Megalithic and surviving binary high-tin bronze traditions in southern India: Tracing binary bronze usage to Harappan times

This paper attempts to trace the development of an unusual and skilled class of alloys, of binary high-tin bronze (i.e. alloys of only copper with a higher percentage of tin), which are found from surprisingly early contexts from Indian antiquity. In particular, the deliberate use was made of binary beta bronze with around 22–24 % tin, specifically exploiting the properties of higher hot-forgability of bronze of this composition due to the formation of the high temperature beta intermetallic compound phase of 22.9 % tin. Quenching resulted in the retention of the beta phase, yielding a musical alloy with golden lustre and improved tensile strength as compared to the as-cast state. Examples of hot forged and quenched high-tin beta bronzes studied by the author from the South Indian Iron Age and megalithic cultures from Tamil Nadu and Maharashtra and Gandharan Grave Culture of Taxila are summarised here ranking amongst the earliest and most finely wrought such finds. There are technological and morphological similarities to surviving high-tin bronze crafts practices documented by the author in Kerala since 1990. Since the 1990’s she has also documented the making of high-tin delta bronze mirrors at Aranmula with a composition closer to the pure delta phase of 32.6 % tin, which instead exploited the specular properties this alloy while managing its brittleness. Although it is difficult to speculate about origins, a long standing practice of using binary tin–bronzes (i.e. only copper–tin alloys) can be detected going back to Harappan bronzes which also seem to be predominantly binary bronzes with not much lead added to them. Though most of these seem to be low-tin bronze, the presence of a couple with higher tin of about 20 % is also notable in terms of the above discussion