Archaeological iron

Project leader: Quanyu Wang

Department: Conservation, Documentation and Science

Project start: 23/08/2004
End date: 21/07/2006

Other British Museum staff: Fleur Shearman

Project funded by: The British Museum

Description:

Archaeological iron objects have proved to be problematic as a result of inherent instability. Corrosion often causes surface flaking and even makes an object disintegrate (Figs. 2-3). No single conservation treatment works for every iron object. 15% has been reported to be the critical RH (relative humidity) level above which further deterioration can occur if chloride ions are present on the object. However, keeping iron in storage at such a low level of RH may be an expensive option and practicably difficult to achieve.

The aim of the project was to identify the main causes of deterioration of archaeological iron in the Museum collection and to identify a suitable storage environment. The project was carried out both on material in the collections of the British Museum and on iron coupons by investigation and experiments.

The investigation of deterioration of archaeological iron objects in the Museum was carried out on archaeological iron from British Roman and Anglo-Saxon sites. Both metal (if possible) and surface corrosion were studied using a variety of analytical techniques, e.g. microscopy, scanning electron microscope with energy-dispersive X-ray analysis (SEM/EDX), Raman spectroscopy, X-ray diffraction (XRD) and X-ray fluorescence spectroscopy (XRF). Archaeological contexts, previous conservation treatments and storage environments were also investigated to help understand the mechanism of deterioration.

In the experiments Industrial powdery FeCl₂^.4H₂O, mixtures of iron coupons with FeCl₂^.4H₂O, and archaeological nails were exposed to variable RH levels, including 75, 54, 44, 33 and 23%, at room temperature for over a year. Changes in these tested materials were examined using the analytical techniques mentioned above and ion chromatography.

Objectives:

• Survey of conditions of objects from different sites to look into the correlation between object condition and archaeological context, e.g. burial environment.
• Investigation of the effects of microstructures and manufacture techniques on the degree and mechanism of degradation of the objects.
• Investigation of the effects of chloride ions on the degradation of the objects.
• Experimental tests on the effects of RH on the corrosion of iron.
• Comparison of the effectiveness of past desalination methods by comparing the current condition of the objects.
• Comparison of the desalination effectiveness using sodium hydroxide and alkaline sulphite by monitoring changes in the treated objects in storage in relation to the environmental condition, e.g. RH levels, of the storage
• Assessment of the level of risk to artefacts and evidence preserved in the corrosion layers incurred by wet treatment approaches to the stabilisation of archaeological iron.
• Recommendation of an acceptable RH level for long-term preservation of the archaeological iron.
• Presentation and publication of the research results at academic conferences and publications.

Publications:

Q. Wang, ‘An investigation of deterioration of archaeological iron’, Studies in Conservation, 52, (2007), pp. 125-134.

Q. Wang, 'Effect of relative humidity on the corrosion of iron: an experimental view', The British Museum Technical Research Bulletin, 1, (2007), pp. 65-73.

Images (from top):

• Investigative cleaning using a microscope, with xray
• Photomicrograph of a Roman nail showing pitting on the cross section caused by Cl containing corrosion