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Minerals and Metals in Ancient India v1.c27: 19. Iron in the Ancient India of the Post-Christian Era

Minerals and Metals in Ancient India v1.c27

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19. Iron in the Ancient India of the Post-Christian Era

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We have discussed in chapter 12 the emergence of the Iron Age in India and its developments during the pre-Christian era. It has been established that: (a) India developed iron-making traditions independently and indigenously in at least three nuclear zones; (b) the Indian sub-continent was one of the earliest sites for the discovery of lamination technique viz., forge-welding of alternate layers of uncarburized and carburized sheets of iron; (c) the technique of quench-hardening and producing the martensitic structure, well-known in steel technology, was practised by the Indians at least during third century BC if not earlier.

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Further developments in ancient India were noted in chapter 14 which records Taxila of the first century AD producing iron swords and adzes containing 1.2-1.7 per cent carbon. During this century, certain Gujarat sites like Dhatwa and Shamalaji produced iron for domestic and overseas consumption. The Roman trade with India had started; *Periplus of the Erythrean Sea* (second century AD) recorded that ‘*sideros indikos kai stomoma*’ or ‘Indian iron and steel’ used to be shipped from the inland part of Ariake or the gulf of Cambay area.

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Bronson (1986) accepts that India might have made steel during the second century AD, but questions whether it was the celebrated wootz of the later period; whether wootz was a super-steel or merely carburized iron, and whether the literary references to Indian iron and steel should be accepted uncritically. Prakash (1989) commented on Bronson’s paper and emphasized on the antiquity of Iron Age in India; however, his comments and Bronson’s rejoinder (1989) do not shed any further light on the subject.

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Carburized iron showing pearlite and ferrite grains was known in ancient India as early as 1200 BC, but this is not accepted as equivalent to modern steel. A high percentage of carbon results in the cementite (Fe₃C) phase which contributes to hardness (BHN 600+) as well as brittleness. Bronson (1986) referred to the ‘dangerous brittleness’ of the wootz weapons. The implement can be made ductile by careful forging at a not-too-high temperature of 650-850°C when the coarse cementite grains are compressed and elongated in one direction, surrounded by pearlite and ferrite matrix.

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To produce steel, reheating and quenching is necessary; such a treatment results in martensitic transformation and high Brinell Hardness number which cannot be achieved by mere carburization. The Damascus steel, swords, produced from Indian wootz, constitute of high carbon steel with (white) cementite streaks in pearlite-graphite-martensite matrix, and possess the ideal combination of hardness (even at the edges) and ductility. (Sherby and Wadsworth, 1985) Bronson wondered when were such steel swords produced in India.

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’.

The Tradition of Forge-Welding

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Ever since 1200 BC (vide Komaranhalli evidence), India developed the lamination technique for forge-welding thin carburized sheets. This practice continued for centuries. Gautama Buddha mentioned about corroded iron undergoing further corrosion (Dhammapada, Malavagga, 240) and took his last meal from Chunda, an iron-smith. Pāṇini referred to ayoghana, a forge-hammer in his Aṣṭādhyāyī (3.3.82). Kautilya’s writing showed his familiarity with the art of forge-welding;

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he commented (in his Arthaśāstra, 7.3.8-9): “Heat is the means of joining together; metal that is not heated does not become joined with metal”. Patañjali wrote a short treatise Louhaśāstra on iron metal.

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In the post-Christian era, Kālidāsa referred to forge-welding in his Vikramorvaśīyam (3.16). Subandhu, a contemporary of Candragupta Vikramāditya II (AD 380-414), and author of Vāsavadattā, wrote about varieties of iron and the Mālwā traditions in metallurgy. We learn from Epigraphica Indica (Vol. 30, p. 478) that in AD 592, Viṣṇusena, a king of the Mālwā region, issued the famous commercial charter for the conscription of iron-smiths by royal officers for state purposes. In his Yuktikalpataru king Bhoja (AD 1000-55) discussed manufacture of iron weapons, and referred to the earlier texts on metallurgy such as Louhārṇava, Louhadspa and Louhapradīpa.

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Let us now present the technical details of the internationally famous forge-welded structure — the Delhi Iron Pillar — the pride of ancient Indian metallurgy.

The Delhi Iron Pillar

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In Meharauli, six miles south of New Delhi, there is the famous 1600-year old Iron Pillar which is a forge-welded wrought iron structure assaying approximately 0.15 per cent carbon, 0.25 P, 0.005 S, 0.05 Si, 0.02 N, 0.05 Mn, 0.03 Cu, 0.05 Ni and balance 99+ iron. The pillar has rusted little over the ages, and thus remains the centre of attraction for historians and metallurgists all over the world.

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The pillar is 737.5 cm high of which 50 cm is below ground level and 45 cm is surrounded by a stone platform. The diameter of the pillar at the ground level is about 41.6 cm tapering upwards to about 30.4 cm just under the decorative capital which tops the column. Below the ground level, the pillar widens to a diameter of 62 cm. On excavation (Ghosh, 1963), a heavy slab of stone was found horizontally on the stone foundation of the yard; on this slab was a thick lead plate (99.2 % lead with little Cu and Ag) on which the iron pillar rested. The base of the pillar was flat, and eight small projections shaped as the toes of an elephant were seen at equal intervals. These eight iron rods protruded and penetrated into the thick lead plate. The weight of the pillar is estimated to be more than 6096 kg. The top surface is flat and a separate cylinder of solid iron forced into a hollow space provided in the main pillar. The central rectangular slot in the iron cylinder probably served to accommodate flag-staff.

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The Sanskrit inscription on the pillar (even now sharp and distinct) states in three verses how the king Candragupta Vikramāditya II (AD 375-413) erected the pillar as a dhwaja or monument in a shrine dedicated to Viṣṇu, on his demitting his office (as king) and his taking to another, i.e., the life of renouncement of the world. On a unique coin-type of Kumāragupta I, son and successor of Candragupta Vikramāditya II, the new ruler is seen saluting his old parents of the iron pillar fame. The iron pillar was placed in the Viṣṇu temple (somewhere in north India — Kashmir or Mathurā?) from where the pillar was brought to the present site during the eleventh century AD.

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Sir Robert Hadfield (1912, 1922 and 1925) first studied the material of the pillar scientifically. A more thorough investigation was presented by Ghosh (1963). Wranglén (1970) has reviewed the various theories for the ‘rustlessness’ of the iron pillar.

The Composition and Properties of the Delhi Iron Pillar

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The material of the pillar is wrought iron obtained by solid state reduction. Regarding the composition of the Delhi pillar, several analyses have been published, and some of these are summarised in Table 19.2.

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Table 19.2 Chemical Analysis of the Delhi Iron Pillar

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| | Ghosh (1963) | Hadfield (1925) | N.M.L. (1956) |
| C | 0.28 | 0.08 | 0.28 |
| Si | 0.026 | 0.016 | 0.056 |
| P | 0.18 | 0.114 | 0.155 |
| Mn | Nil | Nil | Nil |
| S | Trace | Trace | Trace |
| N | 0.0065 | 0.032 | — |
| Cu | — | 0.034 | — |
| | Total | Fe 99.72 | |
| | sp. gr. | 7.81 | |

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According to the latest analyses of the pillar, including the underground parts which assayed 0.44-0.48 per cent phosphorus, the average composition of the material in the pillar may be given as:

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0.15 per cent C, 0.25 P, 0.005 S, 0.05 Si, 0.02 N, 0.05 Mn, 0.03 Cu, 0.05 Ni balance Fe.

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The pillar was forge-welded from a great many sponge iron lumps of different composition, so that the elemental composition and the microstructure are variable from layer to layer. Micro-examination of unetched section of the pillar iron reveals slag particles distributed irregularly. In etched condition, polyhedral grains of ferrite with some slip bands in some grains near the surface are seen. Some ferrite grains have a small amount of pearlite at the grain boundaries. The slag content is high and this contains magnetite and FePO₄, seen as yellow specks on the surface.

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The hardness test for the different zones were related with the microstructures. The data are provided in Table 19.3.

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Table 19.3

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Hardness Test Results of the Delhi Iron Pillar

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| Description | Hardness (Brinell) |
| 1. Coarse ferrite grains-cold worked | 164, 155, 148 |
| 2. Coarse ferrite grains with slag | 84.8, 139 |
| 3. Coarse ferrite grains with nitride needles and slip band | 115 |
| 4. Ferrite with small amount of pearlite | 138 |
| 5. Ferrite with grain boundaries, pearlite and forge-welded zone | 136 |
| 6. Ferrite with slightly more pearlite | 140, 125 |
| 7. Pearlite with ferrite | 162 |
| 8. Increasing pearlite with ferrite | 162 |
| 9. Still more pearlite | 165 |
| 10. Maximum pearlite | 177 |

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The carbon content shows considerable variations, and its average value is not particularly low. The content of carbon and of pearlite are low in the surface layer (increase inwards) apparently as a result of surface decarburization during hammer-forging.

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The phosphorus content is very high and is less varying. While P in steel occurs as a solid solution in the ferrite (up to 1 %), a considerable part in wrought iron may appear as slag inclusions of FePO₄. Naturally, the strongly oxidized parts and the surface layer, depleted in carbon, tend to be rich in phosphorus. In accord with the above, Ghosh (1963) found that of the 0.28 per cent P in a sample from the pillar, 0.18 per cent occurred as elementary P, the remainder occurring as phosphate slag. In oxygen-consuming corrosion, as in water and in the atmosphere, high P-content actually exerts a beneficial influence, probably because phosphorus in the ferrite is oxidized to phosphate which, as an inhibitor, promotes the formation of protective oxide films.

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The sulphur content of the Delhi iron pillar is very low, according to all determinations, probably because charcoal was used in reducing the ore. Coupled with the fact that manganese-content of Delhi iron pillar is also very low, the low content of sulphur means that there are very few centers of iron-rich manganese sulphide in the pillar which could initiate pit corrosion by serving as effective local cathodes. The contents of Si, Cu and Ni in the pillar are also very low, these values being typical of the ancient Indian wrought irons.

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Wranglén (1970) has pointed out that in terms of the carbon and silicon contents, the pillar roughly corresponds to an ordianry construction steel of the type En 3 (AISI 1015) of semi-killed quality, and differs from the En 3 steel in the following ways:

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(i) high slag content and marked heterogeneity
(ii) higher P content, about 5 times as high in steel
(iii) lower Mn content, as compared to 0.5 per cent in steel
(iv) S content, an order of magnitude lower than in steel.

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The composition of the pillar is favourable for a good corrosion resistance in the atmosphere.

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Rusting of the Pillar

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The apparent rustlessness of the Delhi pillar has been investigated by several scholars. (Hadfield, 1912, 1922, 1925; Ghosh, 1963; Lahiri et al., 1963; Bardgett and Stanners, 1963; Wranglén, 1970) The part of the pillar which is below ground is covered by a rust layer, about 1 cm thick, and shows deep pitting. Above ground, the pillar is coated by a protective film, 50-500 µm thick, analysed by Ghosh (1963) as: 67.0 per cent Fe₃O₄, 13.1 per cent FeO, 14.8 per cent H₂O, 1.7 per cent FePO₄, 3.1 per cent SiO₂, 0.2 per cent MgO and 0.1 per cent CaO.

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The much larger proportion of the magnetic fraction in the rust of the pillar — both aboveground as well as underground — compared to the non-magnetic fraction is a unique feature. We find in the rust of the ordinary mild steel much greater proportion of the non-magnetic (Fe₂O₃, nH₂O) over the magnetic (Fe₃O₄). Evidently Fe₃O₄ in the Delhi pillar rust has not fully undergone further oxidation to the Fe₂O₃ stage.

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There is an enrichment of P in the rust (0.35 per cent P in Fe), as compared to the substrate. Wranglén (1970: 766) believes that phosphorus evenly distributed as a solid solution in the ferrite has probably contributed more to the formation of the protective surface film than the heterogeneously distributed inclusions of phosphate slags. The larger proportion of silicon on the surface (compared to that in the bulk) is explained through the proven presence of quartz which is the consequence of sandstorms common in the Delhi area. MgO and CaO on the surface might have arisen due to sandstorm or from the slag in the body of the pillar.

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The Causes for the Rustlessness of the Delhi Iron Pillar

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The remarkable freedom of the Delhi iron pillar from rusting has been attributed to various reasons which can be categorised under the following two headings:

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I. The compositional factors stressed by the Indian investigators, and

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II. The environmental factors stressed by the non-Indian investigators.

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The theorists, while trying to emphasize one category over the other, have tended to be partial, and we feel that the two categories of factors as stated above must have been augmentative rather than exclusive of each other. Let us now enumerate the various factors.

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Compositional Factors

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Ghosh (1963) has stressed that while the factors of drier and uncontaminated atmospheric condition, low sulphur, high cinder or slag coating in the pillar, better forge-welding etc. have played some role in lowering the rate of corrosion, the major factors seem to be (a) absence of manganese and (b) presence of high phosphorus in the iron of the Delhi pillar. We have discussed the mechanisms for high P and low Mn and S lowering the rate of corrosion, and these machanisms are universally accepted.

v1.c27.s2.t2.28

Lahiri et al. (1963) have drawn analogy between the tribal wrought iron made by primitive method and the Delhi iron pillar, and their corrosion resistance, which, according to them, could be attributed to the mode of manufacture and slag or cinder content acting as the protective film. A primitive furnace iron sample was found to stand the SO₂ test better than a mild steel sample of similar carbon content. (Ghosh, 1963: 40). Prakash and Tripathi (1986: 575) have reported Igaki's polarisation studies of some ancient iron sample showing their superior corrosion resistance compared to a modern sample of ultra-pure iron.

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Ghosh (1963: 42-43) has argued that the atmosphere theory advocating primacy of the environmental factors cannot explain why Koṇārka (ancient) iron has not corroded faster in spite of being placed in the humid sea-side area, whereas a few modern iron beams lying in the temple courtyard have corroded so much. The rectangular pit at the top of the Delhi iron pillar must have collected rainwater from time to time and should have corroded mush faster, according to Ghosh, if the environmental factor exclusively determined the corrosion rate.

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Environmental Factors

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Countering Ghosh’s argument about the Delhi pillar, just now cited, Wranglén (1970: 763) pointed out that “the lowest part of the pillar, which is buried in the ground in a moister environment than the atmosphere, is covered by a rust layer more than 1 cm thick, and shows corrosion pits up to 10 cm deep; this indicates that climate rather than composition is the deciding factor in its corrosion-resistance”.

v1.c27.s2.t2.32

A band of the circumference of the pillar at a height 1-1.5 m above the stone platform is very bright as if polished. This is due to the custom of visitors standing with their backs towards the pillar, and trying to clasp their hands around it ‘for luck’. As in the 2000-year old iron chain in a shrine on Adam’s peak in Sri Lanka, contact with human hands and consequent repeated polishing and greasing have practically prevented rusting.

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According to magnetic measurements (Bardgett and Stanners, 1963), the oxide film on the bright, polished section is less than 50 μm; immediately above this section it is 500-600 μm. If the oxide film is scraped off, the exposed iron starts to rust, and after a few years, the newly formed oxide cannot be distinguished from the main oxide of the pillar. This shows that the good state of preservation is mainly due to a protective film of corrosion products.

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According to Wranglén (1970), the deciding factor behind the rustlessness of the pillar is the dry and unpolluted climate of the Delhi area particularly during the 1500 years up to the beginning of this century. The atmosphere has low relative humidity which rarely exceeds the critical 70 per cent (the three monsoon months being exceptions), and is often as low as 20-40 per cent. The rainfall is relatively small. Before the twentieth century, the concentrations of the corroding CO₂ in the atmosphere were low. There has been more corrosion-passivating ammonia in the atmosphere on account of the biological waste products in the neighbourhood areas.

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The Delhi iron pillar has large mass, and absorbs lot of solar radiated heat. This counteracts dew precipitation during the night and results in rapid drying after rainfall.

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Lahiri *et al.* (1963) found a corrosion loss of 5 μm/year after exposure of a small smaple from the Delhi iron pillar for 3-1/2 years at Jamshedpur. The rate is more than that of a copper-containing, slow-rusting steel kept at Delhi and of a normal carbon steel (of the composition of the Delhi iron pillar) exposed in Sweden. (Wranglén, 1970: 766) When a protective film is formed, its thickness increases according to the parabolic law: y² = K.t; with y = 5µm for one year, Wranglén calculated that for 1600 years, y = 200 µm which is in excellent agreement with the measurements by Bardgett et al. (1963).

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Wranglén (1970: 767) concluded by saying that the rate of rusting of one and the same steel varies as 1 to 100, or more in different climates, different unalloyed or low-alloyed steels, exposed in the same location, show much smaller variations, perhaps as 1 to 5 or as 1 to 10 at the most. We are not sure whether the ratio in the second case (1:5 to 1:10) observed for short periods may or may not increase over a much longer period for corrosion.

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The compositional factors such as low Mn and S, high P, high slag content etc. were not denied by Wranglén (1970) altogether, and we have no hesitation in concluding that the unusually low corrosion rate of the Delhi iron pillar has been both due to Delhi (atmosphere) and the pillar (composition)!

Forge-Welding and Dhar Pillar

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The marvel of the Delhi iron pillar is twofold: one, its apparent rustlessness and the other, probably more striking, is that it could be fabricated at all 1600 years back! It shows what excellence Indian smiths had reached in the art of forge-welding. Tylecote (1984) suggested that the ‘pancake’ forging technique was used, but Prakash and Tripathi (1986: 576) wondered how the said technique, possible for forging short circular objects, could be feasible for an iron pillar of the size as in Delhi.

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Judging from weldlines on its surface, the Delhi iron pillar seems to have been built up from a great many lumps, weighing 20-30 kg which have been successively forge-welded together, under firing with a charcoal blast. (Turner, 1893: Hadfield, 1912) The archaeological excavations at Ujjain have revealed the existence of typical forge furnaces although the precise technique of forge-welding in ancient India remains a mystery.

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The surface of the Delhi iron pillar shows marks of hammer-blows. Heavy mechanical working at high temperature is also attested through (a) measured high density (Hadfield’s measurement 7.81; NML measurement 7.747), (b) low porosity, (c) nearly identical yield point (23.5) and maximum stress (23.9 tons per square inch) and (d) the obtained micro-structure. The quality of forge-welding of Delhi iron pillar is much better than that of the Konārka beams. (Ghosh, 1963: 44)

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Another iron pillar which had even bigger dimensions than the Delhi iron pillar, belonged to the period of Bhoja (AD 1000-55). Its three broken parts are found at Dhar near Indore; possibly there was a fourth one in Bhoja's capital.

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Cousens (1902-03) gave a full account of the pillar and its broken parts. The largest piece 24' 3" in length and square in cross-section lies half-buried in Lat Masjid built near the pillar itself in AD 1405. The second piece is 11' 7" in length; its shape is not uniform, 8' 6" being square and 3' 1" octagonal. The third piece lies embedded in the public garden Lalbag; it has a length of 7' 6" and is rectangular in cross-section, the average width being 10-1/4 inches. Several small holes surround the pillar; Cousens was of the opinion that the holes or pits in the body of the Dhar pillar were made as the result of inserting some instruments like crowbars, while the pillar was being forged and welded.

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Possibly, Bhoja, the king, himself had the pillar erected as a mark of victory. The weight of the pillar is estimated to be seven tons, one ton greater than that of the Delhi iron pillar. The total height has been estimated to be 43' 8" — nearly twice as long as the Delhi iron pillar. It is the largest pillar of its kind, not only in ancient India, but also in the entire ancient world. (Neogi, 1914: 21-26; Sohoni, 1983: 106) The eminent historian, Vincent Smith remarked: "Whilst we marvel at the skill shown by the ancient artificers in forging a great mass of the Delhi pillar, we must give a still greater measure of admiration to the forgotten craftsmen who dealt so successfully in producing the still more ponderous iron mass of the Dhar Pillar monument".

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Another iron pillar, probably belonging to the fifteenth century AD, is that in the temple of Acaleśwar on Mount Abu in Rajasthan. It is 12' 9" high with a Śaiva trident on its top. This one as well as the Dhar pillar are made of wrought iron. The foregoing achievements amply illustrate how skilful and imaginative were the metalsmiths in India.

The Orissa Beams

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In the eleventh century AD text Yuktikalpataru, the author king Bhoja mentioned a list of places where swords were manufactured from Indian steel: Sri Lanka, Mysore, Surat, Nepal, Varanasi, Magadha, Anga (Bhagalpur) and Kalinga (Orissa). The Bengal-Bihar-Orissa centres had the benefit of accessibility of good quality iron ores. Orissa particularly specialised in making structural iron pieces such as beams.

v1.c27.s3.2

Iron clamps and beams were extensively used in the joining of long projecting cornices and roof-stones of the magnificent temple at Bhubaneśwar, constructed around AD 640. Beams of quite large dimensions have been found in the temple of Jagannath at Puri (built in the 12th century AD) — both in the main and the garden temple (Gundichā Bādi). As many as 239 iron beams have been counted; these range up to 17 ft in length and 6" x 4" or 5" x 6" inches in cross-section.

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The most thoroughly studied beams are from the Koṇārka temple on the seacoast which was built partly in the later half of the ninth century AD (the main temple), and partly in the first half of the thirteenth century AD.

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Out of a large number of iron beams of great dimensions in Koṇārka, some 29 have been counted and studied. (Friend and Thorneycroft, 1924) The largest one has a dimension of 35 ft in length and 7 to 7-1/2 inches in width with a square cross-section. Its approximate weight would be 6000 lbs. There is another beam measuring 23-1/2 ft in length and 11" x 10 1/2" inches in section, which was possibly a part of a much larger one. (Neogi, 1914; Friend et al., 1924)

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The study of the fractured surface of the Koṇārka beams clearly indicates that it was manufactured by forge-welding square rods of small sections. When a small piece of 8 inches square cross-section was hammered, instead of breaking off crosswise, the piece broke longitudinally and appeared to be laminated, being composed of two square pieces joined together. (Ghosh, 1963: 35) Jena and his associates have found traces of lead between two pieces forged together; it seems that a large lead bath might have been used for uniform heating of a bundle of wrought iron bars to the required temperature, before forging them together. (Prakash and Tripathi, 1986: 576) Some de-carburisation took place on the surface during forge-welding.

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Table 19.4

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Chemical Analysis of Koṇārka and Koraput Iron Samples

v1.c27.s3.8

| | *Koṇārka Samples* | | | | |
| | *KT/1* | *KT/2* | *KT* | *Analysis by Friend* | *Iron Peg from Jiragora, Koraput, Orissa* |
| C % | 0.27/0.32 | 0.45 | 0.27 | 0.11 | 0.23 |
| Mn % | 0.04 | trace | 0.04 | Nil | Nil |
| S % | 0.008 | 0.006 | 0.015 | 0.024 | 0.006 |
| P % | 0.015 | 0.018 | 0.018 | 0.015 | 0.015 |
| Si % | 0.050 | 0.110 | 0.096 | 0.100 | 0.10 |
| Cu % | — | Not traceable | 0.16 | — | 0.10 |
| Ni % | 0.16 | — | — | — | — |
| Cr % | — | 0.0040 | 0.0025 | — | — |

v1.c27.s3.9

Chemical analyses of Koṇārka beam samples were done by Ghosh (1963) and Friend *et al.* (1924). Ghosh selected three separate samples: KT/1, KT/2 and KT. The results are provided in Table 19.4. The similarity in compositions of the ancient Koṇārka samples and a modern tribal sample from Orissa shown in the table is noteworthy.

v1.c27.s3.10

The analytical figures show that as in the Delhi pillar, in Koṇārka samples also, the carbon percentage is not negligible. Percentages of Mn, S (as well as P) are low. There is some silicon; the percentages are variable, presumably because forge-welding was done on smaller pieces obtained from diverse source. Micro-structure and hardness values at different regions of the Koṇārka sample were noted by Ghosh (1963: 37), and a summary thereof is reproduced in Table 19.5.

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Table 19.5

Micro-structure and Hardness values of Koṇārka Samples

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| Position in the specimen | Description of the structure | Hardness V.P.N. |
| 1. | Large grains of ferrite with slag inclusions | 80.8 |
| 2. | Medium ferrite grains with slag inclusions | 81.3 |
| 3. | Variable size of grains in a structure approaching that of 0.20 per cent C steel | 96.6 |
| 4. | Slag inclusions in the midst of ferrite and pearlite structure | 106.0 |
| 5. | Overheated structure of about 0.35 per cent C steel | 115.0 |
| 6. | Weld zone showing imperfect welding by forging | 106.0 |
| 7. | The structure of about 0.20 per cent C steel | 86.5 |

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Friend *et al.* (1924) also found the presence of carburized phase in Koṇārka beam, the constituents were found to be ferrite and pearlite. Decarburisation had taken place during the operation of welding. Ghosh (1963: 37) determined the yield point of the Koṇārka beam to be 14.0 tons per sq inch which is much less than maximum stress of 23 tons per sq inch. The two values are not close, and on this count the Koṇārka sample differs from the Delhi Iron Pillar, which is denser, less porous and must have been subjected to better forge-welding and heavier mechanical working.

v1.c27.s3.t1.3

The rust scales from the Koṇārka beams have been analysed by Ghosh (1963: 41-42) The magnetic portion is about 98 per cent whereas the figures for the Delhi Iron Pillar rust are 80 per cent (above ground) and 99 per cent (underground sample rust). X-ray diffraction analysis showed that the magnetic portion of the rust in the Koṇārka beams mainly consisted of magnetic goethite, and a small proportion of lepidocrocite. There is some quartz in the non-magnetic fraction. The Delhi Iron Pillar rust showed considerable amount of Fe₃O₄ which was not further oxidised to Fe₂O₃.

v1.c27.s3.t1.4

Hudson had concluded that the Koṇārka sample, having composition similar to that of the Delhi Iron Pillar, corroded more on account of the moist and salt-laden air of the Orissan sea-coast (1953: 499). The relative humidity of the air at Delhi seldom exceeds the critical value of 70 per cent. Ray (1956: 213) cited Hudson’s work and commented: “This should lay to rest all speculations regarding the alleged intrinsic superiority of the ancient iron, so far as its corrosion resistance is concerned”.

v1.c27.s3.t1.5

However, Ghosh (1963: 39-40) was not convinced that the effect of the composition of an iron sample on its corrosion-resistance was absolutely negligible. While Koṇārka ancient beam has corroded more than the Delhi Iron Pillar, yet one has to reckon “that a few modern iron beams lying in the yards of the Koṇārka temple got corroded much more (than the ancient beams on the same spot) tho-ugh they have not been there for more than 50-70 years”. Better corrosion-resist-ance of Delhi Iron Pillar could be not only due to environment but also to the facts that it has more (0.25 %) corrosion-resistance phosphorus (Koṇārka beam assays only 0.015 %), and less porosity on account of heavier mechanical working.

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Ghosh (1963) has extended Hadfield’s (1912) idea that ancient wrought irons being purer are more corrosion-resistant, but this view is not accepted by many modern scholars who attach greater importance to the environmental factors. Wranglén (1970: 767-69) favours the environmental theory and yet strikes a reconciliatory note by commenting that while the environmental factor may cause the variation of the corrosion-rate of similar samples by a factor of 100, there could be tenfold variation in the corrosion rates of different kinds of ferrous materials kept in the same environment. He also cites Evans (1955, 1957, 1960 and 1968) explaining the apparent rustlessness of some of the ancient iron samples in environmental terms. In the ancient eras, the environments were much less polluted with acidic oxides (CO₂ and SO₂), there being more ammonia from the biological waste-products contributing a passivating effect on the samples. “The initial conditions of exposure often determine the life of a metal in the atmosphere, and under favourable conditions, a protective layer of corrosion products is formed that protects the metal when the atmosphere becomes more polluted”.

v1.c27.s3.t1.7

We emphasize that apart from their resistance to corrosion, the marvel of the ancient massive iron objects in India is that such structures could be fabricated at all, 800-1600 years ago, with such technical perfection. The precise methods by which forge-welding was done for a huge number of small objects to obtain a large piece of desired size and shape, are yet to be discovered, and shall remain fascinating topics for further investigation.

v1.c27.s3.t1.8

In the course of reviewing iron technology in ancient India, Prakash and Tripathi (1986) emphasized on the tribal metallurgical traditions in the sub-continent, a topic reviewed earlier by Elwin (1942), Krishnan (1955), Ghosh (1964), Prakash and Igaki (1984) etc. The topic is indeed fascinating and we would like to consider and include the vast literatures in our forthcoming studies on pre-modern iron technology in India. However, our present studies are restricted up to the period AD 1200, i.e., the ancient period, and we are far from certain regarding the earliest dates of the tribal metallurgical practices in India, observed and reported during the recent past only.

v1.c27.s3.t1.9

The roots of the tribal traditions must have been buried deeper in the ancient period, i.e., the traditions must have originated before AD 1200; but the exact dates are problematical to find. We tend to believe that before AD 1200, tribal metallurgy was an integral part of Indian metallurgy. The dichotomy probably arose in the eighteenth/nineteenth century when Indians became aware of and started adopting the modern or Western metallurgical techniques. The discarded indigenous methods remained isolated amongst the tribes.

v1.c27.s3.t1.10

During the ancient period, tribal metallurgy was no different from the indigenous metallurgy, then in vogue in the sub-continent. Wrought iron, made and sold by the muṇḍas, was known as mundaloha. 12 to 15 mm diameter rods of 150 mm length was bought by black-smiths and used for manufacturing tools and domestic objects. Some of the black-smiths were employed by the state to manufacture very special objects like the Delhi Iron Pillar, Dhar Pillar of the Koṇārka beams.

v1.c27.s3.t1.11

The ancient India had a long tradition of art and aesthetics finding expression through the objects of diverse materials. Ever since the bronze dancing girl statuette was cast in the Harappan era, terracotta artefacts were made, sandstone pillars of the Aśokan era were chiselled to carry Buddha’s message, and various stones, metals and alloys were used to produce art objects. Iron pillars and beams, discussed in this chapter, were dedicated to the temples of worship. Inside the places of worship of all denominations shone idols and art objects made of various non-ferrous alloys which we would discuss in the next chapter.

v1.c27.s3.t1.12

Bardgett, W.E., and Stanners, J.F., 1963, “The Delhi Pillar: A Study of the Corrosion Aspects”, In *Journal of Iron and Steel Institute*, 201, 3, 1963; also NML Technical Journal, Vol. 5, No. 1, 24, 1963.

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Bronson, Bennet, 1986, “The Making and Selling of Wootz, A Crucible Steel of India”, In *Archaeomaterials*, 1986, Vol. 1, pp. 13-51; also 1989, reply to Bhanu Prakash, ibid, Vol. 3, pp. 95-96.

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Buchanan, Francis, 1807, *A Journey from Madras through the Countries of Mysore Canara and Malabar*, Vol. II, London.

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Coomaraswamy, A.K., 1956, *Mediaeval Sinhalese Art*, 2nd Edition, Pantheon, New York, pp. 189-94.

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Cousens, H., 1902-03, *Annual Report of the Archaeological Survey of india*, 1902-03, pp. 205-212.

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Elwin, V., 1942, *The Agaria*, 1942.

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Evans, U.R. and Chilton, J.P., *Journal of the Iron Steel Institute* 181, 113, 1955; 186, 98, 1957; Evans, U.R., 1960, *The Corrosion and Oxidation of Metals*, Arnold, London, p. 508 and 1968, Supplementary Volume, p. 191.

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Friend, J.N. and Thorneycroft, W.E., 1924, “Examination of Iron from Konarka”, In *Journal of the Iron and Steel Institute*, Vol. 110, 2, pp. 313-15.

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Ghosh, A., 1989, *An Encyclopaedia of Indian Archaeology*, Munshiram Manoharlal, New Delhi, 2 volumes.

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Ghosh, M.K., 1963, “The Delhi Iron Pillar and its Iron”, In *National Metallurgical Laboratory Technical Journal*, Vol. 5, No. 1, Feb. 1963, pp. 31-45. ——, 1964, “Primitive Iron Making in Contemporary India”, In *TISCO Journal*, 11, 3, 1964, pp. 132-134.

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