From Pure Copper to High–Tin Bronzes
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SUPERIOR COPPER METALLURGY OF EASTERN INDIA AND BANGLADESH IN MEDIEVAL PERIOD: FROM PURE COPPER TO HIGH–TIN BRONZES Prasanta K. Datta* Pranab K. Chattopadhyay* Introduction The history of East India and Bangladesh has been pushed back to Paleolithic, Chalcolithic and subsequent early historic times by the findings of archaeological excavations at Paharpur[1], Bangarh[2], Pandu Rajar Dhibi[3], Mangalkot[4], Mahasthan[5], Wari-Bateswar[6] and elsewhere around Singhbhum copper belt. Other than the mass of copper-bearing materials collected from the explored sites, very often a large number of copper tools, in caches, so called “copper hoards” have been obtained at places nearby. Whatever be the source, fortunately metals preserve their history of processing, of their chemical extraction and physical transformation, in their microstructures. Therefore, a study of their microstructures can predict their production processes, if assisted with suitable archaeological evidences of mines, slag, crucibles or molds. Characterisation of four copper samples taken from different sites in chronological order starting with in and around Christian era from 1st millennium B.C. to 1st millennium A.D. to reconstruct the remarkable development of copper metallurgy, in Eastern India and Bangladesh. 2.0 Description of samples 2.1 Fragment of a bar-celt, Khuntitoli, (Ranchi), Manbhum Copper hoard The fragment (Fig.1) is the cutting end of the bar-celt[7] used in scooping the earth of agricultural field, locally known as “khurpi”. This is a pure copper casting (99%) but full of gas holes and seems to be made in open molds with poor gating. The purity (over 99%) is an excellent example of pure copper, though melting and casting techniques deserve more improvement for good casting production. 2.2 A double-ended axe, Khuntitoli, (Ranchi), Manbhum Copper hoard The axe (Fig.2) has in plan, convex lead edge; slightly concave side edges which converge towards the rounded butt[8],[9]. It weighs around 600 gms and has length around 160 mm and thickness 5 mm at the midriff. Made of pure copper (over 99%) the metal was probably deoxidized with arsenic before pouring in a closed mold. The beautifully cast piece was then probably hot-forged in super plastic temperature to develop sharp edges, while residual cast structure were allowed to remain in central section. 2.3 A broken disc of a copper lump, Aguibani, Medinipur A part of the copper disc (Fig.3) solidified at the bottom of a crucible in form of a half-round is the end product of an extraction heat. Though shrinkage is not missing and weighs only around 200 gms. it exhibits the size (diameter) of the crucible in use. The purity (over 99.5% Cu) and the softness achieved in annealed condition suggest a deep knowledge of metallurgy. 2.4 A broken part of a bronze bowl, Gazole, (Rangmahal), Malda The broken fragment of a high-tin bronze bowl (Fig. 4) is an example of copper alloying and copper alloy forging. The half-mm thick section of the bowl demonstrates a high degree of metallurgical skill in complex shaping of a difficult hard phase alloy by open-die forging. The corrosion resistance and superior hardness of tin bronze might be a common knowledge that enhanced its wide use in historical period. 3.0 Microstructure of samples 3.1 Optical Microscopy The results are shown in Fig.5 to Fig.8. In the sample of the bar-celt (Fig. 5), the remnants of dendrites are visible, with wide distribution of micro-voids and inclusions as well as diffusive layers of dendrite arms revealing unsound knowledge either in foundry or in heat treatment. The sample (Fig. 6), of axe shows the break down of coarse dendritic structure in form of equi-axed grains, free from gas holes with segregation of inclusions and second phase particles mostly in grain boundaries. This indicates not only superior techniques of foundry like degassing, closed molds, gating etc. but also the development of good working and heat treatment practice using recrystallization-recovery-grain growth mechanism for producing sound metals. The sound material has become softer (HK 60-61) than that of the bar-celt, as indicated by the micro- hardness (HK 70-71) readings. The copper lump (Fig. 7), which is an ingot, contains mostly equiaxed grains with globular Cu-Cu2O eutectics or deoxidation products and low-melting constituents of insoluble lead widely distributed. The shrinkage void indicates sound metal revealing deoxidation techniques (here by arsenic) and good annealing treatment of hard arsenical copper (HK 183 - 185). The bronze sample (Fig. 8), reveals the preferred orientation of hard second phase alpha- delta in the forging direction in the matrix of β-phase (HK 276 - 283). It also vindicates the capability of those people to hot forge a high tin (over 20%) bronze in a narrow forging temperature range – a remarkable feat indeed in those poor conditions in medieval period. Considering Europeans were not conversant in hot forging of high tin bronzes even as late as sixteenth century as recorded by Vannoccio Bringucchio in Pirotechnica (1540) of Papal Foundry in Rome.[10] This was a great achievement by East Indian metal workers. Good strength of the material proves the soundness as well as excellent manual forging practice . 3.2 Scanning Electron Microscopy Under scanning electron microscope, the bar-celt confirms the wide presence of inclusions of oxides (Fig. 9) and sulphides and micro-shrinkages. Most of the trace elements are Ni, Co, Sn and Bi, which are present in Singhbhum chalcopyrites,[11]thereby confirming the natural sources of copper ore and its extraction. The structure of the axe is the proof of sound metal, which contains arsenides (Fig.10) along with usual oxides and sulphides, as residual deoxidation products. The central portion, after etching, reveals cast structures (Fig 10, RHS), which still retains the dendrites of pure copper undisturbed. The unetched copper lump shows (Fig 11, RHS) the irregular shrinkage cavities. The wide presence of white arsenides (Fig. 11) confirms the intentional addition of arsenic to refine and deoxidise the metal at the ingot stage. EDAX result (Fig. 11, centre) also proves the presence of lead arsenides and copper sulphides, generally present in Singhbhum ores The etched structure also indicates ingot pattern due to slow cooling (~1 K/Sec) obtained from the dendritic arm spacing, calculated of ~100 µ m, as is expected in a shut-down crucible. In case of the bronze sample the interconnected α - phase (Fig. 12) look more roundish in nature, indicating very close under annealing below transformation temperature like process annealing in carbon steel. The SEM study establishes the argument that the production process of pure copper from nearby chalcopyrites was continuously refined over hundreds of years to a useful technology unmatched in the ancient world. The eastern people could also develop the technology of copper alloying and forming of high tin bronze, not easily available else where in India or abroad. 4.0 X – Ray Diffraction (XRD), Differential Thermal Analysis (DTA) and Thermo gravimetric Analysis (TGA) Results. Table I and II provide X – ray diffraction patterns produced by samples. Bar – celt, Axe and Lump show basically α - Cu phase as predominate phase with minor phases, almost non-existent. Bronze sample indicate the / presence of β or metastable - β phase, along with α - Cu phase. Some peaks remain to be identified. For Axe, a prominent endo – peak at (812oC) and a small endow peak (482oC) were observed in DTA. Similarly for copper lump small endo peaks around (351oC) and (682oC) were observed. All these DTA & TGA results of copper samples suggest the presence of some amount of low melting constituents, probably of tin, zinc, lead, arsenic and others in these systems. 5.0 Reconstruction of copper technology Logistics of Production Copper mines (Hazaribagh, Baragunda, Mosabani and Rakha of Jharkhand)[12],[13], quality wood charcoal (Sal = shorea robusta wood), fired crucibles, slag heaps, tuyers, alloying metals like Tin, (Ranchi, Hazaribagh, Bastar)[14], [15] or imports from Thailand or Malay, copper products – copper hoards, copper plaques[16], Pala- Kurkihar- Jhewari Bronzes[17] and lastly copper related names Tamajuri (Heaps of copper), Tamralipta (Pasted with copper), Kansabati (Carrier of Bronze), Shilabati (Carrier of Stone-Copper Ore), Aguibani (Forest of Fire – agun), Mosabani (Forest of crucibles – musa), provide ample evidences for the development of copper metallurgy over two millennia in East India. 5.2 Extraction Process Dry balls (large pellets) of powdered copper (sulphide) ores and wood charcoal, bonded with cow-dung and clay as flux, were stacked in a cylindrical crucible, fitted with a slag notch (in form of a terracotta pipe) at the bottom and a blast pipe at the top to blow air. The charged crucible was placed in a underground hole and the mass was ignited at the top with occasional air blast. The ignition temperature of chalcopyrite is only 3000 C[18]. Iron suphide (FeS) got roasted to iron oxide (FeO) and fluxed with silica were melted out as viscous (viscosity 500 – 1000 cP) liquid fayalite (sp. gr. 3 – 3.7) while remaining FeS – Cu2S mixture forming fluid (viscosity 10 cP), heavy matte (sp. gr. 4.4) at the high temperature percolated to the bottom (Fig. 12). During trickling, on further air blowing FeS again got smelted to FeO and produced fayalite separating it from Cu-phase, due to the excess silica present in the system. The collected ‘white metal’ rich in copper (∼ 80%) (sp. gr. 5.2), afterward, due to air blowing converted itself to ‘blister copper’. The process resembles the copper extraction process of present Nepal[19], (Gajurel and Baidya,1984), and also the Continuous Process Technology of advanced countries like Noranda, Mitsubishi, or Ausmelt process.[20] In Figure 13 reconstructed furnace is shown and Figure 14 includes Thermodynamic reactors as practiced in most modern continuous melt process, which closely resembles, the ancient crucible copper extraction of East Indian metal workers.