Minerals and Metals in Ancient India v1.c27.s1.t4: The Art of Damscening Wootz
Minerals and Metals in Ancient India v1.c27.s1.t4
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The Art of Damscening Wootz
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The literatures written on the scenario after AD 1200 are outside the scope of our dissertation. Yet, we are mentioning some of these evidences merely to show that there has been an uninterrupted sequence of references to Indian iron and steel starting from fifth century BC to nineteenth century AD.
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As recently as 1985, Sherby and Wadsworth studied how Damascus swords, containing more carbon than most modern steel, could have been prepared from Indian Wootz. The typical manufacturing procedure given by them is provided in fig. 19.1.
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High-carbon wootz (crucible steel) cakes were probably forged at a not-too-high temperature in the range of 650-850°C (between ‘blood-red’ 650°C to ‘cherry-red’ 850°C) and then heat-treated by quenching. The European smiths failed to reproduce the result, probably because they were familiar with only low-carbon steel having a higher m.p., and tried to forge wootz at a white heat, when the cementite phase dissolved in the austenite, and the brittle structure ‘crumbled under the hammer’.
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Sherby and Wadsworth (1985) have shown that high-carbon (1.3-1.9% carbon) wootz can be satisfactorily forged up to a temperature of 850°C without producing brittleness; slow cooling allowed a coarse, continuous and white cementite network which could be seen after selective etching preferentially attacking the ferrite background. These damask patterns of cementite network have been poetically described as ‘ten millions of (white) meandering lines in the dull blue sword’.
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Heating and quenching produced martensitic structure providing additional hardness. If the Damascus blade were heated to only just above 727°C before being quenched, it would have been hard as well as tough (non-brittle). On the other hand, heating to much above 800°C before quenching would endow brittleness to the sword, giving a bad reputation to Indian wootz!
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The rates of cooling of the ingot and sword at various stages are critically important; the medieval smiths attributed magical properties to the quenching media such as blood, urine, milk etc. which, possessing different specific heats, ensured different rates of cooling of the hot plunging sword.
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Sherby and Wadsworth further showed that heavy rolling during the cooling of the ingot results in dispersed cementite structure and no damask pattern. Such damask-free ultrahigh-carbon steels are stronger and yet more ductile than conventional steels. With their superplastic properties, they can be shaped with precision into complicated objects. The ancient smiths might have also stumbled on to such high carbon steel without damask pattern, which was hard and yet tough. The ill-reputation of the ‘brittle’ Indian wootz was the result of the ignorance of some of the users, who did not know how to make the finished product of desirable properties. The ancient world however knew that Indian wootz was an unique, if not the only, starting material for making Damascus steel. (Smith, 1983; Pant, 1985; Verhoeven and Jones, 1987b)
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CASTING 1,200°C. IRON ORE CHARCOAL BELLOWS SPONGE IRON WROUGHT IRON 1,200°C. SLOW COOLING WOOTZ CHARCOAL CRUCIBLE
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FORGING 650°C. — 850°C. WOOTZ
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HEAT TREATING QUENCHING HARDENED BLADE
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Fig. 19.1: Typical manufacturing procedure for a Damascus sword from Indian wootz.