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Evidence based management strategies for Heritage iron

Cardiff University Heritage Iron Preservation Research Group

Professor David Watkinson

An international problem

Most museums in Britain contain vast collections of archaeological and heritage iron objects that are rapidly rusting away due to an absence of evidence-based management strategies for their display and storage. Experimental research at Cardiff University has led to the development of clear guidance for devising, implementing and managing preservation strategies for iron. These are now plied in major museums and to preserve Brunel’s steamship ss Great Britain.

Developing corrosion control

Corrosion of marine and buried archaeological iron is supported by chloride derived from these environments. Chloride within objects contributes to their post-excavation corrosion, aided by oxygen and atmospheric moisture as humidity. Either desiccation of the surrounding environment or chloride removal could prevent corrosion. However, the level of dryness required to stop the damaging activity of chloride, known as the no-corrosion’ relative humidity value, and the effectiveness of chloride extraction methods were unknown.  Cardiff University research addressed both these unknowns with the aid of grants from HLF, AHRC and the AHRC/EPSRC Science and Heritage Programme. Experiment determined how hygroscopic chloride-bearing iron corrosion products (akaganéite and ferrous chloride) can corrode iron at humidity below 25%. It was established that a 12% relative humidity was required to prevent corrosion by akaganéite and that corrosion is very slow until relative humidity rises above 30% (Watkinson and Lewis 2005). The effectiveness of treatments to remove chloride and their impact on corrosion rate of objects was also determined (Rimmer et al. 2012; Watkinson, D. and Rimmer, M.  2013).

Desiccation plant within dry dock

A huge desiccation plant controls humidity around the hull of the ss Great Britain

Outcomes of the research

These studies provide managers with options, either to prevent corrosion at the cost of high monetary and supervisory input, or adopt a less stringent approach and accept a small rate of change but ultimately a finite lifespan for their iron objects. For archaeological iron complete desiccation may be sought in large stores such as those managed by the Museum of London. Elsewhere, management have adopted a 20% relative humidity target to desiccate the ss Great Britain and the rationale of corrosion control is offered in guidelines prepared by English Heritage.

Recognition of the research and its concepts is reflected in the 2006 award of the Gulbenkian Museum Prize to ss Great Britain for evidence-based innovative conservation;  chief judge Professor Lord Robert Winston commented ‘… a truly ground breaking piece of conservation…’ making the ship ‘…accessible and highly engaging for people of all ages. In 2010 Professor David Watkinson was awarded the Plowden Medal for ‘innovative research and services to conservation’ as a direct result of this work on iron corrosion and conservation (see page 17 of the following report of the Royal Warrant Holders Association PDF).