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History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s2.t11: (iii) Copper Base Alloys

History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s2.t11

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(iii) Copper Base Alloys

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*Bronze (Cu-Sn)* : Besides the invention of Cu-As alloys, which were not very common in ancient India, bronze is the next copper base alloy acclaiming to have World wide revolution in the metals technology which gave a fresh self-defending power to human race and a boost to the cultural heritage. This alloy gained its popularity owing to its golden colour and added edge hardness in as cast condition (200-250 B.H.N.). Bronze is basically a cast alloy having 0 to 20% Sn in copper, to which Pb, Zn, and Ni are also added either as tramp element or purposely to change its properties. *Fig. 46* shows the binary phase diagram of Cu-Sn alloy which indicates a maximum solid solubility of Sn as 15% at 798°C. On solidification the solubility decreases precipitating out Cu- Sn compound which imparts hardness to the alloy. The chemical analysis of the ancient bronze objects have shown a variety of compositions all falling within the above range and having a cast structure. This reveals that manufacture of designed castings and their molding technology was already fully developed in the bronze age or even earlier.

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Fig. 46. Copper rich side of the Cu-Sn alloy phase diagram

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So far as the availability of tin ore in India is concerned only very lean ore (Casseterite) is found in Chota Nagpur (M.P.) where the tribal craftsmen of Bastar area are still engaged in producing bronze metal and its castings using the age old lost wax process. The popularity and the massivity of the use of bronze alloys suggest that either the ore or metallic tin was brought from Burma or Malaya with which India had very close trade relations. The cast bronze weapons have very hard but brittle nature and these cannot be generally hot or cold worked. The edge of the cast weapon was sharpened by grinding and no heat-treatment was given because the general hardening treatment viz. solutionizing at high temperature and quenching will decrease the hardness due to the increase in the amount of retained solid solution. In order to increase the hardness, solutionizing treatment has to be followed by precipitation hardening and the metallographic studies of bronze objects have not revealed use of such knowledge.

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*Cu-Zn Alloys* : The oldest brass artifact to be found is a cup in the possession of the Iranian king Derius’ dating back to 522 to 486 B.C. This cup had a colour like Gold but from it a foul smell used to arise. Fores (1964) has written that most probably this cup was made in India. The famous Chinese traveller Hiuen Tsang has written that Indians knew the method of preparing brass from a mixture of copper and Calamine. As described by Beel,S., Hiuen Tsang has mentioned the use of brass castings and sheet metal work in India as early as 7th century A.D. and he has particularly described a brass ‘*vihar*’ (unfinished convent) at Nalanda during the period of Shiladitya (Harshvardhan) whose walls, doors and window-sill all were covered with brass. Ray (1956) has mentioned about brass technology to be imported from China but above evidences clearly indicate that brass was first made in India about 7th century B.C. if not earlier and most probably the technique of its production was carried by Hiuen Tsang to China.

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As described by Tylelcote (1976) the first brass could have been made by co-reduction of copper oxide and calamine in a closed crucible but there is higher probability of its production by dissolving zinc vapour in molten copper using a system similar to the vapour distillation, which was known to Indians for a long time *Rasa Ratna Samuccaya*. Such a system has been described also in Chinese literature as shown in Fig 47. *Fig 48* shows the copper rich part of the Cu-Zn binary

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*Fig. 47. Chinese system of production of brass using a bent tube for the conduction of zinc vapour to the crucible containing molten copper* phase diagram which indicates that upto 39 wt.% Zn forms a solid solution with Cu (-brass).

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Fig. 48. Copper rich part of the Cu-Zn phase diagram and change in mechanical properties

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The alloys in this range contain Zn from 5 to 20% and they can be cast and cold or hot worked, and these have a very low M.P. i.e. only 750°C. Brasses having higher Zn content have been rarely reported from ancient sources. These are called α- β brass and they can be generally used only in as cast condition. These alloys are characterised by their Gold like colour and capacity to take very shining polish. These could be easily gilded with Gold. On storage of alkalies or food having organic acids the food gets stale in short time and the surface of the pot tarnishes black, giving out foul smell. This metal is also subjected to a special type of corrosion on storage i.e. Dezincification. Many brass objects found during archaeological excavations have suffered from this type of corrosion and got converted into a lump of copper powder, due to the loss of Zn with time. In RRS two type of brass have been mentioned ‘Ritika’ and ‘Kakatundi’: on coming in contact with hot lemon juice (Kanjika) Ritika used to turn copper red whereas Kakatundi used to become black.

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Brasses have been the alloy of the more civilised agriculture based communical culture and it had found extensive domestic uses. Brasses containing 20 to 30% Zn have very good ductility and they can be shaped by any metal craft technique. In this period more refined technique of preparation of these alloy were developed by direct alloying as described by Kautilya in his Arthasastra and Saraswati (1936). In the Chola period this alloy has been extensively used for production of beautiful idols and icons using ‘Cire-perdue’ casting technique.

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Brass has also been used as Putti (Vajrasanghatt) or soldering alloy. Saraswati has mentioned a typical composition made by melting 8 parts of lead mixed with two parts of bell metal (bronze) and one part of brass. During the Medieval and latter period brass industry has developed many folds and even today it is in great demand, although today stainless steel utensils are replacing brass utensils at a very fast rate.

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