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Minerals and Metals in Ancient India v1.c27.s1: The Evaluation of the Literary Evidences

Minerals and Metals in Ancient India v1.c27.s1

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The Evaluation of the Literary Evidences

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Herodotus wrote that the Indians in the Persian army were equipped with cane arrows tipped with iron. Many such arrowheads were found in the contemporary sites of Ujjain and Kauśāmbī. Some of these were surface carburised. The Greek physician Ctesias referred to ‘two magical swords of Indian iron’, but Pleiner (1971) points out that the text is not clear whether the swords were actually Indian or merely made of Indian iron.

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Quintus Curtius reported that Alexander the Great received one hundred talents, about three tons, of *ferrum candidum* or bright iron from the vanquished Malloi and Oxydrakai (326 BC). Most possibly this was not steel but carburised iron (white cementite or wootz?) which could be converted into steel. In Kauṭilya’s *Arthaśāstra* (later half of fourth century BC), the words *vaikṛntaka*, *kālāyasa* and *vṛtta* have been mentioned several times (2.12.23; 2.17.14; 4.1.35) and translated by several scholars as magnetite, wrought iron and steel respectively. Was *vṛtta* ‘the crucible steel’, so named on account of the approximately circular cross-section of the carburised iron ingot (made from magnetite) solidifying in the crucible, and later known in South India as wootz? It is not clear whether the 108 ingots, obtained in the first century AD site of Taxila, which measured 1-2 inches in cross-section, were circular in cross-section and prepared in crucibles. These were porous, contained 0.1 per cent carbon, and could have been carburized at a later stage; were crucibles used for this purpose?

What is ‘Steel’?

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Many of the above questions are yet to be answered satisfactorily. It should be made clear that there is some controversy about the precise definition of ‘steel’. According to Rollason (1973: 160), “the essential difference between ordinary steel and pure iron is the amount of carbon in the former, which reduces the ductility but increases strength”. This definition does not articulate what are the minimum values of carbon percentage, strength and ductility which an iron sample should possess in order to be considered as ‘steel’. We know that modern steel may be eutectoid or pearlitic (0.87% carbon), hypereutectoid (more than 0.87 % carbon) mixture of pearlite and cementite (Fe₃C) or hypoeutectoid (less than 0.87 % carbon) mixture of pearlite and ferrite. While ferrite is soft (BHN 80), cementite is very hard (BHN 600+) but brittle. Pearlite consists of alternate layers of ferrite and brilliantly white cementite, and thus possesses reasonable hardness and ductility. Many of the ancient Indian iron samples, mentioned in chapter 12, could pass as ‘steel’ by virtue of their carbon content, hardness (BHN) and demonstration of pearlitic phase.

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Today, a key procedure followed for steel-making is hardening by quenching, which produces martensite needles — highly stressed α- iron supersaturated with carbon — with adequate hardness (BHN 200-400). The hardened steel is usually tempered (heated to a temperature below the lower critical temperature and then cooled at a desired rate) to relieve the internal stresses, and improve ductility with slight reduction in hardness. We reported in chapter 12 that a couple of Indian iron samples belonging to third and second century BC showed martensitic structure, proving thereby that the quenching procedure was known to the Indians during the pre-Christian era.

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During the post-Christian era, Yavaneśvara’s book Yavanajātaka (AD 150) mentioned śastram or steel weapon. Varāhamihira’s Bṛhat Saṃhitā (BS) (sixth century AD) contained a separate chapter (No. 50) on swords: Khaḍgalakṣaṇam which contained several recipes for quenching and hardening of steel. The procedures of quenching known as śastrapāna or ‘sword-drink’ involved the use of different liquids such as blood, ghī, water, butter-milk etc., which had different specific heats and thus ensured different rates of cooling of the hot sword. After proper sharpening, the swords ‘did not break on stones, nor became blunt on other instruments’. (BS, 50.26) These swords were certified as being adequately hard as well as tough (non-brittle).

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Salmasius mentioned an early Greek chemical text entitled ‘The Tempering of Indian Iron’. There is a tradition that Diocletian’s (fourth century AD) armory at Damascus was supplied with Indian iron or steel. The circumstantial evidences in favour of Indian steel are indeed weighty, and even Bronson (1986: 18) concedes:

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That Indian steel existed and was traded internationally from at least the second century AD onwards seems indisputable. (evidence from Periplus)

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Yet, Bronson questions “whether Indian steel was made in crucibles during the Roman period, and whether it was outstanding in quality”. The celebrated *Encyclopaedia of Indian Archaeology* (Ghosh, 1989: Vol. I, pp. 320-21) records the different kinds of crucibles made and used in India, starting from the Harappan to the Historical period — their heights ranging from 24 to 164 mm and maximum diameters from 24 to 57 mm. The crucibles in Sambhar, Nasik, Nevasa, Shamalaji, Maheshwar, Ujjain, Takalghat, Nāgārjunakoṇḍa, Kanheri etc. were used during the Historical Period (500 BC-AD 500) for making glass, smelting copper and iron, preparing alloys etc. We are not definite if some of these were used during this period, i.e., prior to AD 500, for converting wrought iron to wootz steel.

The Later-Day Evidences

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Bronson records that the evidences of wootz being made in India come from much later medieval periods. Wrought iron, made from direct reduction of magnetite, used to be heated with wood or organic materials in closed crucibles over charcoal fire, maintained at high temperature by blowing air.

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Bronson (1986: 39-46) has compiled the information on the various processes of making wootz in pre-modern India, studied by the previous scholars. (Buchanan 1807; Heyne 1814; Voysey 1832; Heath 1839; Schwartz, 1901; Coomaraswamy 1956; Rao *et al.*, 1970 etc.) The common denominator of most of the processes was to heat the charge of iron and vegetable matter in a sealed crucible. Voysey (1832: 247) observed a process at Golconda (Hyderabad) in which low-carbon and high-carbon iron smaples were fused together. Additional carbon could have come from rice husk in the crucible wall.

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Coomaraswamy (1956) reported a Ceylonese steel or wootz assaying 1.97 per cent carbon. Verhoeven (1987a) wondered whether wootz was first made in the 2.5-3.5 per cent carbon range and then de-carburized. He favoured the other notion that most wootz had lower as cast carbon level of 1.6 per cent, which meant that the crucible temperature was at least in the 1400-1450°C temperature range. Rao *et al.* (1970: 16) detected cristobalite in the fired crucible proving that the crucible must have been heated over 1470°C at least for a short duration. Such a temperature could be reached in a bellows-blown charcoal fire. The ingots were probably in nearly molten condition, although the distinct carbon concentration profile in some samples suggested solid state diffusion rather than mixing of carbon. Smith (1960: 23) published the cross-section of a nineteenth-century wootz ingot (1.34 % carbon) showing the dendritically-induced microsegregation of the impurity elements: P (0.1 %), Mn (0.06 %), Si (0.04 %) and S (0.02 %). The Oriental Damascus dagger and sword samples contain 0.1 to 0.25 per cent phosphorus. Verhoeven and Jones (1987b) have shown that the high phosphorus impurity levels of Damascus steels could have controlled the mechanism of cementite formation. The formation of primary cementite between austenite dendrites during solidification could lead to the Damask pattern of the final sword.

References to Indian Steel: Sixth-seventeenth Centuries AD

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The carburized metal (often having hypereutectoid composition and pronounced brittleness) used to be converted to steel objects and exported for the manufacture of Damascus blades.

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Export of Indian iron (or steel or wootz) was recorded not only by the Romans but also by the Arab and Iranian scholars. Bronson (1986: 19) himself records several pre-Islamic Arab literatures of the sixth and early seventh century in this connection. The Prophet was compared to a ‘drawn muhenned (Indian) sword in Allah’s armory’. Similar references to ‘swords made in India’ were made by Jabir Ibn Hayyan (eighth century AD), Al-Kindi (ninth century), Al-Biruni (11th century AD) etc., till the knowledge of making swords of steel was widespread in Sri Lanka, Malayasia, Arab countries, Iran, etc. Al-Idrisi wrote in Al-hadid Al-Hindi (twelfth century AD):

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The Hindus excell in the manufacture of iron, and in the preparation of those ingredients along with which it is fused to obtain that kind of soft iron which is usually called Indian steel (Hindiah). They also have workshops wherein are forged the most famous sabres in the world . . . It is impossible to find anything to surpass the edge that you get from Indian steel. (quoted by Bronson, 1986: 20)

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The above reference is clearly related to the organised industry in twelfth century AD. India producing crucible steel of good quality. Some of the adverse comments on the brittleness of Indian steel, quoted by Bronson, can be easily explained. India used to produce good quality steel (medium hardness and ductility), low-carbon soft iron, as well as high-carbon, hard but brittle steel. The last variety gave brittle materials when fabricated, but also served as a good source of carbon (vide Table 19.1).

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As a matter of fact, one of the processess for manufacturing good quality wootz during the medieval period was to mix at high temperature (a) low-carbon soft iron and (b) high-carbon brittle iron to produce intermediate carbon good steel. The commodity (a) used to be exported from India during and after eleventh century AD for consumption by the Arab iron-smiths who had learnt to make (b) high carbon brittle wootz, and knew that this had to be mixed with (a) of ‘magical property’ to get good steel. Thus, India was the source of (a), (b) as well as the best wootz of intermediate property.

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Elemental Composition of Samples of Traditionally Made Indian Steels

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| Steel | Origin | Per cent Carbon | Other Elements |
| Wootz | Sri Lanka | 1.97 | S and Mn 0.07, P 0.02 |
| Wootz | India | 1.642 | S 0.181 As 0.037 |
| Wootz | India | 1.652 | S 0.17 As 0.036 |
| Wootz | Mysore | 0.963 | S 0.02 Mn 0.097 Si 0.13 |
| Wootz | Mysore | 0.15 | S 0.01 P 0.27 Si 0.14 |
| Kheri | Jabalpur | 1.214 | S 0.002 P 0.14 Mn 0.3 Si 0.1 |

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Note : Compiled from different sources by Bronson. The dates of the samples are not given: presumably not before AD 1200 Kheri was prepared from ‘sorted fragments of steely blooms’.

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During thirteenth-fifteenth centuries AD, the Chinese records referred to the import of jewelled swords from ‘Banggela’ or Bengal in East India. From sixteenth century AD onwards, wootz used to be mentioned by many European travellers. Tavernier (1679) was quite explicit about the connection between Damascus blades and Golconda (Hyderabad) steel:

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The Persians are excellent artists at Damasquing with vitriol. But the steel is brought from Golconda, there being no steel in the world but that of Golconda which can be Damasked . . . . This steel is sold in pieces as large as our one-sou loaves.

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Tavernier also mentioned that Golconda steel should be given moderate heat treatment; drastic quenching makes it brittle. Evidently, these were high carbon cementitic materials which had to be mixed with low-carbon soft iron and/or subjected to optimal heat-treatment in order to produce the best varieties of steel. The writings of Jean Baptiste Tavernier (1679) and Jean Chardin (1711) make it abundantly clear that India used to export (a) and (b) varities as well as the third: best quality wootz of intermediate carbon content. The art of preparing the best quality wootz, which could be directly used for sword-making, was a well-guarded secret.

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.

Concluding Remarks on Wootz

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The issue raised by Bronson (1986) as to whether India produced crucible steel much before sixteenth century AD is wide open for further research. The last word on the subject is yet to be written. While we must wait for fresh archaeological evidences, the circumstantial evidences from the literature seem to be re-assuring. Alexander must have been presented with a variety of carburised iron which could be converted to steel. Third century BC Pāṇḍu Rājār Dhibi craftsmen employed quench-hardening technique to produce sharp and hard steel from carburised iron. Taxila hypereutectoid steel assayed as much as 1.23 per cent carbon.

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The Periplus of the second century AD records export of Indian iron and steel. During the Gupta era, there were several methods to quench-harden and produce steel sword. The Indian export item ‘steel’ or high carbon iron could have been produced by ‘lamination technique’, (which we have discussed in chapter 12), or the crucible method. The thirteenth century text Rasaratnasamuccaya refers to several kinds of crucible, including closed crucibles, to be used at high temperatures and reducing environments. Al-Idrisi’s writing (12th century AD) (quoted earlier) clearly shows that ‘Indian workshops fused iron with ingredients’ (presumably in crucibles) and from this ‘forged the most famous sabres’.

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