A Synchrotron‐Based Study of the Mary Rose Iron Cannonballs

Abstract

Post‐excavation iron corrosion may be accelerated by the presence of Cl−, leading to conservation methods designed to remove Cl. This study exploits a unique opportunity to assess 35 years of conservation applied to cast‐iron cannon shot excavated from the Mary Rose. A combination of synchrotron X‐ray powder diffraction (SXPD), absorption spectroscopy (XAS), and fluorescence (XRF) mapping have been used to characterise the impact of conservation on the crystalline corrosion products, chlorine distribution, and speciation. The chlorinated phase akaganeite, β‐FeO(OH,Cl), was found on shot washed in corrosion inhibitor Hostacor IT with or without an additional reduction stage. No chlorinated phases were observed on the surface of shot stored in sodium sesquicarbonate (Na2CO3/NaHCO3); however, hibbingite, β‐Fe2(OH)3Cl, was present in metal pores. It is proposed that surface β‐FeO(OH,Cl) formed in the early stages of active conservation owing to oxidation of β‐Fe2(OH)3Cl at near‐neutral pH.

Influence of microstructure on the corrosion of archaeological iron observed using 3D synchrotron micro-tomography

“…[57,59,60] Loss of Clsur during aqueous washing may be observed by a decrease in the pre-edge feature in the akaganeite Cl k-edge XANES. [14] This is illustrated in Fig. 8-a for a lab-synthesised akaganeite standard immediately after synthesis (red), where a pre-edge can be seen, and following 1 month of washing in dH2O (blue), where the pre-edge has been lost.…”

“…Within the akaganeite crystal, two chlorine sites are present: partially bound chloride adsorbed to the surface of the crystal, Clsur, and structurally bound chloride within the tunnel structure of the crystal, Clstr.. [59] Earlier work has shown that these two sites may be differentiated by Xray Absorption Spectroscopy, [14,59] which is particularly useful for studies of iron postconservation, as only one sitethe mobile Clsuris thought to be removed during a washing based conservation treatment. [57,59,60] Loss of Clsur during aqueous washing may be observed by a decrease in the pre-edge feature in the akaganeite Cl k-edge XANES.…”

“…8-a for a lab-synthesised akaganeite standard immediately after synthesis (red), where a pre-edge can be seen, and following 1 month of washing in dH2O (blue), where the pre-edge has been lost. In the XANES spectrum of the akaganeite from shot 81A6143, plotted with a washed akaganeite from another Mary Rose shot, see [14]. Insets show zoomin of pre-edge feature.…”

“…[1] To address this, conservation treatments for archaeological iron focus on removal chlorine from the artefact. [12,13] A recent investigation into the impact of conservation on cast iron corrosion [14] noted severe structural degradation to cast iron cannon shot from the Mary Rose shipwreck (c. 1545) following a washing-based conservation treatment. Using a destructive sampling methodology, [15] it was concluded that this mechanical damage occurred during object storage and display, arising from corrosion around cracks and casting voids inside the artefact.…”

“…Using a destructive sampling methodology, [15] it was concluded that this mechanical damage occurred during object storage and display, arising from corrosion around cracks and casting voids inside the artefact. [14,15] Previous studies on the corrosion J o u r n a l P r e -p r o o f of iron in indoor atmospheric conditions have shown that in the presence of Clthe iron oxyhydroxide akaganeite, β-FeO(OH, Cl), may form around cracks. [16][17][18] However, as these investigations focused on the interaction of metal with external environments, the cracks studied were restricted to the corrosion layer [16,17] or surrounding medium, [19] rather than inside the metal object itself.…”