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Minerals and Metals in Ancient India v1.c26: 18. Antiquity of Zinc and Brass in Ancient India

Minerals and Metals in Ancient India v1.c26

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18. Antiquity of Zinc and Brass in Ancient India

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THE earliest occurrence of zinc in man-made artefacts is in the form of the copper alloy known as brass. Ever since the discovery of copper and the alloying elements of tin, arsenic, lead etc., different materials including zinc were used to alloy and harden copper.

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The earliest method of making brass was possibly the cementation process in which finely divided copper fragments were intimately mixed with roasted zinc ore (oxide) and reducing agent such as charcoal, and heated to 1000°C in a sealed crucible. Zinc vapour formed dissolved into the copper fragments yielding a poor quality brass, zinc percentage of which could not be easily controlled.

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Fusion of zinc with copper increases the strength, hardness and toughness of the latter. When the alloy is composed of 10 to 18 per cent of zinc it has the pleasing golden yellow colour. It can also take very high polish and literally glitter like gold. For this property, brass has been widely used for casting statuary, covering temple roofs, fabricating vessels etc.

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Reduction of zinc oxide around 1000°C is crucially important; below 950°C no zinc is produced. If there is a trace of oxygen then zinc vapour would be reoxidised and hence the successful operations must have been done in closed crucibles. If the temperature was higher than 1083°C, copper melted and flowed down to the bottom of the crucible forming a puddle there, exposing very small surface area of the metal for alloy formation.

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Brasses containing up to 36 per cent zinc are known as α- brasses, which undergo easy cold work. Brasses containing more than 46 per cent zinc are brittle. With zinc content between 36 to 46 per cent, we have α + β brasses which are lighter, harder and more suitable for casting statuary.

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Werner (1970) and Haedecke (1973: 229-33) demonstrated experimentally that brass produced by the cementation process could not contain more than 28 per cent zinc. The brass founders trying the cementation process have verified this observation. (Day, 1972: 62)

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The materials of antiquity containing more than 28 per cent zinc in copper matrix must have been prepared by mixing of the two metals, which could have been possible only after discovery of zinc as a separate metal and its preparation by a process such as distillation. The antiquity of brass artefacts can therefore be divided into two eras — one preceding and the other following the discovery of zinc as a separate metal.

Brass Before the Discovery of Zinc

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The earliest artefacts noted so far containing zinc is from India. Lothal (2200-1500 BC) showed one highly oxidized antiquity (No. 4189) which assayed: 70.7 per cent copper 6.04 zinc, 0.9 Fe, 6.04 acid-soluble component (probably carbonate, a product of atmospheric corrosion). The material could have been prepared through smelting of zinc-bearing copper ore or the cementation route described earlier. The raw materials might have come from the Ahar-Zawar area.

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Similar materials might have been used for making the brass-bronze items of Atrañjikhera during the PGW era (1200-600 BC). One copper item contained 11.68 per cent Sn, 9.0 Pb and 6.28 Zn; another item assayed 20.72 Sn and 16.20 Zn. Both the samples contained traces of iron and sulphur, indicating the possibility of chalcopyrite and sphalerite-galena having been the source materials which could easily come from the Ahar-Zawar area.

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We have discussed the above-mentioned zinc-containing artefacts in chapters 6 and 11. The author of this monograph was the first scholar to draw attention to the brass item of Lothal and Atrañjikhera and their possible link with the 1136 ± 160 BC. C-14 date of the timber samples in the Rajpura-Dariba silver-lead-zinc mine near Udaipur. (Biswas, 1990: 53-54)

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Craddock (1978) and Craddock et al. (1980) surveyed the evidences of early brass artefacts in the West. The earliest brass artefacts known come from the excavations at the Gordion Tomb in Phrygia, dating from the eighth and seventh century BC onwards. These came after the Lothal and Atrañjikhera traditions. From the seventh century BC the Greeks commented upon brass or oreichalkos, but always as an expensive, exotic metal not produced in Greece. There was no zinc in the early Greek bronzes. Etruscan bronze of the fifth century BC contained 11 per cent zinc.

Zinc Metal and High-Zinc Brass

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The earliest brass containing more than 28 per cent zinc, which could be made only after the isolation of pure zinc metal, came from Taxila. We have noted in our chapter 14 the overriding importance of the vase (BM 215-284) excavated from the Bhir Mound at Taxila and dated to the fourth century BC. This brass sample contains 34.34 per cent zinc, 4.25 Sn, 3.0 Pb, 1.77 Fe and 0.4 Nickel. This is very strong evidence for the availability of metallic zinc in the fourth century BC. (Craddock, 1981: 16-17) Most possibly India was the first to make this metal zinc (rasaka) by the distillation process as practised for the other metal mercury (rasa).

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There are references to zinc and brass in the lost fourth century BC text Philippica of Theopompus, quoted by Strabo in his Geography:

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There is a stone near Andreida (north-west Anatolia) which yields iron when burnt. After being treated in a furnace with a certain earth it yields droplets of false silver. This, added to copper, forms the so-called mixture, which some call oreichalkos. (Jones, 1968)

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The above reference is probably to the process of downward distillation of zinc (‘droplets of false silver’) and its subsequent mixing with copper to make brass oreichalkos (ārakūṭa in Kauṭilya’s Arthaśāstra) described in detail in the post-Christian era Sanskrit texts.

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It is quite possible that the zinc-making technology travelled west from India during sixth-fifth century BC, as it did later again in the eighteenth century AD. The pseudo-Aristotelian work On Marvellous Things Heard mentioned:

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They also say that amongst the Indians the bronze is so bright, clean and free from corrosion that it is indistinguishable from gold, but that amongst the cups of Darius there is a considerable number that could not be distinguished from gold or bronze except by colour. (quoted by Craddock, 1978: 8)

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The Indian emphasis was on the ‘gold-like’ brass and not on the zinc metal. The Greeks however used zinc metal as such in a few cases. In the course of the excavation of the Agora in Athens a roll of sheet zinc was found in a sealed deposit dating from the third or second century BC. (Farnsworth et al., 1949) Analysis showed it to be nearly pure zinc with 1.3 per cent lead, 0.06 Cd, 0.016 Fe, 0.005 Cu with traces of Mn, Mg, Sn, Ag, and Sb. A Hellenistic statuette, also from the Agora, but from a mixed deposit, has also been shown to contain more than 99 per cent zinc. (T. Wheeler quoted by Craddock, 1978: 7) Although Needham and Forbes doubted the above evidences on the ground that ‘the pieces were beyond the contemporary technology’ (Craddock, 1981: 18), it is quite possible that the Greeks had carried the material or the technology which had existed in Taxila as early as fourth century BC and possibly much earlier in Rajasthan.

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India’s monopoly on high zinc (more than 28 per cent) brass has never been questioned. The manufacture of zinc metal by reduction and distillation was a well-guarded Indian secret. The Western countries knew about the cementation route for making low-zinc brass, but after Theopompus referring to ‘droplets of false silver’, the art of making zinc and high-zinc brass was almost unknown in the West till the early part of the eighteenth century when the knowledge again travelled from India, and William Champion of Bristol, England utilised it to manufacture the said materials.

Mining Archaeology Related to Indian Zinc Ore

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In the previous chapter the recent pioneering work on the zinc-lead-silver mining archaeology in the southern part of Rajasthan (Willies, 1984, 1987, 1989) was mentioned and the relevant C-14 dates related to metallic ore mines in ancient India quoted (Table 17.1).

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The ancient workings in the South Lode (100 m depth) of Rajpura-Dariba mine have been C-14 dated as 1260 BC, 1130 BC, 1050 BC and the East Lode workings as 375 BC, 360 BC, 250 BC, 120 BC, AD 150 etc. Thus it is clear that the tradition of underground mining in India goes back to the thirteenth century BC if not earlier. The earliest emphasis was possibly on copper; at Rajpura-Dariba the other targets were lead, silver and possibly zinc which is strongly suggested by the brass artefacts of Lothal and Atrañjikhera.

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The art of smelting zinc ore and recovery of zinc metal by distillation must have been discovered before fourth century BC when Taxila produced the brass vase containing 34.34 per cent zinc. This possibility is reinforced by the facts of mining archaeology. Starting from the fifth century BC we have many C-14 datings in Rajpura-Dariba, Rampura-Agucha (these mines were discussed in the previous chapter) and most crucially in the Zawar Mine systems which would be discussed below.

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Zawar (24°21'N, 73°41'E) is about 30 km south-west from Udaipur, where the ancient mines (earliest C-14 date obtained so far is 430 BC) are found both opencast and underground. The locations and other details of the Zawar Mala, Mochia and Balaria Mines are provided in fig. 18.1. Some of the C-14 datings were given in the previous chapter (Table 17.1).

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Modern 470m level ZAWARMALA MINE Modern Raise 497.6 33 31 32 29 509.8 Wide Stope Timber 28 Chamber 25 26 27 Main Chamber 518.5 15 530.2 16 21.23 24 Double Arches 22 536 546 Promonontory 555 18 558 19 20 560 10 12 14 564 Top Chamber 13 Oldest Workings • Altitude-metres 0 metres 20 ∧ Photo-locations Long Stope Delhi Rajasthan INDIA Bombay Madras River Tiri Mochia Mine Old Zawar Zawar Mala Mine Boroi THE ZAWAR MINES 0 km 1 Ancient Mines STATE OF RAJASTHAN Delhi Ajmer Jaipur Jodhpur RAMPURA AGUCHA Kota RAJAPURA-DARIBA Udaipur AMBAJI ZAWAR Ratlam Devnimori 0 km 80 LW 1987 Fig. 18.1 : Ancient Mines at Zawar and the Details of Zawar Mala Mine.

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The host rock of the Zawar Mines is sheared dolomite, the result of metamorphism of sedimentary dolomites. The ore was geographically deposited syngenetically as disseminated lenses within the dolomite beds. Zinc occurs as sphalerite or as marmatite in which the zinc sulphide is in solid solution with iron sulphide. Also associated with the rock are galena, hydrozincite, pyrite, silver etc.

Journey Down the Zawar Mala Mines

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Ancient workings are found at outcrops on the ridge of Zawar Mala, and as deep as the 470 m (above sea) levels of the modern Zawar Mala mine, some 120 m below surface. The upper parts accessible from there were a few tens of metres below surface, isolated by a roof fall. At the surface, mine openings occur at intervals of 50 m or so. Willies (1987) investigated one mine of more commodious proportion—Pratapkhan or Pratap’s mine—in which Rana Pratap Singh, rival of Akbar took refuge during AD 1595-1600. A flat ‘room’ floored with phyllite slabs is inferred as Pratap’s refuge. The quarried materials used to feed a zinc smelter just below the narrow valley.

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The highest and possibly oldest workings are formed of small arched chambers, some 3 m across. Draughting strongly, the upper area was above water table, and was worked by a form of pillar and stall, with both pillars and openings very characteristically rounded by fire-setting. There was evidence of retreat-working of the pillars, producing wide or high chambers depending on the form of the high-grade orebody. Dumping of waste was systematically done, and debris from fireholes neatly stacked, thus the actual working areas being kept very clear.

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The method of excavation seems to have combined fire-setting and hammer-gad work. Mining tools like iron-chisels and pestle-shaped hammers were found. Lower still in the workings, the mine has been worked almost entirely by fire-setting, with very little gadwork being evident. A sample of timber from a set of steps near altitude of 558 m (32 m depth from the surface) has yielded a date of 170 BC. Nearby a rubble stone-built promontory (vide fig. 18.1) juts out from the waste-heaps. Also found at this spot was a ‘baby-bath’ sized earthenware water-container.

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The path then leads to a spectacular area of double arching (536 m altitude). From here, the path leads downdip across the strike of the beds. In this Wide Stope were found fireset holes, timber props, shallow trenches and several wooden launders to convey mine drainage water. One of these launders yielded a radiocarbon age of AD 60.

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At this 500 m altitude (90 m depth from the surface) was found one 5 litre capacity pottery water vessel. Hegde (1989: 89) identified several such pear-shaped pots (probably used for keeping water) as Ahicchatra 10 A type datable to the last quarter of the first millennium BC.

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At the bottom of the ‘Wide Stope’ (altitude 479.6 m), a ‘finger-stope’ has been developed. These are down-dip (30-40º angle) fireset inclines about 3-5 m diameter. This stope had been cut by the 470 m modern mine level, and the bottom was entirely removed before adequate photography was available.

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Eastwardly along the strike from the double arches area, a path leads to a large ‘Timber Chamber’, which is characterised by many vertical timbers revetting large heaps of small fire-setting debris, to form a platform. This has been used possibly as a hauling position from finger-stopes down-dip (suggestive of some form of haulage apparatus-windlass or man-powered wheel?) and certainly as a means of removing a pillar of an earlier working. An wooden bipod in the chamber yielded an age of 140 BC.

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From the Timber Chamber, the path led via a 1 m high step and circular (firehole) opening to a further large chamber, called the Main Chamber, of the order of 30 m across, and perhaps 20 m high, developed on the dip. A large long stope extends slightly eastwardly across the dip downwards for over 80 m. The forefield going a little lower than 470 m was presumably limited by inflow of water. Burnt wood residues correspond to a date of 200 BC. There were large amounts of bamboo and acacia burnt fragments in the area and also pottery lamps (*pradīpa* or *diyā*) kept on ledges for illumination.

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The use of good paths, ladderways, semi-fixed lamps, neat stacking of waste rock etc. suggested the use of a large labour force. “One has a mental picture of perhaps some hundreds of persons at work, some carrying fuel or support timber in, and ore and rock waste out, probably using baskets carried on the head, with others conveying water in a similar way”. (Willies, 1987: 86)

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The combination of major ventilation problems due to fire-setting, the oxides of carbon and sulphur, particulate materials, high humidity etc. must have made working conditions difficult at best.

Some Other Zinc-Ore Mines

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In Zawar, the Mochia mine (*fig.* 18.1) is separated by the gorge of the River Tiri from Balaria, and has a series of parallel, almost vertical ore-bearing beds outcropping as a high ridge for a strike length of nearly 7 km. Most of the ridge is characterised by trench type workings, or by inclines and shallow shafts, exploiting richest individual beds. Ancient workings were found down almost to river level and showed iron gads, a draw hoe for filling baskets, an iron pestle, woven basketwork, rope and timber. The later part of the working was more recent; Balaria Mine yielded a post-medieval date.

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Boroi Magra Mine ridge (fig. 18.1) has a number of shallow workings, and deeper ones have been revealed by boreholes. Small mortars were cut into the rock near shafts, as a means of crushing ore down to coarse sand size. It is not clear whether washing technique or any means of beneficiation, apart from hand-picking was employed after crushing in the mortars.

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The earliest C-14 datings in the Zawar Mines are 430 ± 100 BC of the PRL 932 sample from the Zawar Mala Mine and 380 ± 50 BC of the BM 2381 sample from the Mochia Mine. (Paliwal et al.) Similar datings from Rajpura-Dariba (e.g. 375 BC), Rampura-Agucha (370 BC) etc. (vide Table 17.1) confirm the widespread underground mining of lead-zinc ores in the southern Rajasthan during the fifth-fourth centuries BC onwards.

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There are subsequent C-14 datings in the said mining area: 250 BC, 200, 170, 140, 120 BC, AD 60, 110, AD 150. The history of Zawar is interwoven with the colourful past of the former state of Mewar (Udaipur). An inscription dating to the seventh century AD found at Samoli in southern Mewar refers to the arrival of a merchant in the area and of his starting of a mine at Zawar, then known as Arandyagire. Inscriptions on old temples constructed in and around old Zawar tell of mining and working of the deposits in the seventh century AD, and of the full flourishing of the industry during thirteenth century AD onwards. (Paliwal et al.)

Making of Zinc and Brass in Ancient India

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As regards the recovery of zinc from the ore, the crucible reduction/distillation method was put to large scale commercial practice in the thirteenth century AD, and this will be described later. Indirect and circumstantial evidences suggest that the distillation method was in vogue much earlier, probably from the fourth century BC onwards, although not in a large scale as we find in the thirteenth century AD context.

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In this connection we recall the evidence from Rampura-Agucha presented in the previous chapter. The zinc-lead-silver ore in the site was selectively mined at least as early as 370 and 250 BC (vide Table 17.1). An appreciable amount of zinc must have been separated from the zinc-rich ore (present-day ore in the site contains 13.5 % zinc) as revealed from the low-zinc content slag. One sample of slag (No. 4 in Table 17. 4) assayed as low as 0.01 per cent zinc. (Tiwari et al., 1984: 85) Near the slag dump area several retort-like pieces were reported. When assembled, their appearance suggested a cylinder approximately 20 cm long with walls about 4-5 cm thick and an innermost pipe-like feature with a coating of dirty white material, mainly zinc sulphate. They could not be mistaken for tuyeres but for their closed pointed ends. This is highly suggestive of a used retort. Along with this, some thin-walled tube-like material containing a thin coating of blister type material was also found. (Paliwal *et al.*)

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It is conceivable that the retorts were being used in the said context for roasting zinc ore to obtain the light, white, smoky zinc oxide, which the ancient Greeks called pompholyx, philosopher’s wool. In the modern zinc plant at Udaipur, roasting of zinc sulphide concentrate produces not only zinc oxide (and sulphur dioxide gas) but also some zinc sulphate which was noticed in the fourth-third century BC retort in Rampura-Agucha*. The said retorts, already found sealed at one end, must have been closed or sealed at the other and also to prevent the escape of smoky zinc oxide into the atmosphere. The retorts were possibly modified to serve as reduction distillation chambers (to produce metallic zinc), the final version of which, noticed in the thirteenth century AD context, would be described later. Very significantly, Tiwari *et al.* (1984: 85) noted that the slag sample (no. 4 in Table 17. 4) containing only 0.01 per cent zinc but as high as 9.30 per cent lead, was “attached to baked earthen materials which could be the part of the earthen appliance used for smelting”. We suggest the possibility that the earthen appliance was a zinc distillation retort.

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We may now turn our attention to the antiquity of brass in ancient India. Before the discovery of zinc metal in India (made by the distillation route) some time during the fifth-fourth century BC, brass could be made only by the cementation route in which one of the following—zinc ore, sphalerite concentrate or the roasted product philosopher’s wool or zinc oxide—was smelted along with copper ore. The traditions of making philosopher’s wool and cementation brass** could have persisted even after the discovery of the distillation process of making zinc. As we have indicated earlier, the distillation route of making zinc and alloying this with molten copper was the only way of making high-zinc (more than 28 %) brass such as the fourth century BC Taxila vase (34.34 % zinc).

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We have noted the compositions of the Taxila brass samples in Table 14.7 and discussed the zinc-containing artefacts of Taxila in the same chapter. Some of the other early brass samples from ancient India have been reviewed by Neogi (1917, 1979: 40-42) and Ray (1956: 94-95), an extract of which is presented below.

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∗ There are also remains of zinc furnaces at Sojat in Jodhpur.

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∗∗ The second century AD text *Rasaratnākara* of Nāgārjuna mentions (1.3) “What wonder is it that calamine (zinc ore) roasted thrice with copper converts the latter into gold”. This is an allusion to the making of yellow gold-like cementation brass. The same text (1.31-32) also refers to the reduction-distillation of calamine yielding zinc : “an essence of the appearance of tin”.

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Brass articles of the first century BC or AD have been found on excavation of ancient stūpas. General Ventura executed operations for the examination of the stūpas at Manikyalaya in 1830. Three deposits were obtained of which the third, at a depth of 64 ft, consisted of a copper box enclosing a brass cylindrical box cast and beautifully turned on the lathe. The lid of the brass casket was found on cleansing to be inscribed. From the inscriptions on the various articles of this deposit and the accompanying Indo-Scythian coins, the great tope at Manikyalaya has been identified to be a mausoleum of the Indo-Scythian king Kaniṣka (1st century BC or AD) (Wilson, Asiatic Researches, Vol. 17, p. 601; Cunningham, Journal of Asiatic Society of Bengal, 1854).

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Another inscribed brass urn of the same date as the former has been discovered in a tope about 30 miles west of Kabul in the district of Wardak. This urn which in shape and size approaches closely the ordinary water-vessels in use in India to this day, was originally thickly gilt and its surface has in consequence remained well-preserved. (Princep’s Indian Antiquities edited by Thomas, Vol. 1, p. 162 quoted by Neogi, 1917, 1979: 40) As regards coins, both brass and bronze were used in ancient India for coinage. Circular punch-marked brass coins of Dhanadeva and Āryavarmā of Ayodhyā (circa 1st century BC) have been found. Brass coins of kings of several other dynasties living at that time have also been collected (vide Vincent Smith’s Catalogue of Coins in Indian Museum). From these arachaeological and numismatic evidence it is clear that brass was in common use in ancient India during the first century BC. A small number of die-struck coins of the Pre-Gupta and Gupta period, including a piece attributed to Candragupta II, are considered to be made of brass.

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Gradually brass was used in making statues of gods and goddesses mainly because it was yellow and looked like gold. An inscribed brass statue of Buddha, 30 cm high and 13.5 cm wide, of the sixth century has been discovered in a dharmaśālā at Fatehpur, a village 20 miles due west of Kangra-Kot (Annual Report, Archaeological Survey of India, 1940-45, p. 107). Another inscribed brass statue of a large size of about eleventh century discovered in Bengal has been preserved in the Dacca museum. We would present some technical details about brass statues in chapter 20.

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The most remarkable and gigantic work of brass has been described by the Chinese traveller Hiuen-Tsang who saw near the famous Nalanda convent “a vihāra of brass built by Śīlādityarāja (Harṣavardhana ruling 606-47 AD); although it is not yet finished yet its intended measurement when finished will be 100 feet”. When completed, this would undoubtedly have been a magnificent example of the skill of the ancient Indians in brass-work.

Literary Evidences on Zinc and Brass in Ancient India

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The literary evidences on the above subject, discussed in several chapters of our monograph, may be briefly summarised at this point. We have mentioned earlier in this chapter the pseudo-Aristotelian work ‘On Marvellous Things Heard’ referring to ‘gold-like bronze’ or brass of the Indians during the reign of Darius. The Greek’s ‘oreichalcos’ was equivalent to Kauṭilya’s ārakūṭa. (Arthaśāstra, 2.12.23, 2.17.14, 4.1.35) Both Kauṭilya and the earliest Indian brass of Taxila belonged to fourth century BC, while the earliest C-14 dating of the Zawar Mine is 430 ± 100 BC.

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The technical term ārakūṭa for brass persisted through centuries and we find this mentioned in the fourth century AD Jaina text Aṅgavijja (as hārakūḍa) and also in Amarakosa (AD 450).

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A more popular name for brass in the ancient India has been riti or ritikā which also meant calx of brass. The word was probably derived from harita or yellow which had been a synonym for gold in the Vedic literature. The word was chosen on account of the yellow colour of gold-like brass. Caraka Saṃhitā of third century BC refers to riti (Sūtrasthān, 5.26) and ritikā (Siddhisthān, 3.4) amongst other metals and alloys. The writings of Manu, Yājñavalkya and Patañjali of the pre-Christian era sometimes refer to bronze (kāṃsya) and brass (ritikā) almost synonymously.

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During the first century AD the Roman traders wrote about Indian brass (vide our chapter 16). While the Periplus used the Greek term oreichalcos (mountain copper) for brass, Pliny mentioned about aurichalcum (golden copper) made in India from cadmia, identified as calamine or the zinc ore. Samuel Beal suggested that the name cadmia came from Calamina, a port at the mouth of the Indus, which negotiated the export of the ore or the alloy of zinc. Ball (1886-87: 331) however suggested that the port was Calliana or Kalyāṇa near Bombay. The sixth century AD traveller Sopater mentioned about Calliana exporting brass.

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Philostratus of Lemons, writing around AD 230, mentioned a shrine in Taxila (presumably Buddhist) in which “the various figures were portrayed in a mosaic of orichalcum (brass), silver, gold and oxidized copper”. A much more significant fact related to Indian brass of this era is the advent of the alchemist Nāgārjuna.

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We would be presenting (in Vol. II chapter 8) our view that Nāgārjuna, the Indian alchemist was a contemporary of the Sātavāhana king Yajñaśrī Gautamīputra (AD 166-203), and therefore his Rasaratnākara must have been originally a second century AD text although what we have today is possibly a version edited during a later period such as the seventh century AD. We have provided in the earlier footnote in this chapter the second century AD. Rasaratnākara reference to cementation and distillation processes for making brass. The details of the said Sanskrit text and similar texts on zinc and brass from Rasārṇava (twelfth century AD), Rasaprakāśasudhākara, Rasacintāmaṇi, Rasakalpa (thirteenth century AD) and Rasaratnasamuccaya (fourteenth century AD) would be presented in the chapters 8 and 9, Vol. II of this monograph.

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The early sixth century AD text Bṛhatsaṃhitā of Varāhamihira (BS, 57. 1-7) described several vajra-lepa or cement recipes, one of them containing ritikā or calyx of brass. During the same century, Sopater, a traveller, mentioned about the port of Kalyāṇa exporting brass.

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When Hiuen-Tsang visited India (629-645 AD) he found many statues made of teou-shih or brass: standing Buddha at Bamiyan, cast in small pieces and then joined together, the image of sun-god in Multan, statues in the kingdom of Takka on the river Vipāśā, some near the Kulu province, Māyāpura or Haridvāra, Garhwal and Kumāun areas, brass statues of Śiva in Vārāṇasī, of Buddha in Vaiśālī, the famous brass statue of Buddha in Bodh-Gayā, ‘ornamented with rare jewels’, and lastly the brass vihāra of Nalanda referred to earlier. We have also mentioned the sixth century AD brass statue at Fatehpur near Kangrakot.

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Thus it is clear that by the seventh century AD India had well-established traditions in brass-making. (We would provide some analytical data in our chapter 20). Hiuen-Tsang recorded the tradition of the alchemist Nāgārjuna having made four brass statues of Buddha for the Sātavāhana empire which had fallen short of gold; we would establish in our Vol. II chapter 8 that the event had taken place in the second century AD.

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Analyses of Indian copper alloys (Werner 1972) and a series of analyses of Tibetan material (Riederer 1979; Craddock 1981) showed that brass was used almost exclusively from the seventh century AD. We have already mentioned the seventh century AD. Samoli inscription referring to fresh initiation of mining at Zawar. The five richly adorned Jain temples of thirteenth century AD Zawar testify the existence of a thriving community which initiated zinc smelting on a commercial scale. Discovery at Zawar of 29 silver coins in the soil burying ancient furnace proves that such coins were minted in AD 1593 during the era of Rāṇā Pratāp and Akbar.

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The Rasaśāstra texts initiated in the seventh century AD were related to the medicines of vegetable origin and those made out of metallic salts. These texts described smelting of metals and preparation of metallic oxides. The detailed references to the reduction of zinc ore and distillation of zinc metal, which were made in the thirteenth century AD Rasasāstra texts and fourteenth century AD text Rasaratnasamuccaya, shall be presented in the chapters 8 and 9 respectively of our Vol. II. It is significant that there was a spurt of smelting activity in the Zawar area around thirteenth century AD, and we would now present the newly discovered details of the ancient zinc distillation practices in Zawar.

Discovery of the Ancient Zinc Smelting Outfit

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Colonel Tod, the great historian of Rajasthan, wrote in 1829 that as recently as 1760 the Zawar Mines had been in full production but that “the miners are now dead, the mines filled with water”. (Tod, 1829, 1950) The mines were abandoned during the reign of Maharana Bhim Singh (1778-1828) when the State of Mewar was repeatedly plundered by Marāthā invaders. In 1850, Brooke mentioned about the ancient art of zinc distillation and an old man who claimed that he knew the technology. (Brooke, 1850: 212-15)

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In the neighbouring villages the used brinjal-shaped retorts have been utilised as components of building-walls and this has always been a familiar sight although people forgot how and where these retorts were used. Attention has been drawn to the huge quantity of zinc extraction debris at Zawar (Carsus, 1960; Straczek and Srikantan, 1967) and the deep mine shafts. (Mookherje, 1964: 657-677)

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Straczek et al. (1967) pointed out that there are two distinct types of smelter residues. One is a sinter or clinker intermixed with fragments of fired clay pots or retorts and contained in unbroken retorts; the retort residue and wall contain substantial amounts of zinc showing that these retorts are possibly zinc smelting crucibles. The other kind of residue is normal glass slag left after the recovery of lead and possibly silver. Straczek et al. (1967) estimated that the quantity of the sinter residues and retorts could be 2.03 to 2.54 lakh tons containing six to seven thousand tons of zinc metal and 400-500 tons of lead. Recently Freestone et al. (1985a) have estimated that the quantity of the extraction debris could be in the neighbourhood of 8 lakh tons.

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In April 1980, the Hindustan Zinc Limited (HZL) sponsored a three-year research project on recovery of zinc from the ancient slags which was successfully conducted at the Indian Institute of Technology (IIT) Kanpur by the author of this monograph. (Biswas, 1981-84) Some of the results related to phase characterisation of and zinc recovery from the ancient slags would be recorded in this chapter. In 1982, HZL collaborated with British Museum Research Laboratory (P.T. Craddock, Lynn Willies etc.) and the Department of Archaeology, Baroda University (K.T.M. Hegde) on archaeological investigations, and this led to the spectacular discovery of the zinc distillation outfit including furnaces and retorts showing the production strategy. Craddock (1984) described the exciting discovery:

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On the third day of the excavation (December 1983), one of the Baroda team (Hegde) spotted the corner of a refractory plate sticking out from a heap of spent retorts beside a goat track in a valley on Zawar Mala . . . . . . . With mounting excitement we cleared a small area above and around it to reveal first, the edges of furnace walls, and then the tops of retorts still in situ.

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Extensive archaeological and archaeo-material investigations followed (Craddock et al., 1983, 1985; Freestone et al., 1985a, 1985b, 1987; Hegde et al., 1986, 1989); we now present the summary of the results of the historical experiments performed in two continents.

Ancient Retorts and Furnaces at Zawar

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During the 1983 excavations two groups of furnaces were uncovered. A single bank of seven furnaces upon Zawar Mala contained small retorts 20 cm long and 10 cm in diameter. In old Zawar there was a more extensive arrangement of furnaces using larger retorts 30-35 cm long and 10-15 cm diameter. In both groups thirtysix retorts in a six by six arrangement were contained within the truncated pyramid of each furnace. The retorts were supported vertically on perforated bricks through which the condenser tubes passed into the cooler zinc collectors beneath (vide figs. 18.2-18.4).

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The furnaces are in two parts consisting of a zinc vapour condensation chamber at the bottom and a furnace chamber at the top. The two chambers are separated by a perforated terracotta plate. The condensation chamber measures 65 cm by 65 cm and 20 cm in height. The perforated terracotta plate that separates the two chambers is a composite unit made up of four equal segments of 35 cm². It is 4 cm thick, well-baked, and sturdy. Its perforations include circular holes of two sizes: larger ones of 4 cm diameter, each of which are surrounded by a number of smaller holes of 2.5 cm diameter. Within the furnace the composite terracotta plate was found to be supported on a ledge in the furnace walls on all four sides and a single solid terracotta pillar placed below the junction of its four segments. (Hegde et al., 1986; Hegde, 1989)

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Up above the perforated terracotta plate is the furnace chamber, in which 36 charged retorts (figs. 18.2-18.3) were arranged, inverted vertically; it may be presumed that 36 vessels were placed, one underneath each retort, to collect the condensed zinc vapour. This arrangement of downward distillation retort with the condensing unit underneath or distillation per descensum, is precisely what had been described in *Rasaratnasamuccaya* text (2.157-166; 9.48-50) to be discussed in chapter 9 of our Vol. II, and what was copied by William Champion in 1748 for his plant in Warmley, England. The brinjal-like retorts in Zawar are also similar to the *vṛntākamūṣā* described in *Rasaratnasamuccaya* (10.22-3).

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Fig. 18.2 : Midpoint Vertical Section Through a Zinc Reduction — Distillation Furnace at Zawar Mala

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Fig. 18.3 : Horizontal Section through the Furnace in Fig. 18.2 at Perforated Plate Level

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Fig. 18.4 : Section Through Small Charged Zinc Reduction — Distillation Retort with Stick Still in Place

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The furnace structures unearthed at Zawar did not include their roof part. However, curvature of their extant brick walls (*fig. 18.2*) suggests that the furnaces probably had domed roofs with a chimney-like opening at the top, similar to those of the vertical up-draught pottery-baking kilns of Mohenjo-Daro found by Mackay (1938). The opening would have served two purposes: (1) It would have acted as an outlet for the flow of hot gases generated within the furnace, and (2) it also facilitated continuous feeding of the furnace with fuel. The use of thin cow-dung cakes could have enabled quick attainment of a high temperature with a natural induced draught of free air coming through the front opening in the condensation chamber and the smaller holes in the perforated terracotta plate separating the condensation chamber from the furnace chamber. It is significant that the banks of furnaces at Zawar face south-west, probably to receive the natural draught during April to September. The dome-shaped superstructure of the Zawar furnace was possibly dismantled after the completion of each distillation process and rebuilt again for the next batch.

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A very few undistorted retorts were found showing the original shape of the larger types to be cylindrical, ending in a cone. The retort walls are about 1 cm thick and of clay, containing large fragments of quartzite and phyllite, both local rocks. (Mookherjee 1964) The retorts were prepared in two parts: a condenser was luted onto the open main body of the retort after it had been filled with the charge. Of the condensers, only the heads survive, usually embedded in the retorts (*fig. 18.4*).

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These heads appear to be of at least two types: a relatively large, thin-walled tulip-shaped variety and a thicker, more robust cone which was formed in a polygonal mould. Droplets of zinc-rich material are found just inside the broken, tapered ends of the heads. (Freestone *et al.*, 1985a : 231 and 243) The heads appear to have tapered into tubes which led the condensed zinc vapour to the collecting vessel underneath.

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It appears that a cylindrical reed of 1.5 cm diameter was inserted into the retort after it was charged and the funnel part was luted on it (*fig. 18.4*); this is evident from the central hole which is preserved in many retort residues. (Freestone *et al.*, 1985b: 4) The reed helped to keep the charge within the retort when it was inverted and placed in the furnace. When ther furnace was fired, the reed burnt away leaving behind a cylindrical flow channel for the zinc vapour to flow freely out of the retort.

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Petrography of the ceramic retort bodies and furnace walls shows that these were made from local (Tiri river bank) clays. (Freestone et al., 1985a) Analysis of the ceramic matrices, using Energy-Dispersive Analysis X-ray spectrometer (EDAX) attached to Cambridge S 600 Scanning Electron Microscope (SEM), revealed that these contain around fifteen weight per cent fluxes (FeO, MgO, CaO, Na₂O, K₂O). Thus the ceramics are closer to house bricks than to modern refractories in composition. (vide Table 18.1 for a complete analysis)

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The temperature to which a ceramic has been subjected may be estimated by the examination of its micro-structure in the SEM. The methodology has been worked out by Tite et al. (1982). As we move from the inside of the retort wall to the outside in direct contact with the furnace flame, we find from the BSE micrographs of the polished thin sections, that the degree of inter-connection of the particles increases and the porosity decreases (indicating temperature 1000-50°C). Finally the pores are isolated and the trapped gases expand, causing the formation of rounded vesicles (T 1100-50°C) and at higher temperatures (1200-50°C) macroscopic bloating. (Freestone et al., 1987: 22)

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Examination of the vitrification textures of the Zawar materials and comparison with refired examples shows that temperatures of C. 1200-50°C were attained during smelting. The furnace walls shows a gradient in vitrification structure from the high temperature, highly vitrified interior to the feebly vitrified exterior. The temperature gradient depends on the duration of the smelt as well as the temperature achieved in the furnace. Comparing the micromorphologies of the furnace wall at series of points with those of a series of refired examples, and using a simple heat-conduction equation, the time for which a furnace was at high temperature may be estimated. For the zinc smelting process at Zawar, duration of around 5 hr has been estimated. (Tite et al., 1982, 1985; Freestone et al., 1987)

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In order to obtain some idea of the bulk composition of the retort, a thin slice throguh the body or the wall was crushed and analysed by atomic absorption spectrometry. (AAS: Hughes et al., 1976) The data (Table 18.1) were compared with the electron probe micro-analysis (EPMA; Freestone 1982) data for the clay matrix. The higher SiO₂ content in the retort body (62.49%) compared to that in the clay matrix (53.82%; vide Table 18.1) may be explained in terms of inclusion of large quartzite grains, as earlier observed by Mookherjee (1964), which is naturally reflected in the AAS analysis. Brooke reported in 1850 that the Rana's experiments in the previous year for smelting zinc had resulted in breaking of the crucibles because, according to an old man, the quartz gangue had not been removed from the ore prior to smelting. (Brooke, 1850: 214) We feel however, that the crucible was not properly made and strengthened by inclusion of quartzite and phyllite, the local rocks, as used to be done in the earlier centuries, and hence the failure of the Rana’s experiment.

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Zawar Zinc Retort Wall: Analysis of Ceramic Materials

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| | (1) | (2) | (3) | (4) |
| SiO₂ | 62.49 | 53.82 | 64.85 | 57.96 |
| TiO₂ | 0.71 | 0.27 | 0.44 | 0.59 |
| Al₂O₃ | 13.90 | 17.49 | 13.75 | 13.97 |
| FeO | 2.48 | 4.16 | 7.49 | 15.39 |
| MnO | 0.04 | b.d | b.d. | b.d. |
| MgO | 1.44 | 1.29 | 0.93 | 1.35 |
| CaO | 0.64 | 0.34 | 0.39 | 0.70 |
| Na₂O | 0.94 | 1.81 | 0.67 | 0.55 |
| K₂O | 3.08 | 3.11 | 9.90 | 7.54 |
| P₂O₅ | n.a. | 0.50 | 0.28 | b.d. |
| Cl | n.a. | 0.16 | b.d. | b.d. |
| S | n.a. | b.d. | b.d. | b.d. |
| ZnO | 2.18 | 3.22 | 0.61 | 0.61 |
| TOTAL | 87.90 | 86.17 | 99.11 | 98.66 |

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(1) Bulk analysis of retort body (crushed slice) by atomic absorption spectrometry; (2) Clay analysis by EPMA or electron-probe micro-analysis; (3) and (4) EPMA analysis of partial melt (glass) from outer surface of retort.

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n.a. = not analysed; b.d. = below detection limits

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The glassy layer on the external surface of the used retort contained higher proportion of FeO and K₂O due to a partial melting of the ceramic which filled the pores and prevented the escape of vapours containing zinc. Some amount of zinc was retained in the retort body, a much smaller proportion appearing in the outer glassy layer (vide Table 18.1). The liquidus temperatures of the K₂O — FeO — Al₂O₃ — SiO₂ glasses on the exterior of the retort body are likely to be of the order of 1100-1125°C (Freestone and Powell, 1983) which are just sufficient for reduction of zinc oxide to the metal. (Smith, 1918; Freestone et al., 1985a : 238)

Thermodynamics of Smelting

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Before we present the composition of the residue in the ancient retorts (left after recovery of zinc by distillation) let us briefly discuss the physico-chemical route possibly adopted in the ancient zinc-smelting process at Zawar.

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In the Zawar mines, zinc occurs mainly as sphalerite, ZnS, although there are other zinc minerals such as smithsonite, ZnCO₃ etc. in minor proportions. The host rock is dolomitic and arkosic (quartz and feldspar) containing a variety of sulphide minerals such as sphalerite, galena PbS, pyrite FeS₂, pyrrhotite Fe₁₋ₓS, chalcopyrite CuFeS₂, argentite Ag₂S, silver etc. (Straczek et al., 1967)

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It is necessary to roast sphalerite before it can be smelted, because sphalerite is not decomposed in a reduction reaction. Roasting sphalerite to zinc oxide 2 ZnS + 3 O₂ → 2 ZnO + 2 SO₂ is a partly endothermic reaction requiring a temperature of the order of 900°C. As soon as the oxidation reaction sets in, sulphur in the ore supports further combustion. However, to produce a dead roast, free from sulphur, and also to decompose any zinc sulphate ZnSO₄ formed in the reaction, a higher temperature of the order of 1000°C is necessary. We detected in the retort residue not only a little sulphur (0.35%) but also some ZnSO₄, indicating that zinc ore had been roasted, but not to a dead heat, prior to reduction-smelting in the retort. (Biswas et al., 1984) Barnes (1973) described a medieval method practised in Persia in which the ore used to be roasted at temperature in excess of 1000°C to obtain pure zinc oxide sublimate, free from Ag, Fe and Pb etc., depositing on iron or ceramic frame above the fire. The zinc oxide sublimate was known as tutia in the Persian language and the metal zinc obtained from tutia as tutenaga. The high Ag, Fe and Pb contents as well as presence of sulphur and ZnSO₄ in the retort residue indicate that the Zawar process was one of simple roasting and not the Persian method.

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Zinc melts at 432°C and under one atmospheric pressure boils at 913°C, which is below the temperature required to reduce the oxide to metal in the presence of carbon. Though the initiation of reduction of zinc oxide to zinc by carbon begins at 950°C, the minimum temperature necessary for successful extraction of the metal in a retort is of the order of 1150-1200°C. The reaction ZnO + C → Zn + CO Δ H = - 57 kcal. is highly endothermic. Therefore, before the advent of modern high-pressure reduction technology or electrolytic extraction processes, zinc had to be produced inevitably as vapour around 1200°C and then condensed as liquid much below 913°C.

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Furthermore, to overcome re-oxidation of zinc vapour, the partial pressure of oxygen had to be kept very low. The obvious method of doing this was to use excess of charcoal or other suitable carbonaceous matter in the retort to maintain a strongly reducing atmosphere and excess of carbon monoxide. Temperature-pO₂ relation *Fig.* 18.5 shows that the production of metallic zinc requires extremely reducing conditions below the pO₂ required to reduce wüstite (FeO) to metallic iron. Some metallic iron has indeed been detected in the spent charge. Taking mole fractions of 0.05 FeO and 0.10 ZnO in the glassy portion of the spent charge and making simple thermodynamic assumptions, it has been found (Freestone 1985a: 238) that the FeO-Fe and ZnO-Zn reactions are displaced to the dashed curves (*fig.* 18.5). Both the FeO and ZnO contents of the glassy portion of the residue are consistent with operation of the process 1-2 log units of pO₂ below the Zn-ZnO reaction curve, within the temperature range of the interest.

The Retort Residue or the Spent Charge: Results from the British Museum Research Laboratory

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The retorts were found filled with somewhat less that 1 kg of spent charge most of which could be tipped out but some of which was fused to the walls, more towards the centre of the retorts than at the rear or front. The material was retrieved typically in the form of cindery pellets one cm³, most representative of the charge, and a fine dust fraction, which is essentially a mixture of the pellets and material that has spalled off the retort walls. This is evident from the analysis of the different components of the retort residue (Tables 18.2 and 18.3). The analysis of residue from different parts of the retort and also ash heap are provided in Table 18.4.

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X-ray diffraction and polarised light microscopy of polished thin sections (Freestone *et al.*, 1985a: 236) show the pellets to be composed primarily of carbonates (of Ca and Mg) but their state of aggregation and rigidity is imparted by a skeleton of silicate glass, in which are embedded numerous crystals of diopside. Fine grains of metallic iron were also seen. The EPMA or Electron-Probe Micro-analysis data (columns 1 and 2 in Table 18.2) show association of diopside CaMgSi₂O₆ crystal with some Fe, Mn and Zn. The glass is rich in zinc, containing up to fifteen weight per cent ZnO, and also contains higher proportions of Al, Mn, Na, K, Cl and P₂O₅.

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Fig. 18.5 Plot of Partial Oxygen Pressure versus Temperature for Selected Reactions. The Stippled Area represents the likely conditions within the zinc Reduction-Distillation Retorts.

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Zinc is markedly depleted at the closed end or top of each retort (vide Table 18.4), reflecting the movement of zinc vapour during the smelting process. In contrast lead does not appear to have been very mobile during smelting (Tables 18.3 and 18.4). Substantial amount of ZnO (2.18 %) in the retort body or wall (Tables 18.1 and 18.3) is due to the entry of zinc vapour and zinc-rich glass into the pores. The pore volume decreased in the outer and hotter surface of the retort and that is why the zinc content in the outer surface is much less (columns 3 and 4 in Table 18.1).

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Sodium in the spent charge could have been derived from gangue such as albite NaAlSi₃O₈, but high chlorine content (0.53 %, vide column 5 in Table 18.2) is less easy to explain. The fourteenth-century AD text Rasaratnasamuccaya (2.157) recommended the inclusion of salt in the zinc-smelting charge, an idea which was incorporated by William Champion in his AD 1748 British patent. Common salt would have served the zinc distillation process in two ways. Firstly, the soda vapour would help to sinter calcium and magnesium oxides and hence keep the charge within the retorts in a porous open texture. This Analysis of the Spent Charge from Zawar Zinc-Smelting Retorts

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| | (1) | (2) | (3) | (4) | (5) |
| SiO₂ | 54.36 | 50.50 | 20.82 | 40.48 | 42.48 |
| TiO₂ | b.d. | 0.28 | 0.15 | 0.27 | 0.17 |
| Al₂O₃ | 0.45 | 8.97 | 3.61 | 6.42 | 5.42 |
| FeO | 4.42 | 5.72 | 8.30 | 8.97 | 8.12 (Fe₂O₃) |
| MnO | 1.93 | 3.11 | 1.60 | 0.96 | 1.64 (MnO₂) |
| MgO | 14.10 | 0.28 | 10.16 | 5.94 | 11.16 |
| CaO | 21.85 | 6.43 | 24.54 | 15.25 | 24.54 |
| Na₂O | 0.34 | 7.13 | 0.34 | 0.80 | 1.28 |
| K₂O | b.d | 5.07 | 0.51 | 1.06 | 0.31 |
| P₂O₅ | b.d | 0.27 | n.a | n.a | 0.27 |
| Cl | b.d | 0.70 | n.a | n.a | 0.53 |
| S | b.d | 0.43 | n.a | n.a | 0.23 |
| ZnO | 3.44 | 12.45 | 0.61 | 1.22 | 2.28 |
| PbO | — | — | — | — | 1.72 |
| TOTAL | 100.89 | 101.34 | 70.65* | 81.37* | 100.15 |

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Silicate fractions: (1) diopside, from carrier material, EPMA electron-probe micro-analysis; (2) glass, from carrier material, EPMA; (3) example of carrier fragment, AAS atomic absorption spectrometry; (4) fine fraction from within retort, AAS; (5) overall analysis of the spent charge (Hegde, 1989: 92).
* Most of CaO and MgO present as carbonates, accounting for low totals.

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would minimize the chance of zinc vapour getting trapped in any one part of the retort and forcing that part to burst open. Little bit of the glassy phase helped in sintering by acting as skeletal adhesive; 0.7 per cent chlorine in the residue glass, with 7.13 per cent Na₂O (column 2 in Table 18.2) is probably close to saturation. (Freestone *et al.*, 1985a: 237) Secondly, soda vapour would react with the silica and alumina of the clay in the interior surface of the retort wall to produce a thin film of glaze. This glaze would prevent the loss of zinc vapour through the micropores in the thin clay wall of the retort. (Hegde, 1989: 92) The low level of lead oxide (1.72 %, *vide* column 5 in Table 18.2, also 18.3 and 18.4)

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