A marvel of medieval Indian metallurgy: Thanjavur’s forge-welded iron cannon

Abstract

Over the last 40 years, there has been a discernible increase in the number of scholars who have focused their research on early industrial organizations, a fi eld of study that has come to be known as Archaeotechnology. Archaeologists have conducted fi eldwork geared to the study of ancient technologies in a cultural context and have drawn on the laboratory analyses developed by materials scientists as one portion of their interpretive program. Papers for this department are solicited and/or reviewed by Michael Notis, a professor and director of the Archaeometallurgy Laboratory (www.Lehigh.edu/~inarcmet) at Lehigh University.In this article, metallurgical aspects of a 17th century forge-welded iron cannon at Thanjavur are addressed, including an analysis of manufacturing methodology based on careful observation of its constructional details. Microstructural examination of iron from the cannon reveals that the iron was extracted from ore by the direct process. Thus, the cannon was fabricated by forge welding and not by casting. Electrochemical polarization studies indicate that the corrosion rate of the cannon iron can be compared to that of 0.05% carbon mild steel under complete immersion conditions. However, the atmospheric corrosion resistance of the cannon is far superior to that of modern steel and can be attributed to the formation of an adherent protective passive fi lm. It is concluded that this cannon constitutes a marvel of medieval Indian metallurgical skill.

Microstructural studies of composite Mughal period cannons of Daulatabad Fort, India, by electron backscattered diffraction and scanning electron microscopy

“…The hot metal pieces, with inclusions and particles of liquid slag, were then forge welded. [24][25][26] This was the manufacturing practice for iron cannons, [18] and appears to be the case for the bimetallic cannons as well. The signatures of plastic deformation in the copper barrel ( Figure 4) and the iron sleeve ( Figure 6) clearly indicate forging.…”

“…Forge-weld bloomery iron cannon was reported in southern India. [18] Ancient Indian iron-steel technology did not involve liquid metal processing. [6] The bloomery iron was formed by direct reduction of appropriate iron ore.…”

“…The actual manufacturing practice, grossly rationalized as forge weld, raises many questions. The famous Delhi iron pillar [5] or the Thanjavur's massive iron cannon [18] were made by forge welding, where hot pieces of bloomery iron were placed together and were forge welded aided by external heating. The manufacturing of the bimetallic cannons appears to involve a similar process.…”

“…Metallographic examination were carried out by preparing chip samples through normal standard procedure [17] using submicron colloidal silica polishing. These samples were further subjected to detailed characterization using X-ray and electron diffraction [18] and analytical microscopy. [19] X-ray measurements were made on a Brukers D8-Discover™ system with Vantec-500™ area detector.…”

“…[20,21] EBSD data above 0.1 CI (confidence index) were used for subsequent analysis. CI or confidence index is a statistical measure [18] of accuracy in automated indexing: CI >0.1 represents above 95% accuracy. An EDX™ silicon drift detector, on the same SEM, was used for the analytical microscopy: namely, energy dispersive X-ray spectroscopy.…”