Minerals and Metals in Ancient India v1.c20.s5: Iron Metallurgy at Rajghat, Varanasi
Minerals and Metals in Ancient India v1.c20.s5
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Iron Metallurgy at Rajghat, Varanasi
v1.c20.s5.1
By 600 BC, ferrous metallurgy was quite widespread and diversified in north India. The investigations on the Rajghat samples were conducted by Bhardwaj and others (1979: 147-56) and may be briefly recorded here. Six iron objects — blade, arrowhead, broken nail, piece of a rod, a fragmentary piece and a bent nail — from the NBPW phase of Rajghat (600-400 BC) were chemically and spectrographically analysed. Four of them were metallographically evaluated. Two iron slag samples from the horizon were chemically analysed.
v1.c20.s5.2
The arrowhead showed ferrite and slag mixture. There is a banded structure due to forging. Glassy and oxide slag particles and central cavities (due to gas evolution during solidification) are clearly visible. The broken nail also revealed banded structure, slag-rich areas and glassy inclusions. The bent nail (Sl. No. 6) showed equiaxed ferrite grains of uniform size. Although there is no evidence of pearlite, a small amount of iron carbide at the grain boundaries is clearly visible.
v1.c20.s5.3
From the chemical analysis, it appears that silica (0.42-3.8 %), alumina (0.15-2.01), lime (trace to 1.2) and magnesia (.08-0.80%) are invariably present as impurities in the iron samples. Also present are oxides of phosphorus (0.15-0.22%) and sulphur (0.03-0.19%). Absence of nickel shows that the samples did not have any meteoric origin. The carbon content of specimens varied between 0.12 to 0.42 per cent except for (No. 6) the bent nail which assayed 1.1 per cent carbon and showed iron carbide at the grain boundaries and deliberate effort for carburization.
v1.c20.s5.4
The iron slag samples assayed 72-74 per cent Fe₂O₃, 16-17 per cent SiO₂ (probably silicates such as fayalite-phase analysis was not done), 4-5% Al₂O₃, 1.2-1.7 per cent CaO and 0.4-0.8 per cent MgO. High percentages of Fe₂O₃ in the slag indicate that the attempt for reduction was inefficient and far from being fully successful.
v1.c20.s5.5
At Ujjain, calcium compounds such as lime or aragonite were used as flux. There is no evidence of flux having been used at Rajghat. The alkalies in the slag might have appeared from the charcoal ash.
v1.c20.s5.6
Rich hematite deposits around Mirzapur might have been used for smelting in the Rajghat site. Bhardwaj (1979: 155-56) has noted that the Rajghat iron is associated with Ti and P₂O₅ and therefore could have been produced from the Singhbhum ore assaying much more than 72 per cent Fe₂O₃, 2-4 per cent Al₂O₃, 0.6- 2.4 per cent SiO₂, trace to 0.35 per cent CaO, 0. 17-0.20 per cent MgO as well as 0.2-1.0 per cent P₂O₅ and appreciable amount of TiO₂.
v1.c20.s5.7
The site of Taxila which went through the vicissitudes of history — rise and fall of empires — across nearly a millennium (520 BC to AD 470), showed a large number of iron samples which would be discussed in chapter 14. The Sirkap stratum showed iron swords and carpenter's adzes assaying 1.2-1.7 per cent carbon and exhibiting cementite and pearlite grains. However, there was no evidence of hardening and tempering. 108 ingots found in Sirkap measuring 5-6 inches in length, 1-2 inches in cross-section and 1-1/2 to 4 lb. in weight were found to be porous wrought iron specimens with 0.1 per cent carbon. Such ingots must have been subsequently carburized and forged to produce harder materials.