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History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s3: Metal Casting Technology

History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s3

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English

Metal Casting Technology

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Generally it is believed that the first metal artifact must have been produced by cold working i.e. forging, virgin metal nuggets found in nature but the early knowledge of the art of metal casting also cannot be denied. The description of this craft in the *Ṛgveda* and *Yajurveda* is a clear indication of its knowledge in that period. It is definite that the idea of shaping metal by handling it in molten condition must have originated from the observation of the flow characteristics of molten Gold, Silver or Copper and their capacity to take the shape of the channel on resolidification. Even some of the earlier-square coins have revealed the cast dendritic structure confirming their manufacture by casting technique. These coins belonging to 4th to 7th century B.C. were most probably made by first casting molten metal in open channels and then hammering it flat before cutting the strip into square sections of uniform size and weight.

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The exact weight was obtained by cutting the metal from the corner and finally the coin was punch marked. *Caraka Saṃhitā* has mentioned the process of making sculptures and idols of Au, Ag, Cu, Sn etc. by the ‘Cire perdue’ process.

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From the study of the bronze statue of the dancing girl it is obvious that metal casting technology had reached its perfection during this period (2500 to 3000 B.C.). The idea of making metal statues and idols probably arose from the manufacture of terra-cotta toys and the method adopted for their mass manufacture using a baked clay die, made into two halves. Archaeological evidences so far collected have indicated that after learning the process of melting metal into crucibles and its casting into earthen channels man started using preshaped molds of clay or cavities made in sand stone or soap stone pieces, which gave rise to the first concept of die-casting. Later-on man must have learnt to make clay or wooden pattern and to produce a replica of the same by making consumable molds in sand and clay mixtures leading to the development of the modern molding method and foundry technology.

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‘Madhuchib-bhist’ or ‘Cire Perdue’ process is a refined technique where pattern of the desired shape is made from bee’s wax and then a mold is prepared by applying coatings of prepared clay slurry on it. Later-on when the refractory clay slurry has dried the wax is removed by baking the refractory shell. Thus only one casting could be made from one wax pattern. Before casting the molten metal the refractory shell was generally embedded in a box filled with the sand clay mixture for further support to the refractory wall. After pouring the metal and its solidification the mold was carefully broken to take out the metal casting which was given finishing touch by filing, chiselling, engraving and polishing etc. Saraswati has given a vivid description of this ancient technology and it has been refined during Calukya dynasty to produce innumerable brass and bronze pieces of art and idols of Gods and Goddesses which have kept the World wondering about the rich technological heritage of India as reviewed and described by Pal. (1983) Shama Shastry (vide *Arthaśāstra* of Kautilya) has referred in two books on this ancient art of metal casting viz. *Manasollāsa* and *Abhilasita Cintāmaṇi* of 12th century A.D., which give a detailed account of the method of preparation of wax pattern and slurry coating, provision of sprue, riser and runner etc. The composition of the clay slurry coating as given by them consisted of clay mixed with charred rice husk and sodium chloride all mixed in a definite proportion and finely ground. Recently Kumar and Saxena et al. have prepared a detailed report on the lost-wax technique currently being used by the tribals of Bastar for producing various brass and bronze objects. *Fig. 49* shows a bronze chandelier cast by this process in one piece where even the chain links were also cast along with the other parts in one mold. Indian and foreign museums are full of such intricate castings produced by the Indian craftsmen in the past. *Fig. 50* shows the five steps of the traditional process of making brass and bronze icons by this process as practiced even today in South India.

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*Fig. 51* shows the famous Icon of Lord Nataraja made by this process. Today this process has been adopted by the modern foundries for the mass production of precision castings of brass, bronze and even steel.

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Beel, S., has quoted from the travel documents of Hiuen Tsang, that at Varanasi a copper icon of Maheshwar Deo having a height of 3000 mm was found. Kulkarni (1989) has written that from Sangharam of Bhagalpur distt. a 2286 mm high copper statue (11th century A.D.) has been obtained which was made by casting in parts and then rivetting them together with the help of iron strips. Joshi (1970) has described the use of this technology two decades back for making a 1820 mm high statue of Lord Hanuman (weight 350 Kg.) by casting it in parts and then brazing them together. The study of the ancient brass and bronze icons have revealed that ancient craftsmen were capable of producing solid as well as hollow castings, which were probably made by pouring out the remaining liquid metal after partial solidification has taken place near the mold wall.

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Fig. 49. Photograph of a brass chandelier of 8 century A.D. showing the chain cast in one piece

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Another example of the skill of the ancient Indian is the process of casting of bronze coins. For producing these cast coin, as described by Sharma and Bhardwaj (1987) and Mukherjee and Lee (1988), the impression of a number of coins was made on Terracotta disks as shown in Fig. 52 with a central sprue connected to all the molds by small runners. The cope and drag of these disks were stacked to form a ‘Jhari’ type mold. This stack was carefully packed from all the sides in a sand molding box and then molten metal was poured through the central sprue. As the 1. Wax model of the deity is prepared. The core is made of clay and sand.
2. The wax model is covered with clay.
3. Preparing the mould with the heating wick on.
4. The next stage is heating the mould to drain the wax and baking the mould in sand.
5. Pouring molten metal into the wax emptied mould. After cooling the mould is broken.
6. And the result comes to light with the master's touch.

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Fig. 50. Five steps of the traditional method of casting brass and bronze idols in South India using ‘cire perdue’ process

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molten metal entered the mold the air from these cavities escaped through suitably provided air vents. Thus a large number of bronze coins of consistent composition, size and weight were cast in one mold and taken out after solidification. The risers and runners were broken away and the finished coins were sent to the mint. This technique of casting large number of small castings in one single mold is also being used with certain modification even today.

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During the Mughal period the casting technology was much developed and it was used for casting huge Guns like Malikai Maidan. The mould for this heavy and beautifully designed gun was prepared below the ground level by using moulding sand mixture and sweep moulding process. A solid core prepared from rice paddy, sand and clay mixture or sand and molasses mixture was first dried and baked and then fixed in the centre of the mould to make the bore hole of the gun. In the absence of heavy crane and machinery for handling such large amount of molten bronze the melting furnace was constructed at a higher level. The molten metal was melted in a large shaft or crucible prepared insitu, and when it had reached the desired composition and fluidity the molten metal was poured into the mould by preparing a channel connecting the crucible to the mould and piercing a hole in the crucible wall. After the casting had solidified over a period of few days the mold was broken to take out the casting and finish it by cleaning, chiselling and polishing. In a like manner huge cannon balls were cast either in non-ferrous metals or cast iron. This industry has also faced a set back during the British Rule.

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Fig. 51. A Bronze casting of Lord Natarāja made by ‘cire perdue’ process

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