This study examined the impact of relative humidity (RH) on the corrosion rate of 129 archaeological iron nails from two sites. Oxygen consumption of individual nails in controlled RH was used as a corrosion rate proxy to deliver quantitative data on corrosion rate as a function of RH. This was negligible at 20% RH, slow up to 40% RH for both sites, and increased rapidly at 60% RH for Roman nails from Caerleon (Wales) and at 70% RH for medieval nails from Billingsgate (London). The nails were digested and their chloride content was determined and related to their oxygen consumption at specific RH values. While a generic pattern of corrosion as a function of chloride was identified, for any single concentration of chloride corrosion rate was not predictable. Desiccation is in common use to control post-excavation corrosion of archaeological iron; quantifying how differing levels of desiccation changed corrosion rate provided a scaled tool for identifying corrosion risk, estimating object longevity, and calculating cost benefit for storage options.

Controlling the corrosion of historical and archaeological ferrous metal objects presents a significant challenge to conservators. Chloride is a major corrosion accelerator in coastal areas for historic ferrous metal structures and for the many chloride-containing archaeological objects within museums. Corrosion reactions involve the formation of akaganéite (βFeOOH) which incorporates chloride within its crystal structure and adsorbs it onto its surface. The mobility of the surface-adsorbed chloride in aqueous systems and atmospheric moisture means βFeOOH can itself cause iron to corrode. The extraction of chloride from βFeOOH by aqueous Soxhlet hot wash and aqueous room temperature washing is measured. The impact of this washing on the ability of βFeOOH to corrode iron is quantitatively investigated by determining the oxygen consumption of unwashed, Soxhlet-washed and room temperature-washed samples of βFeOOH mixed with iron powder and exposed to 80 % relative humidity. This acts as a proxy measurement for the corrosion rate of iron. The results are discussed relative to climatic factors for outdoor heritage objects and the treatment of archaeological iron in museums. Delivering better understanding of the properties of βFeOOH supports the development of evidence-based treatments and management procedures in heritage conservation.

The conservation of heritage wrought iron mostly relies on corrosion prevention by preparation of surfaces and application of protective coatings. In contrast to industrial and engineering treatment of modern steel, conservation practice is not regulated by accepted national and international standards or underpinned by empirical evidence. This paper presents the results of oxygen consumption rate testing (as proxy corrosion rate) of historic wrought iron samples prepared by five commonly applied surface preparation methods and subjected to high humidity environments, with outcomes assessed by use of international standards employed in industrial contexts. Results indicate that choice of surface preparation method has a direct influence on corrosion rate of the uncoated wrought iron, which impacts on performance of the protective coatings that may ultimately determine survival or loss of our rich wrought iron heritage. By implication, more extensive empirical evidence is required to underpin and develop heritage standards for treatment of wrought iron which encompass specifics of the historic material, heritage context and the ethics of conservation practice. The introduction of such standards is called for in order to bring treatment of historic ironwork in line with highly regulated engineering and industrial practices.

Since the early 1970s, archaeologists, conservators and curators have turned to published guidance for methods of safeguarding archaeological metals in the short and long term. Much of this has appeared in editions of the ubiquitous handbook First Aid for Finds, the most recent of which dates to 1998. A central message across all guidance has been to dry metals post-excavation and prevent their corrosion during storage by desiccating the environment around them. This has been proven crucial for iron, with reports of catastrophic loss of collections arising from inappropriate storage environments.There has long been agreement that an effective, low cost option for desiccation is the packing of artefacts in boxes with a desiccant. Though advice on the detail of how to achieve this has evolved over time, what has not changed is the lack of quantified evidence for the methods proposed.Informed by a survey of current practice in the sector, this study investigated quantitatively the many variables involved in creating desiccated microclimates for storage of heritage metals. This paper presents the results of an evaluation of the airtightness of a range of commercially available boxes, the optimum mass of desiccant, the effect of external environments, and preliminary data on the efficiency and risk of drying damp objects with silica gel. A second paper will report the effect of stacking boxes on their airtightness and present data on the accuracy and long-term reliability of humidity indicator cards.The guidance arising from laboratory testing of the hardware and methods for desiccated storage is designed to be achievable and adaptable. Combining these findings with corrosion rate data for iron artefacts allows the escalation of risk to objects from decisions in the storage process to be calculated.Together, these papers offer an evidence-based and practice-focused update to First Aid for Finds.

Since its introduction to the field of heritage science, oxygen consumption has found multiple applications in the assessment of artefact stability and the examination of conservation treatment efficiencies. Early papers identified various factors affecting oxygen measurements and produced preliminary data on their effects. This paper follows on from the early research by other workers and presents guidance on the application of oxygen consumption as a proxy corrosion rate measurement based on a decade of experience at Cardiff University. It examines the rationale for key aspects of the Cardiff protocols for oxygen consumption and presents data to underpin these. Finally, it provides instructional guidance for heritage scientists adopting oxygen consumption for their research applications.