History of Chemistry in Ancient and Medieval India, Incorporating the History of Hindu Chemistry by Acharya Prafulla Chandra Ray c12.s1.2

c12.s1.2

Technique employed in the Indus Valley metallurgy included both casting and forging In the former process the metal had to be previously melted before being poured into a mould. Pure copper, when cast in a closed mould, becomes spongy due to the bubbles of free oxygen. But the addition of a very small percentage of tin or of arsenic acts as a deoxidizing agent. As most of the copper articles found at the Harappa and Mohenjo-daro sites contain these foreign metals in small amounts, their composition was quite suitable for production by casting But curiously enough the closed mould casting was never used for the production of tools in the Harappan Civilization. Most of the bronze objects were products of casting; some, however, had to be shaped and finished by hammering. This indicates that the Indus Valley metal-smiths were quite familiar with the properties of bronze-that enable it to be forged at a temperature just below redness The analysis of bronze objects reveals an advanced state of knowledge of bronze-making on the part of their makers. Copper and bronze vessels have been found together which supports the assumption that they were in simultaneous use. This was presumably due to the scarcity of tin. Copper alloyed with arsenic was also used at Mohenjo-daro and Harappa, as copper-arsenic alloy is comparable with low grade bronze in hardness. As already stated, this alloy might have resulted accidentally from the use of arsenical copper ores for the extraction of the metal. There are evidences that copper articles, such as axes, were made both by casting and hammering; the edges were usually hammered out after plain blades had been cast.