Investigation of a Broken Pile-Shoe from a Roman Bridge

Abstract

A Roman pile-shoe made from four iron bars had breaks in three bars. One break was a recent impact fracture. A sample containing one of the fracture surfaces was broken into large fragments with a hammer. These were investigated fractographically, metallographically, and by surface and bulk chemical analyses. The fractures were brittle and primarily intergranular. The metal was a coarse-grained phosphoric wrought iron (0.52wt% P) with very low silicon, manganese and sulphur contents, and extremely lowcarbon content (0.0033wt% C). This extremely low carbon content and coarse grain size indicate decarburisation during smithing. Furthermore, the combination of extremely low carbon and high phosphorus contents is concluded to be the most probable reason forthe impact brittleness. This could have been facilitated by a notch effect due to surface corrosion. The significance of the embrittlement and surface corrosion is considered with respect to conservation of archaeological iron objects, including similar pile-shoes.

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