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History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s2: Metallurgy of Pure Metals

History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s2

Language
English

Contents

Metallurgy of Pure Metals

(I) Gold

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According to Gowland (1912) and Bhardwaj (1979) it is extremely probable if not absolutely certain that Neolithic men would have first become acquainted with native Gold but they would have been disappointed in its use to shape hunting tools and war weapons. In our country not many Gold artifacts dating to Pre-Harappan period have been reported although in *Ṛgveda* Gold and its alloys with Ag and Cu have been mentioned in many places, such as, I-85.9, 88.5, 67.3; II-37.9; V-54.15, 57.1, 60.4; VIII- 7.27-32 etc. In Vedic literature this metal has been mentioned as *Hiranya* probably because of its high corrosion and oxidation resistance and the practice of its preservation through generations. The recurrence of this metal in the Chalcolithic period is more common and many artifacts have been reported from South India.

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This metal has its mesmerising power in its shining bright deep yellow colour which has not got tarnished even by the stains of human blood. In human civilization it has been named as the most sinister and deadly metal although due to its ductility and softness it had proved worthless for making any war weapon. This is so ductile that one gram of the metal can be drawn into more than 1.24 km long wire without any intermediate annealing.

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Marshall (1902-1903) has remarked that the Gold frontlets or dead man’s mask of the Adichanallur excavations are made of so flimsy gold leaf that in all probability they could not have been used in real life and these have been most probably used as substitutes for the dead. This also indicates the economic aspects of this metals use in the ancient times. The custom of tying gold plates or ‘Diadams’ is still prevalent in some parts of Madurai, and in North India it is customary to put a small piece of the metal in the mouth of the dying person.

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Native Gold has been found in this country in three forms :

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(1) Alluvial sedimentary deposits at shallow depth in the river basin. In Nilgiri hills such deposits have been worked upto a depth of 25 m or more.

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(2) As fine particles mixed with the river sand. Rivers of Sindh, Sutlaj and other rivers of Kashmir, Ganges, Swarna Rekha, Godavari and many other rivers of South India have been reported to carry gold fines in their sand. Kuppuram (1983) has reviewed such deposits in great detail.

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(3) As metal nuggets and streaks in quartz veins at Kolar and Hatti Gold Mines.

(1.1) Process of Extraction

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The most common process of separation of native Gold from the sand and alluvial soil or quartz rock has been by gravity separation or panning. In this process the quartz rock or the hard soil is first crushed to fine size to liberate the gold particle and then separate it by suspending the mixture in water.

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When the pan containing the suspension is agitated under water gold particles settle at the bottom of the pan and the sand and soil are washed away. For large scale working big pans were hung in the river water through a sling so that the mixture could be easily agitated. After the Gold particle was separated from the gangue (Qz and clay etc.) it was sold to the Goldsmiths for melting, refining and shaping of the metal. Dikshitar (1951) has mentioned that the labourers employed for Gold washing were known as ‘Sonwals’. McCrindle (1901) has mentioned that in Assam Gold Washing was being carried out at Suparnakudya (now Sonkudita) even in the 18th century and Sonwals used to pay custom duty annually to the Government in the form of Gold fines worth Rs.5/- During the Ahom’s rule about 10,000 people were employed in this trade. Sharma (1992) has reported that gold panning is being carried out even today by tribals of Rajgarh from the Eeb river, a tributary of Mahanadi and they are able to extract 4200 kg of gold per annum.

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The second process of extraction of this metal is by the process of amalgamation with Mercury and recovery of Gold by evaporation and distillation of Hg. This process has also been known to the ancient craftsmen and it was also used for gilding of objects made of copper, bronze and brass etc.

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Kolar and Hatti Gold mines were being worked even in the prehistoric times and as mentioned by Rao, S.R. (1973) the spectral analysis of the Gold objects of Indus cities and Kolar have shown high degree of similarity. Both the metals are in alloy of Au-Ag known as ‘Electrum’ instead of pure Gold. It has been proved beyond doubt that the South Indians had learnt the technology of mining Gold at these mines and established the smelting technology on commercial scale and the metal was being traded through Lothal by sea to Harappa, Mohenjodaro and even Egypt, Greece and Rome, Africa etc. even as early as 2500 B.C.

(1.2) Technology of Gold Working

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Most probably the first Gold piece used for shaping into a simple ornament like spherical or cylindrical bead or a wire for making bangles was a small nugget but latter-on smelting and refining techniques were developed. Pure Gold melts at 1063°C which could be achieved easily in charcoal fire. The Gold sand obtained by panning was melted in a crucible and refined by the addition of fluxes like borax and a mixture of Ammonium Chloride, Borax and Ammonium nitrate in the ratio 6:1:3. The method of separation of Silver from Gold was known as ‘Niyaria’ and the craftsmen practicing the separation of Gold and Silver were known by this name. Even today about 300 families in Mirzapur are practicing this trade for separation of various metals like, Au, Ag, Cu, Ni, Zn, brass etc. from the dross obtained from parent metal industries.

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The purified metal bar or ingot was worked by forging, punching, embossing etc. for the production of various ornaments having intricate designs. Even rolling and wire drawing was known to the ancient craftmen. Gold coins were either made by die casting in clay moulds or by the technique of punch marking and the quality of the metal was judged with the help of ‘Touch Stone’ and coin’s weight was accurately measured. *Arthaśāstra* of Kauṭilya gives detailed account of the knowledge of minting of coins and production of other objects and the technology of gem setting. They had also learnt to produce Au-Ag and Au-Cu alloys and the use of gold solder (Au:Ag, 8:1) and the ways to improve the golden colour finishing of the joints as early as the vedic period. Today it is well known that Au-Ag have a Eutectic type phase diagram whereas Au-Cu have complete solid solubility and alloying with these metals impart strength, hardness and various shades of colours to the metal as well as lower the melting point of the alloy. As mentioned earlier the technique of Hg amalgamation and gilding was also known to the Indian craftmen from the very early times.

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As regards the trade with foreign countries Prakash, S.(1965) quotes Greek and Roman historians giving an account of the Indian Gold and Silver. Curtius Strobo and Philostratus mention the use of these metals in Ancient India. The accounts of Old Testaments show that Ophir mentions the export of Silver and Gold from India. Megasthanese has also mentioned the mining of these metals in various parts of India in the Neolithic-Chalcolithic and Megalithic period. Allchin (1968) has mentioned that Greek Drahams and Roman Dinars made from Indian Gold were in circulation for trade and often smelted for reuse. During the period of Asokan Empire its capital was known as *Suvarnagiri* and it was known for the Golden splendour.

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*(II) Silver and Lead*

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Sneed et al.(1954) and McDonald (see Butts and Coxe, 1967) have mentioned that just as *Svarṇa* (Gold) has been related to Sun or Sanskrit word *Jval* (to shine), the brilliant white metal silver has been related to Moon and earlier it was called luna. Silver nitrate is still called Lunar Caustic. The Latin name ‘Argentum’ also has its origin in Sanskrit and it comes from the Aryan origin, meaning shining white metal.

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This is the second metal to have attracted the fancy of ancient Indians as a domestic metal which had no value for the manufacture of tools and weapons. The introduction of this metal in the ancient Indian Culture is relatively late because of its non-availability in the nature as virgin metal, i.e. very rarely it is found in the native form. McDonald writes (see Butts and Coxe, 1967) that the discovery of virgin Silver was probably due to the natural fire in some forest where the exposed Argentiferous Galena (mixed sulphide mineral) was roasted and then reduced resulting into the separation of this metal. Archaeologically speaking very few silver artifacts have been found dating back to Harappan period. A silver dagger belonging to 2700 B.C. from Harappa is being preserved in the National Museum, Delhi.

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Since Silver (M.P. 962°C) has been found to occur in nature either as AgCl (Cerasgerite) or Ag₂S (Argentite) associated with Galena and/or Sphalerite or mixed sulphide ores, the extraction of this metal is said to be started in 3rd or 4th Millenium B.C. with the development of the technique of treatment of sulphide minerals viz. roasting and reduction smelting. During the extraction of lead from PbS or PbO, Ag₂S also gets decomposed and reduced easily (at 470°C) and it readily alloys with metallic lead at 1000°C hence the metallurgy of lead is being described along with this metal.

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Lead artifacts have been reported from the Harappan period, although, due to its softness, it did not find much use in this period. It is one of the softest and heaviest metal having shining grey lustre which leaves a black streak on paper.

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It is so soft that it can be scratched even with nail hence it’s use has been mostly for the manufacture of cheap jewellery dies or pattern for shaping other metals, and as a hardner in copper alloys. In South India lead coins have been in use in Ist-2nd century B.C and it was the chief coinage metal for considerable time. It melts at 327°C and its B.P. is 1525°C. Besides PbS (Galena) it also occurs in nature as PbO i.e., Massicot and Litharge (Yellow in colour), PbCO₃ or Cerussite and white lead PbCO₃.Pb(OH)₂ (white powder) or PbSO₄.PbO (white pigment). Lead might have first been recognised as these coloured pigments used for body decoration and during use it might have fallen in the compfire producing metallic lead, because these minerals get decomposed easily to PbO and subsequently reduced to liquid lead at 500°C. The reactions occuring during the roasting of Galena are -

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2PbS + 3O₂ → 2 PbO + 2SO₂ ...(1)

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PbS + 2PbO → 3Pb + SO₂ ...(2)

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and finally PbO is reduced by C at 500°C or by CO even at lower temperature.

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PbO + C → Pb + CO ...(3)

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PbO + CO → Pb + CO₂ ...(4)

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Since liquid lead readily alloys with silver one of its major use was to first produce this alloy by coreduction of Pb and Ag minerals and then produce pure Silver by the process of cupellation. Bhardwaj writes that these processes were known during the Harappan period.

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*(II.1) Occurrence of Silver and Lead Minerals*

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In Indian subcontinent Galena is found in Rajasthan at Zawar and in small amounts in Kashmir, Garhwal in Uttar Pradesh, Bihar and Kerala. One of the biggest deposits of Galena and Ag₂S (Argentite) is found in Burma at Bawduin.

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Such deposit of Argentiferous Galena is found in Zawar and it has been reported to occur in Bihar, Orissa and Kamalpura. Sarkar (1971), and Butts and Coxe (1967) have reported large deposits in Burma at the Namtu and Bawduin mines where signs of ancient working of silver and lead have been found, and Burma is one of the major silver producer even today. Silver has been also reported to be produced at Ananthpur Gold field in Andhra Pradesh. There has been also evidence of the trading of Spanish Silver in exchange of Indian species.

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*(II.2) Production of Lead and Silver*

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As discussed earlier PbS after roasting gets converted into PbO and this can be easily reduced at 500°C to molten lead. The process used in ancient India at Zawar was most probably ‘Ore Hearth Process’ where the roasting and reduction of rich Galena is carried out in an open pit furnace. Fig.5 shows a sketch of such a furnace as reported by Raistrick (1927) and referred by Tylecote (1976). In such a furnace temperature as high as 1000°C can be easily obtained under natural draft and the reduced metal flows out and gets collected in the front pit. At this temperature Ag₂S also gets decomposed, reduced and forms an alloy with molten lead. The details of this process has been mentioned by Sevryukov (1975).

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Silver metal was obtained from the alloy of Pb-Ag by reheating it in a shallow hearth furnace having the bottom prepared with bone charcoal. During remelting of the alloy it is subjected to oxidizing condition by blowing excess air when lead gets preferentially oxidized to PbO (litharge) which has a very low melting point. This molten litharge is skimmed off and partially absorbed by the furnace hearth. This process is known as ‘Cupellation’. At the end of the refining process purified liquid silver is tapped out and cast into ingots. Although no clear evidence has been found at Zawar of any such furnace or the remains of Litharge but it has been known in ancient times for the production of lead and silver. Litharge might have been reduced in another ore hearth furnace to produce metallic lead. Kautilya’s Arthasastra states the following details of the process of cupellation for the purification of silver.

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“The pure and impure silver may be heated four times with CuSO₄, mixed with powdered bone (*Astithutha*), again four times with equal quantity of lead and again four times with CuSO₄ (*Suskatutha*), again three times in a skull and lastly twice in cow dung. Thus purified silver is obtained. This process has been also quoted by Ray (1956) and Bhardwaj (1979). Bhardwaj (1979) has written that this process of cupellation for the production of Silver and Lead was well developed in the 2nd half of 3rd M.B.C.

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*(II.3) Mechanical Working of Pb and Ag and their Uses*

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As mentioned earlier Lead being a very soft metal it did not find much use in the early days but in the latter period it has been used for the manufacture of sanitary pipes, moulding dyes, thin wire or rod for applying surma and probably also as a Fig. 5. Early ‘Ore Hearth’ Lead Smelting furnace as described by Raistrick

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SW NE Bayle Hill Gunners Beck faggots, peat and ore stone wall round fire 2ft 0in C puddled clay melted lead openings for wind section on AB 5ft 0in A bowl shaped hollow pig B dry walling section on CD

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base material for engraving and embossing Gold and Silver sheets and foils. Jeyaraj (1990) has mentioned the use of cast lead coins in South India in the 2nd to 3rd century A.D. possibly belonging to Sangam period. Lead compounds also have been used for glazing potteries and in the manufacture of glass. Evidences have indicated the use of this metal to prepare Pb-Sn alloy used as solder to join pieces of copper and bronze. In latter period lead has been extensively used for preparing projectiles for slings and catapults, bullets, cannon balls and cast shots etc.

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The shining white colour, high corrosion resistance and the non-tarnishing properties of silver attracted the attention of the primitive men and they found its use in making ornaments, utensils, decorative pieces and other house hold articles. It has also been used for the manufacture of various weapons for decorative as well as ceremonial purposes. The high ductility of the metal permitted it to be forged into sheets and foils used for making utensils having intricate embossed designs, as a sheet covering on various wooden objects studded with jewels for the use of rich men and kings. Its very thin foil has been used to cover various food dishes. This metal has been found also to have medicinal properties and used in the ancient Ayurvedic medicinal system.

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The use of this metal as coins has been rather late. It is only in 600 B.C. that punch marked coins have appeared and in the latter period Silver has been popular as coinage metal. In the Gupta period Copper has been deliberately added to economise the consumption of silver as well as increase the wear resistance of the coins. Kauṭilya’s Arthaśāstra gives a vivid account of minting such punch marked coins. In the latter period other coins have been produced by rolling silver pieces into sheets of uniform thickness and cutting it into equal square pieces. The identification marks were punched on these pieces and their weight was controlled by cutting away the metal from one corner. In recent years Saran (1985) and Prasad, B.(1990-91) have analysed and studied the microstructure of a number of Ag-Cu alloy coins from different heads and they found the alloy composition to be consistent in each group.

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*(III) Copper*

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In lithic societies the use of coloured minerals like iron oxide, lead oxide and blue and green coloured Azurite (Hydrated copper carbonate) has been very common as cosmetics, colours used for decorating hutment walls and pottery, and their use in religious rituals and funeral practice. It is possible that accidental fall of the copper mineral in the fire has given rise to the copper age around 4000 B.C. According to the archaeological evidences this metal is probably the fourth in the series to attract the attention of mankind and give a new dimension to the human culture by providing new fighting and self-protecting power in the hands of mankind.

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The discovery of this metal marks the beginning of the Chalcolithic period and the Copper Age when man started using metal instead of stone and clay to fabricate his hunting tools, domestic utensils and many other things like ornaments, decorative pieces, mirror etc. The properties of this ductile reddish coloured metal having a melting point of 1083°C were further improved in the latter period by alloying with Pb, As, Sn and Zn, and some of these produced yellow coloured alloy similar to that of Gold. Copper sulphate mineral was given the name ‘Fools Gold’ owing to its shining yellow colour.

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The first and oldest evidence of copper artifacts in India come from the Nal Cemetery dating back to 3rd M. B.C. i.e.. the same period as in Iran. Tylecote(1976) is of the opinion that the technology of production and use of copper in this country was imported from South Iran along the Persian Gulf i.e. by sea route rather than over the difficult land route through Northern Baluchistan or Afghanistan. If this is true the Indian society developed at Lothal had trade links as early as 3rd M. B.C. i.e. even earlier than Harappan period. Forbes(1964) has mentioned that King Devius of Iran had a brass cup dating to 522 to 486 B.C., and the origin of this cup most probably belonged to India. This leads to the conclusion that the process of extraction of copper was probably known to Indians much earlier than 3rd M. B.C. and it contradicts the views of Tylecote. The theory of indigenous discovery of copper production technology is further supported by the presence of 4.8% Ni and 2.14% Pb in the copper artifacts found at the Harappan sites, whereas the Iranian copper is rich in Arsenic. the copper sulphide ores found at Khetri and Maubhandar have been found to be rich in nickel.

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Rai(1965) has mentioned the discovery of hollow copper, Gold and Silver beads belonging to the Harappan period which were manufactured in two halves and then soldered together. These beads have been threaded in copper wire to form a necklace. At Chinkudaro three types of copper bangles belonging to the same period have been reported. Out of these one type was made from 3.5 to 5.05 mm round copper wire, the second type was semicircular in cross-section with the flat side making the inner side of the bangle. The third type had been hollow from inside and it was made by shaping copper sheet. At Lothal a thin hollow copper pipe has been found which was probably used to drill holes in Gems and glass beads. The massivity of copper objects made in the past can be judged from the tapered copper bolt 620.0 mm in length and having 49.15 mm diameter at the two ends and 56.9 mm at the centre. This bolt has been recovered from an Ashokan pillar found at Rampurva near Nepal dating back to 3 century B.C.. and it is now preserved in the Calcutta National Museum. This bolt requires atleast two persons to lift it. The copper Chariot from Daimabad Hoard has been cast in one piece by the cireperdue process. It is now preserved at the Prince of Wales Museum, Bombay.

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The use of copper has been mentioned as Lohitya and Loha in Vedas and other ancient literature. This has been reviewed in great detail by Kulkarni(1989) and Joshi(1970), Bhardwaj and many others. Rasa Ratnasamuccaya gives a vivid account of the processes of extraction of copper and its use in Ayurvedic Medicinal system. In this book copper has been classified into two qualities i.e. one the pure (Red) copper from Nepal and the other impure or black copper (Malecch). The Nepal copper was of high purity (99.5%), brick red in colour and very ductile whereas the impure copper contained copper oxides and other impurities like Pb, Sn, As, Zn etc. which made it hard and brittle.

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*(III.1) Natural Resources and Extraction of Copper*

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Copper occurs in nature in native form and most probably man's acquaintance with this metal was in this form, which could be hammered i.e. cold worked to make various hunting tools etc. which could replace the crude stone objects. In India the occurrence of native copper is very rare and there is a possibility that Indian craftsmen got introduced to this metal by chance reaction between copper bearing minerals like Malachite, Cuprite and Chalcopyrite and charcoal fire. The chemical formula of the copper minerals is given below:

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Malachite - CuCO3.Cu(OH)2

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Azurite - 2CuCO3.Cu(OH)2

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Cuprite - Cu2O Chalcopyrite - Cu.FeS2 Hydrated Blue Vitriol - CuSO4.5H2O

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Besides these there are many other minerals bearing copper as chloride etc. The most common mineral of copper found in India is chalcopyrite but others are also found as placer deposits. Chalcopyrite has been reported to occur at Khetri in Rajasthan, Surda, Mosabani and Rakha in District Singhbhum of Bihar, Almorah in U.P., Sikkim and Nepal.

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The archaeological survey has indicated that copper was being extracted at Udaipur, Debari, Dilwara, Kedolai, Jhunjhunu, Khetri and Alwar in Rajasthan and Ahmednagar, Daimabad and Ahar in Gujrat and Maharastra region. Singhbhum copper deposits were also being worked in the past and most probably the copper in the Gangetic plains came from here through the river route, whereas the copper objects found in Sindh, Haryana and Rajasthan came from the ancient mines of Khetri region and Aravali hill deposits. Fig. 6(a & b) shows picture of the ancient mining sites located near the modern Surda mine of Singhbhum. Copper working site dating back to 1000 A.D. has been reported by Mohammad (1961) at Agnigundala in Andhra Pradesh. The deposit extends over 3 km and upto 3 km depth and it is surrounded by the entire working starting from mining to pounding stones, washing tanks and melting furnace as well as slag dumps etc.

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The first mass scale copper extraction process might have been carried out by carbothermic reduction of Cuprite in a bowl or small shaft furnace because this does not involve the complicated treatment of sulphide minerals. A comparative stepwise flowsheet is given below :

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(i) Cu2O+C (wood charcoal) heat 2Cu+CO (Reduction of oxides) (ii)(a) CuS.FeS+O2 → CuO +FeS+SO2 O2 → CuO+Fe+SO2 (Roasting), and (iii)(b) 2CuO+2FeO+2C+SiO2 → Cu+2CO +2FeO.SiO2 (Reduction).

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Thus we see that the production of copper from Chalcopyrite is a two step process involving precise control of the thermochemistry. Moesta and Schlick (1989) and Yazawa (1980) have discussed the thermodynamic limits of the roasting and reduction of copper sulphide in great detail. Fig. 7 gives the phase diagram for the Cu-Fe-S-O system as calculated by Yazawa (1980). These studies have confirmed that for dead roasting a temperature higher than 850°C is required and during this process iron acts as sulphur scavenger helping the conversion of CuS into Cu2O and CuO, and during reduction FeO reacts with SiO2 to form 2FeO.SiO2 (Fayalite) slag which has lowest melting point of 1177°C. The calculations have also shown that although at 1000°C copper oxides gets reduced to metal but the slag phase remains solid, and that molten metal separates out only at temperature above 1250°C. Based on the occurrence of 1.08 to 1.88% iron in the copper objects from Ahar, Chakraborti (1983) and Tripathi (1985) have formed the opinion that copper and iron oxides can be simultaneously reduced easily, and in presence of copper the reduction of iron oxide is possible only at very low pressure of oxygen (log PO2 = 10-3 atmosphere) normally not possible to attain in the low shaft furnace. Maddin (1982) and his group have conducted some experiments on the ancient reduction technology and they have found that during the reduction of roasted Fig. 6. Photograph showing an ancient copper mining (a) trench, (b) mouth of shaft at Surda Mines, Singhbhum Fig. 7. Cu-Fe-S-O equilibrium diagram (at 1300°C) indicating the instability of metallic iron in presence of CuO, and ultra low oxygen pressure required for its production

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Sulfur-oxygen potential diagram for the system Cu-Fe-S-SiO2 at silica saturation produced under the thermodynamic conditions of the ancient copper smelting furnace of the Ahar culture.

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chalcopyrite all the iron oxide forms 2FeO.SiO2 slag with silica, and they did find some prills of iron embedded in the slag.

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The chemical reactions taking place during the production of copper are as given below:

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i) Roasting reactions

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4CuFeS2 → 2Cu2S + S2 + 4FeS ..(1)

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2FeS → 2FeS + S2 ..(2)

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4CuS → 2Cu2S + S2 ..(3)

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S + O2 → SO2 ..(4)

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4Cu2S + FeO → Cu2O + FeO ..(5)

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2FeS + 3O2 → 2FeO + 2SO2 ..(6)

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ii) Reduction reactions

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Cu₂S + 2CuO → 6Cu + SO₂ ..(7) Cu₂O + C → 2Cu + CO ..(8) 2FeO +SiO₂ → 2FeO.SiO₂ (slag) ..(9)

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Percy (1861) and Tylecote (1962) have mentioned that in Sikkim evidence of old copper working has been found and there powdered ore was mixed with cow dung, balled and roasted. Then the roasted ore was melted with charcoal in a shaft furnace to produce copper. At Khetri also similar process was in practice as quoted in Gleaning in Science (1831) and by Anon Brook(1964).

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The analysis of copper smelting slag at various sites have been found to contain Fayalite as the major constituent and the slag from Rajghat contained CaO.SiO₂ also, besides other impurities like Al₂O₃, MgO, the CuO content of the slag was 1.05 only.

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*Rasa Ratna Samuccaya* has mentioned a number of processes for obtaining pure copper from chalcopyrite by smelting small charges in crucibles as reviewed by Jha (1990). He has conducted detailed experiments on the process of extraction of copper from chalcopyrite following the Ayurvedic route and using Lemon juice as the reductant. In this process after purification (sodhan) of chalcopyrite following the Ayurvedic process the ore mineral was roasted at 750-900°C adding lemon juice during the process. The process of roasting was alternated by tritaturation of the charge. In this process 100 c.c. of lemon juice was added per 100 gm of the ore. During the roasting process Cu and Fe get converted to citrates. The roasted ore was mixed with 25% Borax (flux) and more of lemon juice and the mixture was pressed into 20-30 mm balls. These balls were dried in Sun and then melted in Silimanite lined crucibles at 1250°C. There are four stages of the process of production. On carrying out the melting at 1250°C almost sulphide and gas free copper button separated from the liquid slag was obtained. *Fig. 8* shows the microstructure of the copper button showing dendrites of copper and some inclusions of CuS, CuO and slag.

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Another process mentioned in the *Rasa Ratna Samuccaya* is based on the precipitation of Cu from blue, vitriol solution (CuSO₄) by the process of cementation. In this process concentrated CuSO₄ solution was kept in an iron bowl, where on the surface a copper layer was formed following the reaction given below:

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CuSO₄ + Fe → FeSO₄ + Cu (10)

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After some time when a thick layer of pure copper particles was deposited on the iron surface the solution was removed and then copper was collected by scrapping it away from the iron surface. This pure copper was carefully washed with water free from (SO₄) and used for the preparation of copper *bhasma*.

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(III.2) Thermo-mechanical Treatment of Copper

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Copper, a brick red coloured metal has a melting point of 1083°C and it is very ductile in nature. This can be easily cold or hot worked to give any shape like rod, plate, wire and sheet etc. Hence, it has been extensively used in the past for the manufacture of domestic utensils, jewellery as well as for covering the wooden surface and engraving it for decoration and preservation of wood pannelling. The work hardening effect can be easily removed by annealing at 700-800°C in cowdung fire to produce metal having stressfree recrystalised grain having high ductility. The archaeological evidences have shown that this metal was shaped into various objects by casting into stone and clay moulds, hot and cold forging and soldering or rivetting to join two metal pieces. The ancient craftsmen had a flare to produce beautiful designs on the copper objects by embossing, engraving and gem setting. Beautiful objects having carvings inlayed with Gold and Silver wire and hot enamelled have been also found in latter stages. Copper being soft metal the sharp edge of the hunting tools and war weapons were hardened by cold working till the technology of production of bronze and Cu-As alloys were developed, which were much harder than the cold worked copper. On polishing the surface of copper could be given a highly shining polished finish which was being used as a mirror in the royal families. The Gold coating of copper objects by the process of amalgamation was also in the practice to produce various decorative cheap articles made from copper but having a golden finish.

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Fig. 8. Microstructure of copper button produced by the Ayurvedic method showing dendrites of copper and presence of CuS, CuO and slag inclusions, Mag. x 100

(IV) Iron and Steel

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The discovery of iron and its technological development on the Indian subcontinent has raised many questions because of the lack of archaeological proof dating back to those existing at Lustrian in Iran and other centres of ancient civilization. There is a total absence of iron objects at the Harappa and Mohenjodaro sites but the recent excavations at Lothal,Ahar(1300 B.C),Gufkral (1300 B.C.),Atranjikhera (1025 B.C.),Tadkanhalli(1000 + 150 B.C.) Hallur (1000 to 900 B.C.) and Khairadih (1000 + 100 B.C.) have pushed back the beginning of the iron age to the same period as those outside India. *Fig. 9* shows the 14C dating of the iron objects found at some of the important excavation sites of India and its shows a definite trend of simultaneous development and appearance of this metal all over India. *Table 2* gives the chronology of development of iron tools and other objects during the early, middle and later stages of the development of iron age in the country. This table clearly shows a gradual widely spread development of the use of iron which is possible only under the conditions where the knowledge of the use of these artifacts is lacking. This indicates that in all probability the iron technology in India is independent of the copper technology and it might have been developed even before copper.

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This hypothesis is further supported by the nature of the deposit of the mineral of the two metals, and comparatively easy method of obtaining iron from the oxide

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Fig. 9. 14C dating of some of the iron objects found at different archaeological sites in Indian subcontinent Table 2 — Various tools and implements appearing at the three stages of iron technology growth

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| Tool Type | Name of Tool | Early Stage | Middle Stage | Late Stage |
| **Hunting Tools** | | | | |
| | Spear heads | * | * | * |
| | Arrow heads | * | * | * |
| | Points | * | # | # |
| | Socketed tangs | * | # | # |
| | Blades | * | * | # |
| | Spear lances | # | * | # |
| | Dagger | # | * | * |
| | Sword | # | * | * |
| | Elephant goad | @ | * | * |
| | Lances | @ | @ | * |
| | Armour | @ | @ | * |
| | Helmet | @ | @ | * |
| | Horse bits | @ | @ | * |
| | Caltrop | @ | @ | * |
| **Agricultural Tools** | | | | |
| | Axes | * | * | * |
| | Sickles | * | * | * |
| | Spade | @ | * | # |
| | Ploughshare | @ | * | # |
| | Hoe | @ | * | * |
| | Chisel | @ | * | * |
| | Pick | @ | # | * |
| **Household Objects** | | | | |
| | Knives | * | * | * |
| | Tongs | * | # | # |
| | Discs | @ | * | # |
| | Rings | @ | * | # |
| | Spoons | @ | * | * |
| | Sieve | @ | @ | * |

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| Tool Type | Name of Tool | Early Stage | Middle Stage | Late Stage |
| | Cauldron | @ | @ | * |
| | Bowls | @ | @ | * |
| | Dishes | @ | @ | * |
| Building Materials | | | | |
| | Rods | * | # | # |
| | Pins | * | # | # |
| | Nails | * | * | * |
| | Clamps | * | * | * |
| | Pipes | @ | * | # |
| | Sockets | @ | * | # |
| | Plumb bob | @ | * | # |
| | Chains | @ | * | * |
| | Door hooks | @ | * | # |
| | Door handle | @ | @ | * |
| | Hinges | @ | @ | * |
| | Spikes | @ | @ | * |
| | Tweezers | @ | @ | * |
| | Anvils | @ | @ | * |
| | Hammers | @ | @ | * |
| | Scissors | @ | @ | * |
| | Saw | @ | @ | * |

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* indicates definite existence; @ indicated non-existence;

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indicates that confirmed data is not available.

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ore than copper from the sulphide ores. In India while iron ore Fe2O3 and Fe3O4 are occurring as surface deposits embedded in the Quartz and Dolomite minerals below the gound level. This aspect of the discovery and development of iron technology has been discussed in detail by Prakash, B. (1990) and from this view point the earlier attempts of correlating the discovery of iron to P.G.W. and N.B.P. in the Northern India seems to be inaccurate, although iron implements appear along with these potteries at many sites.

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The survey of ancient literature has provided ample proof that iron and steel making technology and their difference has been known in the Vedic period (vide reference on Rgveda and Yajurveda) and may be even earlier. These evidences also support the hypothesis of Sharma R.S. that Aryans were the original inhabitants of India and this also supports the use of iron both by Aryans and Dravidians even in the pre-historic period. As cited earlier Indians of the Vedic period were using steam powered transport vehicles and they were capable of producing many types of war weapons and agricultural tools which is possible only by the use of iron and steel. Prakash and Tripathi(1986), and Kulkarni(1989)have published a detailed review on this metal and listed chemical composition of many of the iron objects. These objects have carbon content of 0.03 to 1.3% and they are basically Fe-C alloys. The *Suśruta Saṃhitā* has mentioned the method of fabrication (700 B.C.) of more than 100 surgical tools made of Fe-C alloys and the process of heat treatment to obtain razor sharp edge capable of splitting hair into two halves longitudinally.

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*Fig. 10* gives the sketch of some of these tools prepared on the basis of their description in the text. *Fig. 11* shows the collection of iron swords and daggers etc. collected from the excavations at Tinnevelley dating back to 400 + 100 B.C., and *Fig. 12* and 13 show various iron objects found at Atranjikhera dating back to 1200-1000 B.C.

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Some of the massive iron objects manufactured in India in the Christian era have no parallel in the world, such as the famous Iron Pillar at Delhi (4th century A.D.) weighing 6000 kg, Iron Pillar at Dhar (12th century A.D.), weighing 7000 kg, 29 iron beams at Konark in Orissa (9th century A.D.), of which the longest one measures 11000 mm in length and 175 by 197 mm in cross section weighing about 3000 kg, the iron trident at the temple of Achaleshwar on mount Abu, the iron pillar at Kondachari and the iron trident at the temple of Tanginath near Bishunpur in Bihar. Some of these are shown in *Fig. Nos. 14 to 18*. The massivity of these objects and their high corrosion resistance have kept the modern metallurgists and technologists wondering about the manufacturing and processing details as well as the cause of their such high corrosion resistance.

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*Fig. 10. Sketch shows some of the surgical tools designed by Suśruta* (IV.1) Iron Minerals and Smelting Technology

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Fig. 11. This photograph shows some of the Iron swords and daggers found at Tinnevelley (400 B.C.)

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Iron occurs in nature mostly as Fe₂O₃(Hematite) and Fe₃O₄ (Magnetite) and some Pyrite deposits (FeS) have also been known. Besides this, in some parts of the world Meteoric Iron has also been found and used for the early war weapons. Iron of Meteoric origin is easily distinguished by its high Nickel content.

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India has been bestowed with one of the largest and richest deposits of Hematite, Magnetite and their hydrated forms like limonite etc. and these deposits are spread all over India. These deposits have been worked all over India to produce sponge iron or wrought iron using variety of furnaces and archaeological remains of a number of furnaces have been found at Nalanda, Naikund, Khairadih, Ujjain and many other sites. Fig. 19 shows a sketch of the reconstructed furnace dating back to 700 B.C. As can be seen this furnace has been constructed from prefabricated refractory clay blocks or bricks, whereas the furnaces used in Western Asia and Europe in the same period were either made by digging a hole in the earth or by arranging stone pieces to give the desired shape. These furnaces were generally used only once and then discarded after taking out the hot bloom. That is the [Figure: Iron Weapons : Period III: 1-8, Arrow heads; Spearhead; 10. shaft 1/2 Fig. 12. Some of the iron objects excavated at Atranjikhera (~1000 + 200 B.C.)]

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[Figure: Iron Tools and Implements: Period III: 1-3, clamps; 4-6, chisels; 7-8 , Bars; 9-10, Borers; 11, Needle 1/2 Fig. 13. Some of the iron tools and objects found at Atranjikhera (~1000 + 200 B.C.)]

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probable reason for the occurrence of a cluster of furnace remains at the sites excavated in the West, whereas in India the furnaces seem to be reused after relining and repair with refractory clay. Buchanan (1807), Hadfield (1912) and Verier (1942) have given detailed description of the working of a large variety of iron smelting furnaces and the steel industry prevailing in the country during 17th to 19th century. Ray (1956), Joshi (1970), Banerjee (1965), Krishnan (1955), Prakash B. (1983) Chatterjee and Altekar (1973), Bhardwaj (1982) and many others have surveyed the available information and published excellent reviews and books on the subjects.

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Fig. 14. Iron Pillar at Delhi and its microstructure at the bottom of the pillar

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The smelting of iron in the past was done only by a special group (caste) of persons in each tribe and they all worshipped the God ‘Asura’. This may be because in the early days of the discovery of this metal it was being used only to make war weapons and hunting tools (refer *Table 2*), and the general public being peace-loving the handling of this metal was considered the work of ‘Asura’ who gave devilish power in the hands of man. The belief of the ancient iron smelters in lord ‘Asura’ has been so strong that his image was put even in the ‘Mandala’ of ancient iron making in Japan (Buddhist period) as can be seen from *Fig. 20* which shows the painting of ‘Kāla Bhairava’ and ‘Asura’ working for the iron smiths (Prakash, B. 1990). In Bihar, Orissa and Eastern part of Karnataka the iron smelting was carried out, by these ‘Asuras’ and in Central India by ‘Āgārias’. Verier (1942) and Joshi (1970) have written in great detail about the ‘Āgāria’ tribe and their iron smelting practice spread through the central province, Rewa, Udaipur, Sirguja, Ranchi and Koraput etc. *Fig. 21* shows the region of India in which according to Verier (1942) Āgārias were smelting iron. In some places they are confused with ‘Lohārs’ or ‘Lohārins’(blacksmiths) but as Verier (1942) has described, while Āgārias smelted the iron ore to produce wrought iron blooms ‘Lohars’ shaped the rough iron to manufacture various tools and objects and carried out suitable heat treatment operations. In the year 1982 Prakash and Igaki (1984) surveyed the district of Bastar where iron making by Mundia tribe was mentioned and they could locate one furnace in the interior of the jungles of ‘Bastar; at Loharpara as reported in their paper.

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Fig. 15. Dhar Iron Pillar broken into three pieces. The change from square to octagonal section at the top can be seen

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At Bishunpur on the road between Lohadāga and Netārhat (near the Bihar, Madhya Pradesh border) Sharma, M. has located the site where iron smelting was being done earlier, and his group has got it revived under a rural development plan ‘Vikas Bharati’. There the smelting is done by Birjias and Lohars both belonging to the Asur tribe. A detailed report on this revived iron smelting has been published by Prasad, K.K. (1990) and his group. For recent survey, vide Sahasratudheya et al (1993) of P.P.S.T. Foundation.

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IRON TEMPLE SAMPLE MICROSTRUCTURES

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[Figure: AVERAGE COMPOSITION C .27 S .05 O18 Cr NIL Co .09 Mn TRACE .04 Si .50 Ni NIL W .16 P .006 O15 Cu NIL Ti NIL Al .040 HARDNESS 80 B 115 VPN IRON BEAMS]

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*Fig. 16. Iron beams (12 century A.D.) lying at the Sun temple of Konark*

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The ancient iron making practice consisted of the following operations and each one of them was performed under the strict supervision of the oldman or master craftsman of the tribe. The operation of the furnace was considered to be a sacred ritual and they prayed to the tribal God ‘Asur’ for the success of the smelting. Many of the ‘Mantras’ and verses have been preserved as folk songs and these have been vividly described by Joshi (1970). The secret of iron smelting process was very religiously guarded and passed on only to the younger generation of the family or tribe. Women had active role to play in the process and certain specific jobs were to be done only by them. In *Rasa Ratna Samuccaya* a full chapter has been devoted to describe the characteristic features of the female helper ‘Kalini’ and her duties during such religious rituals.

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The whole operation of the furnace can be divided into five parts

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i. Collection and preparation of the raw-material

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ii. Making of the furnace and its dressing

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iii. Making of air bellows and their fitting

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iv. Ignition of the furnace and its operation

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v. Handling of the sponge iron.

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Following is a short account of the furnace operation :

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Fig. 17. Iron Pillar at Kondacheri

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(i) Collection and preparation of the raw-material: Iron ore - mostly weathered soft Heamatite ore was used but in some regions like in Assam and South India magnetite sand has also been used for the extraction of iron. The operation was generally carried out in the vicinity of the ore deposit or in the nearby forest where suitable charcoal could be made from the wood. It was the duty of the experienced men to locate suitable deposit of the ore and dig it out after performing the puja. Women used the help in transporting the ore and breaking it into pea size (3-5mm). In Mandla before starting for the collection of the ore a ‘mantra’ was recited for the success.

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The translation of this ‘mantra’ is as follows:

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“Go and seek Lohār Mātā, God lead on bed, trusting thee I follow”

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Evidences have been found of mining even at a depth of more than 10m.

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Charcoal – Charcoal was prepared from wood like Irool (Xylia dolaberiformis) Teak, Babul (Acacia arbica), Sal (Shorea rubusta), Bamboo (Caleotropis gigantica) etc. yielding dense and heavy charcoal. It was prepared by cutting the dried branches and trees and burning them into an open heap or inside a pit and when the wood was charred the fire was extinguished by covering the burning wood with green leaves and twigs, and sprinkling water on it. After the fire has completely died out the charcoal was collected and used for smelting. In the latter period Beehive type of firing chambers were introduced for controlled charring of the wood.

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Fig. 18. Iron Trident (Trishul) at Tanginath temple near Netarhat (M.P.)

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Flux – No evidence of the use of lime stone or lime as flux has been found except the SiO2 and CaO present in the ore and the charcoal ash. Some times SiO2 has been added to the charge to make the fluid 2FeO-SiO2 slag. Ayurvedic charges for ‘Satvapatan’ i.e. extraction of metals from minerals have used Borax as flux. The first use of lime as flux in iron smelting has been found at Ujjain (M.P.) and Bairut (Rajasthan), where this technology went through a faster growth.

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(ii) The iron making furnaces and its dressing: Variety of small and large furnaces have been described by Joshi (1970), Banerjee (1965), Krishnan(1955), Prakash(1983), Chatterjee et al. (1973) and Ghosh (1964) and many others, which have been in operation during the 17th and the 18th century in various parts of the country. These furnaces were called Bhaṭṭi, Kothi or Koṣṭhi and they were made from locally available clay, and at many places internally lined with natural china clay or a refractory mixture generally used for crucible manufacture. More details [Figure: CIRCULAR CLAY BRICKS BLOOM CHARCOAL TUYERE TAPPED SLAG CINDER Fig. 19. Iron smelting furnace found at Naikund (700 B.C.)]

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about this mixture will be given later. These furnaces were either made below the ground level or on the sloping face of a hillock or above the ground level.

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They can be classified into three main groups :

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(a) the furnaces of South India having bowl shaped hearth and conical shaft. The furnace was made generally below ground level. Fig. 22 shows a picture of a furnace from Salem which has been made above the ground level.

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From the study of this furnace it seems that a former or inner mould made of read or other thin wood was being used to give the desired contours to the interior of the furnace.

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(b) the second type was cylindrical furnace of Madhya Pradesh, where the bloom was removed from the top,

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(c) Kothi type furnace of Bhopal and Ujjain zone were much larger as compared to the others and had square or rectangular cross- section.

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Figs. 23 to 25 show photographs of different types of furnaces which were in operation during 17th to 19th century. All these furnaces had two holes at the bottom, one for introducing the blast and the other for removing the molten slag. Generally a slanting platform of bamboo, lined with mud, was prepared at the mouth of the furnace to preheat the charge and to push it inside the furnace. A picture of a bowl type furnace which the author studied in 1981 is found at Lohārpārā in the district of Bastar. The line diagram shown in Fig. 26 gives the details of this furnace made in one of the walls of a pit below the ground level. Fig. 27 shows a line diagram of a twin hearth furnace described by Buchanan which was capable of Fig. 20. Reproduction of wall painting from an iron working site in Japan. This painting shows that Japanese craftmen also worshiped ‘Lord Asura’ as in India producing 250 kg of iron per day in the Malabar area. The furnace was fitted with more than one pair of bellows, and peep holes were suitably located to monitor the temperature inside the furnace. Mahamud(1988) has described a furnace from Assam which is asymmetrical in design as shown in Fig.28. One of the walls of this furnace is slanting and this was probably designed to smelt Magnetite sand. Table 3 gives the detailed dimensions of most of these furnaces as available in the literature.

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Diagrammatic Illustration of the Wall Hanging Stature of the Blacksmith God

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Moon

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Sun

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Sanbo-Kojin (A God of mixture of Buddhism and Shintoism)

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God's suite

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God's suite

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Two Devils (employed by Sanbo-Kojin) drive away a woman (symbol of carnal appetite and worldly thoughts)

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Bellows

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Mandarine Orange

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Rice cake

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Wife of Mester

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Master

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Assistan Devil

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仏教金剛力士像奉納日 野崎準次郎

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三寶大荒神 鍛冶刀匠元祖 敬具

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Fig. 21. Map of Central India showing the region in which ‘Āgārias’ were active in the production of iron. Agarias carrying iron ore for smelting

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Fig. 22. Iron smelting furnace from Salem, showing the use of former made of reed to make the inner contour of the furnace

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Fig. 23. Artists impression of the operation of iron smelting furnace using two men to operate a pair of 'Bhathi'

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Fig. 24. Primitive blast furnace of Nagpur region

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Table 4 gives technical analysis of the design features as compared to those of a modern Indian furnace producing more than 1000 tonnes of iron per day. These design features have been calculated on the basis of the design principles of the modern furnaces as discussed by Bashforth(1973).

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A study of Table 4 shows that the ancient furnaces listed in Table 3 were not merely dug out holes or shafts built above the ground level but they were made [Figure: MALABAR IRON FURNACE CAPACITY : 54 kg. TIME - 40 TO 60 HRS SCALE - 1'= 1" (.. .. .. .. 1800)]

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Fig. 25. Photograph of the iron smelting furnace found at Lohārpārā in Bastar

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Fig. 26. Line diagram of the Bastar furnace

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*Fig. 27. Line diagram of the twin shaft furnace from Malabar (as described by Buchanan)*

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based on certain design parameters and principles although they have been of different shapes, sizes and from different parts of the country, and these design principles are very similar to those followed in the design of modern furnaces. For example the throat to bosh diameter (d:D) ratio was 0.28 to 5 in almost all the ancient furnaces while the modern furnace has a value of 0.7 to 0.77. The maximum diameter at the bosh or tuyere level determines the combustion rate of fuel in the furnace and the d:D ratio takes care of the thermal as well as reduction expansion of the ore. The difference between the values of ancient and the modern furnace may be due to the change in the nature of the charge i.e. ore, etc. and the use of coke instead of charcoal in the modern furnace. In a similar manner the stack angle and H/D/2 ratio also shows a good similarity with those for the modern furnace. The useful volume of the furnace has changed from furnace to furnace and for the larger furnace (1000 tonnes) it is 760 m³, but there is a good agreement on the volume of the furnace chamber (cm³) required per kg of iron produced by the direct reduction technique.

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