thermal history through temperature-dependent decomposition [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. These curves illustrate the combined effects of physically adsorbed water loss, combustion of organic matter, breakdown of carbonates, and potential high-temperature silicate transformations. Altogether, the TGA-DSC analysis provides comprehensive insights into the material characteristics of the ancient pottery [\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e\u003cb\u003e3.2 Low‑Temperature Region (25\u0026ndash;300\u0026deg;C\u003c/b\u003e)\u003c/h2\u003e \u003cp\u003eIn the initial stage, all the samples show an initial minor weight loss ranging from 0.9\u0026ndash;3.9%, which is attributed to the removal of physically adsorbed moisture, and volatile compounds often originate from ancient food residues, adhesives, resins, or natural waxes that were once part of the pottery's period of use or post-burial environment [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Samples such as Red Ware (KLD-2) and Black-and-Red Ware (KLD-3) exhibit higher early weight loss (3.3\u0026ndash;3.9%). It suggests greater porosity and water retention, characterised by less-fired or raw clay fabrics. In contrast, the Red Slipped Ware (KLD-1) shows very low initial loss (~\u0026thinsp;0.9%) with a weak endothermic DSC peak at 49\u0026deg;C (2.6 mW), indicating limited adsorbed moisture and a well oxidized slip [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Medium‑Temperature Transformation (300\u0026ndash;600\u0026deg;C)\u003c/h2\u003e \u003cp\u003eThe dehydroxylation, combustion of organic matter occurred in the temperature range of 300\u0026ndash;600\u0026deg;C. The endothermic peaks at 553\u0026ndash;557\u0026deg;C in all samples correspond to kaolinite dehydroxylation (Al₂Si₂O₅(OH)₄ \u0026rarr; Al₂Si₂O₇ + 2H₂O). The most intense dehydroxylation event was recorded for the Black-and-Red Ware (KLD-3) with 55.3 mW. This was followed by the Red Wares, KLD-5 with 54.9 mW and KLD-2 with 52.1 mW [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Both samples show evidence of a high kaolinite content and organic/volatile loss in those samples. The Red Slipped Ware (KLD-1) and Red Ware (KLD-4) showed moderate endothermic peaks, indicating more advanced clay transformations during firing. The Red Ware (KLD-2) experienced up to 4.8% mass loss during dehydroxylation, confirming incomplete firing and classifying it as low-fired earthenware [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Meanwhile, KLD-1 and KLD-4 retained higher residual masses (~\u0026thinsp;93\u0026ndash;94% at 600\u0026deg;C), consistent with well-developed metakaolinite and improved firing efficiency. These variations correspond to differing clay refinement levels and firing intensities\u0026mdash;from low‑temperature earthenware to highly processed, dense ceramics [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.4 High‑Temperature Phase Formation (600\u0026ndash;1200\u0026deg;C)\u003c/h2\u003e \u003cp\u003eWhen the temperature exceeds 600\u0026deg;C, sample‑specific differences were evident:\u003c/p\u003e \u003cp\u003e \u003cb\u003eRed Slipped Ware (KLD‑1)\u003c/b\u003e develops minor weight gain (600\u0026ndash;800\u0026deg;C) possibly due to the oxidation of ferrous iron (Fe\u0026sup2;⁺ \u0026rarr; Fe\u0026sup3;⁺) to form Fe₂O₃, the hematite pigment for the red slip. The breakdown of carbonate occurs between 800\u0026deg;C and 1000\u0026deg;C (-4.7%) is consistent with decarbonation of CaCO₃ into CaO\u0026thinsp;+\u0026thinsp;CO₂ [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. DSC features at ~\u0026thinsp;1060\u0026ndash;1147\u0026deg;C are consistent with mullite crystallization and partial vitrification. Overall weight loss (~\u0026thinsp;10%) and DSC data of KLD-1support the idea of original firing at temperatures likely in the \u0026asymp;\u0026thinsp;500\u0026ndash;900\u0026deg;C range under oxidizing conditions, and consistent with a red-slipped finish [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cb\u003eRed Ware (KLD‑2)\u003c/b\u003e shows primary dehydroxylation near 557\u0026deg;C (DSC\u0026thinsp;\u0026asymp;\u0026thinsp;52.1 mW). The TGA curve shows mass loss and TGA inflection at \u003cb\u003e\u0026asymp;\u003c/b\u003e\u0026thinsp;937\u0026deg;C (residual\u0026thinsp;\u0026asymp;\u0026thinsp;92.7%) and an end point near 1194\u0026deg;C (residual\u0026thinsp;\u0026asymp;\u0026thinsp;88.3%). The absence of previously formed mullite suggests that firing was limited to lower temperatures during production. Strong endothermic peaks at 1065\u0026ndash;1070\u0026deg;C during analysis indicate new mullite formation in the laboratory re-heating. This confirms that the sample is a lower fired (incomplete mullitization) earthenware with original firing temperature likely in the \u003cb\u003e\u0026asymp;\u003c/b\u003e\u0026thinsp;400\u0026ndash;700\u0026deg;C range [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cb\u003eBlack‑and‑Red Ware (KLD‑3\u003c/b\u003e) shows multiple higher-temperature DSC peaks at \u0026asymp;\u0026thinsp;688\u0026deg;C, \u0026asymp;\u0026thinsp;782\u0026deg;C (\u0026asymp;\u0026thinsp;69.9 mW), and \u0026asymp;\u0026thinsp;844\u0026deg;C (\u0026asymp;\u0026thinsp;67.9 mW). These correspond to organic combustion, iron oxide oxidation, and carbonate decomposition during progressive laboratory heating. The black core and red surface of pottery indicate firing under multiple atmospheres (reduction in the core, oxidation at the surface) during original firing. The last peak at 1098\u0026deg;C marks the formation of mullite. Based on the profiles and comparative studies the original firing temperature of the ware is estimated to be in the range of the \u0026asymp;\u0026thinsp;800\u0026ndash;900\u0026deg;C This reveals the complex redox control involved in the firing of this ware [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cb\u003eRed Ware (KLD‑4)\u003c/b\u003e shows dehydroxylation near \u0026asymp;\u0026thinsp;555\u0026deg;C (DSC\u0026thinsp;\u0026asymp;\u0026thinsp;49.6 mW) and advanced high-temperature events at \u0026asymp;\u0026thinsp;1048\u0026deg;C, \u0026asymp;\u0026thinsp;1106\u0026deg;C, and \u0026asymp;\u0026thinsp;1147\u0026deg;C. The latter two peaks indicate progressive mullite formation, glass-phase development, and advanced sintering. The small total weight loss (~\u0026thinsp;8%) and strong high-temperature DSC activity indicate an optimum firing range closer to \u0026asymp;\u0026thinsp;800\u0026ndash;1000\u0026deg;C in original production process [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cb\u003eRed Ware (KLD‑5)\u003c/b\u003e shows dehydroxylation near \u0026asymp;\u0026thinsp;553\u0026deg;C (DSC\u0026thinsp;\u0026asymp;\u0026thinsp;54.9 mW). The sample shows similar behaviour to KLD-4, with peaks at 1037\u0026deg;C and 1065\u0026deg;C (\u0026asymp;\u0026thinsp;41.6\u0026ndash;46.6 mW), indicating strong vitrification and mullite/glassy phase development. The total weight loss of around 11% and high thermal stability confirm an advanced production process. It was likely fired around \u0026asymp;\u0026thinsp;900\u0026ndash;1050\u0026deg;C range, which is typical of dense fine-ware [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Analysis of Firing Behaviour and Ceramic Technological Development\u003c/h2\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e indicates the progressive firing technological development of Keeladi pottery. The findings highlight a clear technological progression from simple, open-firing practices to a more controlled, high-temperature kiln technology. The Red Ware (KLD-2) and Black-and-Red Ware (KLD-3) were fired at low temperatures with incomplete vitrification, showing simple earthenware production. The dual red-black coloration of KLD-3 indicates controlled redox atmospheres during firing. The samples (KLD-4 and KLD-5) exhibit strong mullite formation, vitrification, and minimal weight loss, evidencing kiln-based, high-temperature firing approaching 1000\u0026deg;C. The Red Slipped Ware (KLD-1) represents an intermediate refinement stage, showing oxidizing conditions and fine slip control. KLD-5 shows a more pronounced vitrification away from modern high-performance ceramics. Red-based mineralogy explains hematite red colouring, while the dual-redox nature of Black-and-Red Ware is in accordance iron oxidation and localized reduction phases [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eFiring Behaviour and Ceramic Technological Development of Keeladi Pottery\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFiring Temp (\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eFeatures\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTechnological Interpretation\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKLD-1\u003c/p\u003e \u003cp\u003e(Red Slipped Ware)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;500\u0026ndash;900\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eModerate dehydroxylation; carbonate decomposition\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMedium-fired earthenware,\u003c/p\u003e \u003cp\u003eKiln firing\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKLD-2\u003c/p\u003e \u003cp\u003e(Red Ware)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;400\u0026ndash;700\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003elow structural maturity; incomplete mullitization\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eLow-fired earthenware,\u003c/p\u003e \u003cp\u003eOpen firing\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKLD-3\u003c/p\u003e \u003cp\u003e(Black-and-Red Ware)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;500\u0026ndash;900\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eOxidized slip, dense surface, limited moisture\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eControlled dual-atmosphere firing,\u003c/p\u003e \u003cp\u003eKiln firing\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKLD-4\u003c/p\u003e \u003cp\u003e(Red Ware)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;800\u0026ndash;1000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMullite formation, glass-phase development, advanced sintering\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMature fine pottery\u003c/p\u003e \u003cp\u003eAdvanced kiln firing,\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eKLD-5\u003c/p\u003e \u003cp\u003e(Red Ware)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e~\u0026thinsp;900\u0026ndash;1050\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eIntense mullite/glassy phase; vitrification; low porosity\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eDense fine-ware\u003c/p\u003e \u003cp\u003eAdvanced kiln control\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.6 Archaeological and Cultural Significance\u003c/h2\u003e \u003cp\u003e \u003cb\u003eKLD-1: Medium-Fired Pottery Tradition\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe KLD-1 sample represents the technological maturity of traditional pottery production. It thus confirms a well-established craft tradition, able to manage a controlled firing in the medium-temperature range of 500\u0026ndash;900\u0026deg;C [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Thermal stability and preservation of carbonates indicate a modest yet intended technological setup. This all aligns with local pottery productions at the community level, typically in the early urbanization of Keeladi. Overall, the firing precision in the KLD-1 sample marks a notable technological advance that would be characteristic of the later Iron Age or Early Historic periods, a time when ceramic production became regionally standardized and kiln operations were managed with stable and repeatable firing conditions [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cb\u003eKLD-2: Low-Fired Earthenware Technology\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe KLD-2 sample represents a traditional earthenware produced under a low-temperature firing profile ranging from 400\u0026deg;C to 700\u0026deg;C. This represents pottery produced in rural or small-scale production units. The high moisture absorption and dehydroxylation behaviour indicate a fabrication process oriented toward workability and thermal adaptability [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. These characteristics suggest that the ceramics were primarily intended for everyday domestic use such as cooking or water storage where the natural porosity of the material aided in cooling and allowed for vapour exchange. This pottery reflects an earlier or more conservative phase of ceramic production, during which kiln temperature control was limited [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. This indicates the technological constraints and resource optimization within the community firing tradition. From a cultural context perspective, this Keeladi sample aligns with utilitarian ceramic industry production that preceded or coexisted with advanced kiln technologies [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cb\u003eKLD-3: Well Managed Variable Atmosphere Firing\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe KLD-3 sample represents a variable-atmosphere firing stage in ceramic production [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The remarkable clay purity and complex decomposition characteristics suggest that the clay was derived from naturally occurring kaolinite-rich deposits. The presence of a black core and red surface suggests that the pottery was fired in a kiln atmosphere that alternated between reduction (within the core) and oxidation (at the surface), reflecting deliberate control of redox conditions during firing [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Such pottery represents expertise in ceramic production. Archaeologically, this material reflects a technologically sophisticated and intentionally fired product. It serves as evidence of careful raw material selection, skilled shaping methods, and innovative kiln techniques developed to achieve both aesthetic and functional qualities.\u003c/p\u003e \u003cp\u003e \u003cb\u003eKLD-4: Advanced High-Temperature Ceramic Production\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe KLD-4 sample represents an exceptional advancement in ceramic technology that includes applied kiln control, compositional refinement, and technical sophistication [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The firing range of 800\u0026ndash;1000\u0026deg;C and the required complete formation of ceramic phases represent a turning point in pottery production, as it corresponds to specialised workshops and organised systems of manufacture [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. This sample provides compelling evidence of advanced socio-technical systems capable of producing dense, high-performance ceramics for functional and possibly high-status purposes [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The technical sophistication shown in KLD-4 corresponds to a point of cultural complexity in which craftsmanship and a division of labour were consolidated to fuel a market of standardized ceramics for communal or economic use, suggesting a fully matured and industrialized pottery. This high-performance pottery supports the view that Keeladi was a mature urban centre with integrated craft traditions. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cb\u003eKLD-5: Dense Advanced Fired Ceramic\u003c/b\u003e.\u003c/p\u003e \u003cp\u003eThe KLD-5 sample represents the highest level of technological sophistication among the analysed samples. The vitrification and densification are estimated to be in the range of 900\u0026ndash;1050\u0026deg;C. The strong mullite and glassy phase development indicates careful control of the firing temperature [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. This does not seem to be accidental, but the artisan definitely had the technological knowledge to maintain such high temperatures consistently. Archaeologically, these types of ceramics were typically from advanced production centers or workshops [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The technical quality of KLD-5 implies a society with established resources and technical expertise for bulk production [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThe integrated TGA-DSC analyses of the Keeladi pottery provide valuable insights into the technological advancements in firing methods, raw-material preparation, and ceramic production quality. The thermal results show consistent kaolinite dehydroxylation near 553\u0026ndash;557\u0026deg;C for all samples. However, their high-temperature behaviour is different. KLD-2 (low-fired) and KLD-3 appear to be variably fired in their original production. In contrast, samples KLD-4 and KLD-5 show high-temperature sintering and vitrification. The study identified that the region's pottery tradition includes lower-fired earthenware below 700\u0026deg;C and well-made ceramics in the range of 900\u0026ndash;1050\u0026deg;C. Our Thermoanalytical data provides the first thermogravimetric indications of high-temperature potteries in Keeladi. This confirms a clear technological progression from simple, open-firing practices to a more controlled, high-temperature kiln technology. The red colour in the pottery is attributed to hematite mineralogy, while the dual-redox nature of Black-and-Red Ware corresponds to limited-oxygen reduction in the core during the production stage. Overall, the results of this study strongly support the archaeological view that Keeladi was a highly urbanized region characterized by sophisticated, high-quality ceramic production units.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eTo enhance readability and improve grammar, AI-assisted tools were used in this article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors thank the State Department of Archaeology, Government of Tamil Nadu, Egmore, Chennai, for providing the samples. They also thank Mr M. Ramesh and Mr R. Ajay Kumar, Archaeological Officers, for their support and assistance. The first author gratefully acknowledges the guidance and encouragement of Dr R. Sivanantham, Joint Director, Tamil Nadu State Department of Archaeology.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eGokul Vijay:\u003c/strong\u003e Conceptualization, Methodology, Investigation, Formal analysis, Data curation, Writing \u0026ndash; original draft.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eT. Arun Luiz:\u003c/strong\u003e Conceptualization, Supervision, Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAkhil Krishnan:\u003c/strong\u003e Software, Data curation, Writing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eV. P. Yatheesh Kumar:\u003c/strong\u003e Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\n\u003cp\u003eAll authors have read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorresponding author\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCorrespondence to \u003cstrong\u003eT. Arun Luiz\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data are included within the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthical Approval\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors did not receive support from any organization for the submitted work.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eWu, X., Zhang, C., Goldberg, P., Cohen, D., Pan, Y., Arpin, T. and Bar-Yosef, O.: Early pottery at 20,000 years ago in Xianrendong Cave, China. 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Archaeometry 59(2), 222\u0026ndash;238 (2017). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/arcm.12258\u003c/span\u003e\u003cspan address=\"10.1111/arcm.12258\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Keeladi, TGA-DSC, Firing temperature, Kiln, Black-and-Red Ware","lastPublishedDoi":"10.21203/rs.3.rs-8132400/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8132400/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAncient pottery samples from Keeladi, Tamil Nadu, dated to the 6\u003csup\u003eth\u003c/sup\u003e century BCE, were analyzed using Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). The main objective of this study is to investigate the thermal behavior of the samples and to evaluate the physical and chemical transformations that take place during heating. The findings highlight a clear technological progression from simple open-firing practices to more controlled, high-temperature kiln technology. The samples vary from lower fired earthenware fired below 700°C to high-temperature ceramics fired between 900-1050 °C, demonstrating a technological evolution toward dense, high-performance ceramics in Keeladi’s pottery tradition. These results indicate a more pronounced vitrification process, with qualities comparable to those seen in modern high-performance ceramics. The red colour in the pottery is attributed to hematite mineralogy, while the dual-redox nature of Black-and-Red Ware corresponds to limited-oxygen reduction in the core during the production stage is an example for well managed variable atmosphere firing. Altogether, the results identify Keeladi as an advanced pottery production center with notable kiln control, raising an important question about how this 2500-year-old community achieved such high firing temperatures.\u003c/p\u003e","manuscriptTitle":"Thermogravimetric and Differential Scanning Calorimetric Analysis of Archaeological Pottery Samples from Keeladi, Tamil Nadu, India","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-30 09:02:10","doi":"10.21203/rs.3.rs-8132400/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"27c7f834-2a7f-46e7-aa2a-ceffd2f5b760","owner":[],"postedDate":"January 30th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-04-09T12:12:35+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-30 09:02:10","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8132400","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8132400","identity":"rs-8132400","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}