David Watkinson

Desiccated Storage of Chloride Contaminated Archaeological Iron Objects

Abstract Desiccation has long been used to store chloride-contaminated archaeological iron but there are no precise guidelines on the degree of desiccation required to prevent corrosion occurring. Akaganéite (β-FeOOH), ferrous chloride tetrahydrate (FeCl2·4H2O) and ferrous chloride dihydrate (FeCl2·2H2O) have been recorded on archaeological iron. Iron corrodes in the presence of FeCl2·4H2O and β-FeOOH but not in the presence of FeCl2·2H2O. The rate of desiccation of FeCl2·4H2O at various levels of relative humidity (RH) was determined by experiment and found to be an exponential relationship. The point at which FeCl2·2H2O first becomes a stable hydrate was established. Rates of corrosion for iron mixed with FeCl2·4H2O and with β-FeOOH were examined for a range of RH. The hygroscopicity of β-FeOOH and the RH at which it ceases to cause iron to corrode were established. Corrosion of iron in contact with FeCl2·4H2O and β-FeOOH speeds up as RH rises and is appreciable at 25% RH and above. On the basis of these results, recommendations are made that 12% should be the maximum allowable RH for long-term storage of archaeological iron from chloride-bearing soils. Low RH requirements raise problems for long-term monitoring of storage microclimates.

Indexing reliability for condition survey data

Abstract This paper discusses the application of existing methods for measuring reliability to collection condition survey data. The essential requirement of reliability is understood by conservators but has never been quantified. The relatively recent introduction of computer methods that calculate reliability is mentioned as a significant step towards introducing the practice of ensuring reliability during pilot studies. The paper discusses different methods of quantification, and their qualities, as well as the aims of assessing reliability for condition surveys. The most satisfactory methods, and choosing an appropriate level of reliability for conservation applications are suggested. However, no single number is offered as a standard. It is recommended that more than one index should be used. The paper also describes techniques to use indices to determine causes of disagreement, so reliability can be increased.

The efficiency of chloride extraction from archaeological iron objects using deoxygenated alkaline solutions

Abstract Chloride-contaminated archaeological iron is unstable and problematic to store and display within museum collections. Reducing its chloride ion content using aqueous desalination followed by storage in controlled relative humidity offers one treatment option. This study reports a quantitative assessment of chloride extraction by aqueous deoxygenated alkaline desalination solutions from 120 individual archaeological iron nails. The three treatment methods comprised alkaline sulphite solution (0.1 M NaOH/0.05 M Na2SO3) at room temperature and at 60°C and sodium hydroxide solution (0.1 M) deoxygenated using a nitrogen gas positive pressure system at room temperature. Chloride extraction was monitored using a specific ion meter. The nails were digested after treatment to measure their residual chloride content. A wide range of extraction patterns emerged, with the majority of individual treatments extracting 60–99% of the chloride present. Residual chloride levels for 87% of the objects fell below 1000 ppm and 42% were below 200 ppm. Although no treatment extracted 100% of the chloride in the object, alkaline desalination produced very significant reductions in chloride content. The impact of this on future corrosion of the objects is discussed. This quantitative and statistically viable assessment of deoxygenated desalination treatments provides evidence to support their use in conservation practice, which will impact on procedures for the preservation and management of archaeological heritage.

Towards quantified assessment of aqueous chloride extraction methods for archaeological iron: De-oxygenated treatment environments

Abstract The advantages and disadvantages of treating objects by aqueous washing methods are discussed. Chloride bearing corrosion products on iron are related to aqueous chloride extraction treatments. Alkaline sulphite; de‐aerated sodium hydroxide; de‐aerated Soxhlet; de‐aerated water; sodium sulphite and aerated water washing methods are applied to archaeological iron, measured and quantitatively compared. De‐aerated alkali was the best and most predictable chloride extractor. This is attributed to prevention of corrosion during treatment and the action of hydroxyl as a counter ion.

Preservation of metallic cultural heritage

The ethics and rationale that underpin the preservation of cultural metals are outlined and their influence on the conservation process is examined via a review of treatment strategies illustrated by example. Citing current and formative research into treatments illustrates the role of corrosion science in conservation. Topics that merit further study and research are highlighted wherever appropriate.

ss Great Britain: Teamwork as a platform for innovative conservation

Abstract Bruneis 1843 ss Great Britain was a technological milestone of world importance. It now rests in its original dry dock in Bristol. Research established the significance of the ship; identified its inherent instability and reviewed conservation options to support a successful 8.5 million pound Heritage Lottery Fund bid. The complex preservation project involved innovative use of desiccation to preserve the hull, along with a large scale conservation programme for the fabric of the ship and dockyard structures. The input of architects, engineers, conservators, corrosion scientists, historians and many other specialists was managed directly by the executive director and a qualified project manger who maintained timetables and coordination. Research into the effect of relative humidity on the corrosion of chloride infested iron provided data for use in a design that changed the dry dock into a climatically controlled envelope around the unstable hull.

The use of neutron analysis techniques for detecting the concentration and distribution of chloride ions in archaeological iron

Chloride (Cl) ions diffuse into iron objects during burial and drive corrosion after excavation. Located under corrosion layers, Cl is inaccessible to many analytical techniques. Neutron analysis offers non-destructive avenues for determining Cl content and distribution in objects. A pilot study used prompt gamma activation analysis (PGAA) and prompt gamma activation imaging (PGAI) to analyse the bulk concentration and longitudinal distribution of Cl in archaeological iron objects. This correlated with the object corrosion rate measured by oxygen consumption, and compared well with Cl measurement using a specific ion meter. High-Cl areas were linked with visible damage to the corrosion layers and attack of the iron core. Neutron techniques have significant advantages in the analysis of archaeological metals, including penetration depth and low detection limits.

The impact of chloride desalination on the corrosion rate of archaeological iron

Abstract Although desalination of archaeological iron reduces its chloride concentration and enhances object stability, the reduction in corrosion rate that this produces has never been quantified. This study measures post-treatment corrosion rates in accelerated corrosion environments to identify the impact of removing chloride ions on corrosion rate. Thirty-five archaeological iron nails, treated individually in either alkaline sulphite or nitrogen-deoxygenated sodium hydroxide, were exposed to 75°C and 75% relative humidity together with 31 untreated objects from the same archaeological sites. Object weight change and visual examination of physical change before and after the test period were used to monitor corrosion. 77% of treated objects showed no weight gain and no visible signs of corrosion, while 90% of untreated objects did corrode. The impact of chloride on corrosion of untreated objects was clearly established by a significant linear correlation between chloride content and weight gain. Treated objects with <400 ppm chloride content showed no corrosion behaviour. Corrosion of treated objects was attributed to incomplete treatment: 93% of objects treated to <5 mg/l Cl− in the final solution bath displayed no corrosion behaviour. Based on these results, desalination of iron objects to enhance their stability offers a valuable option for reducing corrosion rates of archaeological iron, which should increase object lifespan. The results also raise the question of whether low levels of post-treatment residual chloride produce corrosion of any significance. Answering this will be an important step forward for managing the preservation of archaeological iron.

Measuring effectiveness of washing methods for corrosion control of archaeological iron: problems and challenges

Abstract The individual chloride content of 116 archaeological iron nails from Romano British and Medieval sites in Wales is reported. The meaning and value of chloride concentration recorded as weight of chloride in object/object weight is discussed in relation to reporting the effectiveness of washing methods designed to remove chloride from archaeological iron. This is theoretically compared to the concentration value weight of chloride in object/metal surface area of object and the difficulty of quantitatively determining the success of washing methods as stability enhancers is discussed. It is concluded that assessing the impact of residual chloride on post-treatment corrosion of archaeological objects has the potential to offer the most significant guide to treatment success.

ss Great Britain: conservation and access - synergy and cost

Abstract Conservation and access are integrated within a unique scientifically-researched, cutting edge, desiccated storage system for preserving the iron hull of Brunels steamship ss Great Britain. As part of the visitor attraction, conservation develops a synergy with the ship that effectively contributes to preservation costs by enhancing visitor experience and numbers. The ethical implications of a desiccation preservation strategy are discussed and the measures taken to mitigate its carbon footprint considered. The cost of conservation to society in energy terms is broached.