Archaeological iron

Department: Conservation and Scientific Research

Project start: October 2007
End date: September 2010

Other British Museum staff:
Quanyu Wang
Fleur Shearman

External partners:
David Watkinson, School of History and Archaeology, Cardiff University

Project funded by:
Arts and Humanities Research Council (Collaborative Doctoral Award)
The British Museum

Description:

The corrosion of archaeological iron objects after excavation can be rapid and severe. This is due to the presence of chloride ions, which accelerate corrosion and can lead to the complete destruction of objects in a short space of time. Although it is possible to stop this process using dry storage conditions, these are difficult to maintain and require significant resources. A number of methods have been developed in the past to extract chloride ions from iron objects, which would reduce the corrosion rate and therefore the need for such rigorous storage conditions. However, although the treatments were developed several decades ago, much of the evidence of their success is anecdotal and not supported by quantified experimental data. Furthermore, the risks associated with treating objects in aqueous solutions and the variable performance of the treatments when used on archaeological material mean that conservators are often reluctant to apply them to their museum collections. As a result, museums across the UK have large amounts of unstable iron objects in storage for which they have no viable treatment plan.

This AHRC-funded Collaborative Doctoral Award with Cardiff University will provide rigorous and quantitative experimental data which will be used to assess the effectiveness of chloride extraction using two different treatment solutions. It also seeks to measure the extent to which treatment can reduce the corrosion rate of objects, and will assess the risks associated with treatment and how these might be reduced or eliminated. This research will provide clear, quantified data to conservators, allowing them to make more informed choices about how to approach the conservation of archaeological iron.

Objectives:

The main objectives of the project are:

To test two chloride extraction treatments, deaerated sodium hydroxide and alkaline sulphite, on archaeological iron and provide quantitative and statistically valid information about their efficiency
To assess ways in which the corrosion rate of archaeological iron objects can be measured, and the effect of chloride extraction on corrosion rate
To assess the risks of treatment, including the formation of chemical residues, possible transformation of corrosion products and the physical effects of treatment on the information-bearing corrosion layers of objects
To provide clear information and guidelines to conservators regarding the treatment and storage of archaeological iron as a reference for best practice.

This work will be submitted as a PhD thesis in 2010, and published in a number of journals and presented at international conferences.

Publications:

Q. Wang, 2008. ‘Evaluation of methods of chloride ion concentration determination and effectiveness of desalination treatments using sodium hydroxide and alkaline sulphite solutions’, with S. Dove, F. Shearman, and M. Smirniou, The Conservator, 31 (2008), pp. 67-74.

D. Watkinson & A. Al-Zahrani, Towards quantified assessment of aqueous chloride extraction methods for archaeological iron: de-oxygenated treatment environments, The Conservator 31 (2008), pp. 75-86

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.

D. Watkinson & M.R.T Lewis (2005) The Role of beta-FeOOH in the Corrosion of Archaeological Iron. In P.B. Vandiver, J.L. Mass & A. Murray (Eds.) Materials Issues in Art and Archaeology VII. Warrendale, PA, Materials Research Society of America Symposium, pp. 103-114

Image:

An Anglo-Saxon spearhead ferrule. The outer corrosion layers are flaking off due to the formation of fresh corrosion products, bright orange in colour, after the object was excavated. If left unchecked, this process can lead to the complete disintegration of iron objects.

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