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History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s2.t3: (1.2) Technology of Gold Working

History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s2.t3

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(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

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