Minerals and Metals in Ancient India v1.c11.s7: How Were the Chalcolithic Objects Made?

Minerals and Metals in Ancient India v1.c11.s7

Language: English

v1.c11.s7.t1 Metallurgical Work in DK Area — 1 part
v1.c11.s7.t2 Copper Ore and Lead — 9 parts
v1.c11.s7.t3 Cast Bronze — 1 part
v1.c11.s7.t4 Metallurgy of Copper Axe — 13 parts

How Were the Chalcolithic Objects Made?

v1.c11.s7.1

After getting the raw metals, the next step is the manufacture of objects from these metals. So the question automatically arises that how those chalcolithic objects were made? It is found that the common metals in the Indus Valley are copper and bronze. In the case of the copper objects, the process was a simple one i.e., to raise all the copper vessels from the sheet metal as their shapes are simple. But in the case of the bronze vessels there are several methods which could have been employed. Firstly, beating them out of sheet bronze, which however would call for constant annealing to prevent the metal from cracking, and the second process is called cire perdue. This process is exceedingly difficult for large and thin castings owing to the risk of flaws and crevices that would render the vessel made in this way non-watertight. The varying thickness of the metal of some of the bronze vessel suggests that the cire perdue process was used; on the other hand it may also be possible that this inequality arose on account of corrosion. Another probable cause for this varying thickness may be that the bronze vessels were raised from sheet metal which being unrolled, would in the first place not be of equal thickness throughout. (M-II,488)

v1.c11.s7.2

At Harappa this use of bronze in place of copper brought one of the revolutionary changes in the metal technology. Copper is ductile and could not resist the heat; bronze being less ductile and considerably stronger and less liable to damage, occupied the position of copper. In this context we may note the opinion of G. Childe, when he says that tin was added to copper not to harden it, but to lower the melting point.

v1.c11.s7.3

From the few bronze objects it is proved that the art of casting bronze was an established industry at Mohenjo-Daro. The dishes and their covers are so clearly made that it is apparent that these must have been either carefully rubbed down or turned up on a lathe. Although no lathe marks are visible, these could have been removed by subsequent honing and polishing. Even sometimes the ‘running on’ process was also employed to make handles which requires skill to perform. In this process the handle was secured to the cover not only by a rivet, but also by pouring molten metal around the base of the rivet for additional security. (MS-II, 489)

v1.c11.s7.4

In the case of blade-axes it is found that in comparison to bronze, copper has been used more. In such a case the axes were cast in open mould and then hammered to give them the requisite hardness. From Central Provinces a good number of copper implements have been found and there are ridges on the edges of several implements which suggest that these were cast in a closed mould. However copper is difficult to cast in this manner. There is a possibility that these implements corresponded to dates later than those of Mohenjo-Daro and a more advanced technique was employed in their manufacture. The absence of hammer-marks on all axes, suggest that these were carefully rubbed down after being hammered. Their straight cut sides and butts and only slightly rounded angles indicate some such treatment for these features, which were probably also present in the mould, but were destroyed in the process of hammering. The bronze blade-axes would have required very much the same treatment, though cleaner castings could be obtained with this alloy and therefore not so much hammering would be required to harden them. Their tin content served this latter purpose. (MS-II, 494)

v1.c11.s7.5

Chisels, rectangular or square in section, were frequently found and these were made from square or rectangular rods of metal, which were probably cast in this shape, as it would have entailed considerable labour to hammer them out. The intentional thinning was probably produced by hammering, for both butt and edge would have required the additional hardness that can be produced in a copper tool only by the hammer. (MS-II, 501-2)

v1.c11.s7.6

When Mackay took the charge of excavation in the DK and HR areas of Mohenjo-Daro, he found a number of ingots and castings there, which threw considerable light on the metallurgical activities at Mohenjo-Daro. But no typical kiln or furnace (which would have been used for smelting the ores) was found from there. So it may be that this work was done in the vicinity of the mines and the crude metal ingots (and not the ore) were supplied to the workers to make the refined metal. It could also have been possible that the ore was smelted in an open hearth with charcoal over a cavity in the ground into which the metal ran. As Childe was pointed out, this same process was formerly used in Japan for smelting copper-ore, except that the pits were lined with clay, lighted charcoal being first placed on the floor of the pit and then the ore and charcoal in alternate layers, the reduction being assisted with the aid of a blast'. (MCK, 450-51)

v1.c11.s7.7

According to Mackay, the metal workers of Mohenjo-Daro apparently obtained their raw material from the smelters, ready in the form of ingots; to work the metal up from such solid masses was difficult, so they usually broke up these ingots for better working; indeed, several very irregular pieces of copper and bronze have been found there. For example, one irregular piece of copper seems (from its appearance) to have been broken from a larger mass of metal. It is quite possible that such pieces of metal were sold by weight. This is the usual custom today in those parts of India where metal working is more or less untouched by outside influences. The fairly close proximity of the two kilns used for metal working also substantiate this theory. (MCK, 452-53)

Metallurgical Work in DK Area

v1.c11.s7.t1.1

The number of copper melts (in House VI, block-2 of G. Section) in the DK area indicates that a part at least of this building was devoted to metallurgical work, perhaps in connection with the large building to the north and west of it, which appears to have been a palace: but no kiln of contemporary date was found in the close vicinity of these melts. That the metal was sometimes smelted at Mohenjo-Daro seems to be proved by the find of copper ore together with a small piece of lead in a brick lined pit in the middle of a room (51) adjacent to the palace. The presence of the lead is particularly interesting, as it may have been used to assist in smelting the ore. (MCK, 41)

Copper Ore and Lead

v1.c11.s7.t2.1

Such type of copper and lead ore was also found from Room 51 of DK area, section G. There the sides of the wall were one thickness of brick and its floor was neatly paved. In it was found a quantity of copper ore in small pieces, together with a little piece of lead. The excavator thought that it is quite possible that a metalsmith worked here with a furnace in the close vicinity, which has entirely disappeared. (MCK, 54)

v1.c11.s7.t2.2

A specimen of lead was found from DK area which contained no silver at all. A large circular piece of lead (L 394), 6.7 inch diameter 1.4 inch thick with bored holes was found in chamber 88 of L area. This net-sinker was probably cast in sand. (MS-II, 464)

v1.c11.s7.t2.3

Samples of copper ore and the piece of lead were found from DK area in Block I, house VI, room 51 and 52. The ore, a red oxide of copper with very little foreign material gave on analysis the following results:

v1.c11.s7.t2.4

v1.c11.s7.t2.5

Lead : traces Insoluble matter : 4.80 per cent

v1.c11.s7.t2.6

The sulphur was present as copper sulphide. The insoluble matter consisted of silica and iron oxide. The ore is thus a copper oxide, no doubt a surface ore at the outcrop of pyrites, as it contains some sulphide. This would be easily smelted, and is particularly interesting as it contains both the key elements, nickel and arsenic, found in most of the earliest copper samples which have been examined. Tin and other metals were absent.

v1.c11.s7.t2.7

The lead sample was in the metallic form, coated with a thin layer of lead sulphate which was removed before analysis. The composition proved to be:

v1.c11.s7.t2.8

Lead : 99.70 per cent Copper : 0.15 per cent Silver : traces

v1.c11.s7.t2.9

This was a very pure specimen and must have been smelted from a pure ore. It could not have been obtained accidentally. (MCK, 599-600)

Cast Bronze

v1.c11.s7.t3.1

Few bronze specimens (figures mainly) have been unearthed from Mohenjo-Daro which indicates that these were cast bronze. Among these, the most noteworthy was the dancing girl from Mohenjo-Daro and the other bronze figures. Even the animal figurines were also of cast bronze. In spite of corrosion it is clear that these were carefully modelled. The excavator was of the opinion that the method employed in casting most of these animal figures was probably the cire perdue process, though a few of them were cast direct in a mould. (MCK, 283-84; MS-I, 345).

Metallurgy of Copper Axe

v1.c11.s7.t4.1

In terms of Brinell hardness, the axe of Mohenjo-Daro was 85 close to the cutting edge (the figure is much above that of mild steel), decreasing to 63 towards the middle of the blade. H.C.H. Carpenter of the Royal School of Mines suggested that it is possible that the ancient axes do not retain their original harness; it is also possible that such an axe was once harder.

v1.c11.s7.t4.2

Mackay concludes from the examination of the rejected castings that the axes of Mohenjo-Daro were cast very much in the same shape as the finished specimen that we have found, even to the curved cutting edge; these would therefore have required not only hammering to shape them, but also probably a great deal of hardening. (MCK, 594-95)

v1.c11.s7.t4.3

Silver was used more extensively than gold at Mohenjo-Daro. It was not a rare metal as in ancient Egypt or Sumer. Pending further analysis of the silver objects made by the Indus Valley people, it is not possible to say anything about the sources of the metal and the ores which were used to make it. Lucas has pointed out that silver is found in nature in three conditions — metallic, non-metallic in ores and combined with gold. Metallic silver is not likely to have been the source of the silver used at Mohenjo-Daro because it is rare now and was still rarer in the ancient times. Silver is commonly found in the form of sulphide or chloride, mined with other metals, i.e., lead, zinc or copper, but a certain amount of knowledge is necessary to reduce it to its metallic state. Silver frequently occurs in small quantities in connection with lead as argentiferous galena. Lead was by no means rare at Mohenjo-Daro, and it may be that silver was extracted from the ore of lead.

v1.c11.s7.t4.4

The main silver objects which have been found from here are silver vase which contained gold and silver jewellery, ear-rings, rings, fillets etc.

v1.c11.s7.t4.5

M.A. Hamid, Assistant Archaeological Chemist, reported as follows on the fragments of silver. One of these assayed:

v1.c11.s7.t4.6

v1.c11.s7.t4.7

(MS-II, 523-24)

v1.c11.s7.t4.8

Hamid attributed the presence of copper in this sample to deliberate alloying, whereas Sana Ullah suggested that the parent ore contained Ag, Pb as well as Cu. One of a number of pieces of scrap silver found in a copper vase together with a number of other vessels and tools has the following composition: