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The details of the furnace described and sketched by Buchanan [8] indicate that crucibles were packed in rows of about fifteen inside a sunken pit filled with ash to constitute the furnace which was operated by bellows of the buffalo hide, fixed into a perforated wall which separated them from the furnace probably to minimize fire hazards (Figure 6). The fire was stoked from a circular pit which was connected to the bottom of the ash pit. The crucibles themselves were conical and could contain up to 14 oz. of iron, along with stems and leaves. The wootz steel process in general refers to a closed crucible
Figure 4. View of newly identified old dump for high-carbon wootz crucible steel production from South Arcot, Tamil Nadu (photographed by S. Srinivasan)
Figure 5. Fragments of newly identified remains of fired wootz crucibles from Gulbarga, Karnataka (photographed by S. Srinivasan)
Figure 6. Furnace for production of crucible steel production sketched by Buchanan (1807) during his travels, indicating that crucibles were packed in a pit with the furnace being operated by bellows of buffalo hide (reproduced from K. N. P. Rao, unpublished monograph)
process and Lowe [32] has remarked that the processing of plant and mineral materials in closed crucibles is often described in Indian alchemical Sanskrit texts of the 7th-13th c. AD.
Investigations by Craddock [34] indicated the wootz ingot itself had a dendritic cast structure. Lowe [32, 33] has investigated particularly well the refractory nature of the crucibles of the crucibles which indicate that they were robust enough refractories to withstand the long firing cycles of up to 24 hours for the process. The formation of mullite and cryistobalite was detected in the crucible fragments studied by Lowe [32, 33] suggesting they had been well fired to high temperatures of over 1300-14000 C, while Rao et al [31] also observed the formation of mullite and cryistobalite in crucibles.
However the microstructures investigated by Lowe [32] of the metal remnants within the particular Deccani crucibles studied by her from Konasamudram could only be related to a failed process of crucible steel production at that particular site or context since they related more to white cast iron, a brittle and not very malleable material formed by over-carburisation, rather than ultra-high carbon steel. In fact based on these findings Lowe [32] has preferred to cautiously aver that it was a white cast iron ingot that was produced by the Indian crucible process. Craddock [34] has also opined that the product of the Indian crucible steel process was probably a general homogenous steel rather than specifically a high-carbon steel.
On the other hand investigations by Srinivasan [3] and Srinivasan and Griffiths [5] indicated the presence of solidified metal droplets in the crucibles with a typical micro-structure and micro-hardness corresponding to a good quality hypereutectoid steel with the formation of hexagonal grains of prior austenite with fine lamellar pearlite within the grains, with the precipitation of pro-eutectoid cementite along the grain boundaries of prior austenite: which is in fact the classic structure of ultra-high carbon steels of about 1.5% C which were made under laboratory conditions by Wadsworth and Sherby [17}and Verhoeven et al. [29]. The findings reported in Srinivasan [3] and Srinivasan and Griffiths [5] are hence significant in that they prove beyond doubt that high-carbon steels were indeed made by crucible processes in south India. Studies by Srinivasan and Griffiths [5] also indicated that temperatures of over 14000 C had indeed been reached inside the crucibles to melt the wrought iron and carburise it to get a molten high-carbon steel with the typical hypereutectoid structure on solidification.
Conclusions
The above review indicates that the reputation of wootz steel as an exceptional and novel material is one that has endured from early history right into the present day, with the story of the endeavours to study it in recent history being nearly as intriguing as the story of its past. The archaeological findings indicate that crucible steel does have an ancient history in the Indian subcontinent where it took roots as suggested by literary references, while the analytical investigations indicate that a high-grade ultra-high carbon steel was indeed produced by crucible processes in south India. Recent investigations on the properties of the ultra-high carbon wootz steel such as superplasticity justify it being called an advanced material of the ancient world with not merely a past but also perhaps a future.
Acknowledgements
The authors would like to acknowledge the Indian National Academy of Engineering. Srinivasan would like to acknowledge the support of British Council, New Delhi for a British Chevening Scholarship for doctoral research, and the interest of Dr. D. Griffiths, Institute of Archaeology, University College London, Dr. J. A. Charles, Cambridge University, late Dr. C. V. Seshadri, founder-President, Congress of Traditional Science and Technology, and Hutti Gold Mines Ltd. for assistance with fieldwork and the support of the Homi Bhabha Research Council.
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