David Thickett

A New Methodology for Accelerated Corrosion Testing

Abstract Accelerated corrosion tests have been used for almost 30 years to exclude corrosive materials from museum display and storage. The method reported by Green and Thickett in 1995 has been developed to allow the evaluation of three metals (silver, copper and lead) simultaneously in a single test. Comparison of a wide range of materials has shown that the differences in results introduced by this modification are less than the differences encountered with replicate tests using the same material employing the previous method. Problems encountered with corrosion in lead control tests have been investigated and the method has been modified to remove this effect. The cleaning method for the test vessels has been modified to exclude the hazardous chemical, chromic acid.

Damage functions in heritage science

Abstract This contribution critically examines various concepts related to damage functions in current use; it does not represent a comprehensive review, however. In heritage science, damage functions can be defined as functions of unacceptable change, dependent on agents of change. Unlike in other domains of science, the reference to unacceptable change implies that a value-based decision needs to be applied to the analytically determinable change, or dose–response function. Since there are a number of values associated with heritage, there can be a number of damage functions describing one and the same physical or chemical process of change. For practical reasons, it is thus useful to decouple the value function from the dose–response function, as the latter can be independently deduced (empirically or experimentally). While the behaviour of single materials and objects is often modelled deterministically, the probabilistic approach can be used to describe the degradation of complex structures, e.g. buildings, although not commonly in heritage research. There is a case to be made for more in-depth exploration of incremental and stochastic processes of degradation of heritage materials and structures, and the associated values, with a view towards developing more damage functions. Damage functions find use in modelling for predictive maintenance. However, predictions are associated with uncertainty due to the need for extrapolation, and this is also rarely analytically examined. Validation of modelling processes is possible using accurate methodologies of asset monitoring, which would enable an effective appraisal of preservation strategies.

Damage function for historic paper. Part I: Fitness for use

BackgroundIn heritage science literature and in preventive conservation practice, damage functions are used to model material behaviour and specifically damage (unacceptable change), as a result of the presence of a stressor over time. For such functions to be of use in the context of collection management, it is important to define a range of parameters, such as who the stakeholders are (e.g. the public, curators, researchers), the mode of use (e.g. display, storage, manual handling), the long-term planning horizon (i.e. when in the future it is deemed acceptable for an item to become damaged or unfit for use), and what the threshold of damage is, i.e. extent of physical change assessed as damage.ResultsIn this paper, we explore the threshold of fitness for use for archival and library paper documents used for display or reading in the context of access in reading rooms by the general public. Change is considered in the context of discolouration and mechanical deterioration such as tears and missing pieces: forms of physical deterioration that accumulate with time in libraries and archives. We also explore whether the threshold fitness for use is defined differently for objects perceived to be of different value, and for different modes of use. The data were collected in a series of fitness-for-use workshops carried out with readers/visitors in heritage institutions using principles of Design of Experiments.ConclusionsThe results show that when no particular value is pre-assigned to an archival or library document, missing pieces influenced readers/visitors’ subjective judgements of fitness-for-use to a greater extent than did discolouration and tears (which had little or no influence). This finding was most apparent in the display context in comparison to the reading room context. The finding also best applied when readers/visitors were not given a value scenario (in comparison to when they were asked to think about the document having personal or historic value). It can be estimated that, in general, items become unfit when text is evidently missing. However, if the visitor/reader is prompted to think of a document in terms of its historic value, then change in a document has little impact on fitness for use.

Designing accelerated ageing experiments to study silk deterioration in historic houses

Abstract An accelerated ageing methodology to study silk deterioration is presented. This draws on approaches used in paper and photographic conservation, although the references are not widely available within conservation. The experiments were designed based on the English Heritage silk collection and its display environment. The selection of different light and humidity conditions for ageing will be discussed, including the experimental parameters and equipment. Details on how the ageing time was related to an equivalent display time will be demonstrated. Some of the key observations and problems which occurred during the project will also be outlined. Some of the advantages and disadvantages of using accelerated ageing experiments to study deterioration reactions will be reported.

Safe display parameters for veneer and marquetry objects: A review of the available information for wooden collections

Abstract Furniture collections can be key in the interpretation of historic houses to visitors, providing a link to their own homes and making houses seem inhabited. The ornate and highly decorative surfaces reflect not only the aesthetic taste of the period, for example chinoserie inspired lacquerware, but also the design and craft expertise and manufacturing skills required to produce the objects. These surfaces can demonstrate the effects of time, having been subjected to the surrounding environment within the property. For highly decorated surfaces, such as veneer and marquetry, this can include loss of material, lifting veneers, fading of wood and its finishes, along with cracks, splits, flaking, and warping. Preventive conservation seeks to minimize the risk to collections by optimizing the display and storage parameters. However, research on these materials to determine the optimal display conditions can be limited, insufficient, or even entirely absent. This review presents the available information for related wooden collections within the literature and highlights the limitations of these data for preventive conservation decision making. The focus of the review is on the effects of relative humidity, as there are significantly more data available on this parameter than others such as temperature, light, or pollutants.

Note on the identification of an unusual pale blue corrosion product from Egyptian copper alloy artifacts

Abstract Examples of a pale blue corrosion product and associated corrosion on copper alloy artifacts from the collections of the British Museum Department of Egyptian Antiquities have been analysed and identified. A previously unreported compound, sodium copper acetate carbonate, has been identified.

Statistics of insect catch within historic properties

IntroductionCertain species of insect are known to cause damage to historic collections. For more than a decade insects have been identified in traps set out in English Heritage properties, the data from which has been used in this paper. Descriptive statistics have been used to explore the data structure and insect distribution.ResultsAbout 55% of the traps of the more than 30000 traps examined over that period contained insects that were readily categorised. The rate of catch (insects/trap) was highest in London and the Southeast. Booklice (Liposcelis bostrychophila) and silverfish (Lepisma saccharina) were the most frequent catch. Woolly bear (Anthrenus spp.) larvae and crustacean woodlice (Porcellio spinicornis) were also common. A higher frequency of furniture beetles (Anobium punctatum) is notable in the Southeast and West. Despite this overall pattern, catch varied greatly between individual properties. The general view that insects have increased over time was not universal, although the dominant booklouse showed an increased catch over the last decade. The insects did not appear to be randomly distributed, but clustered onto traps in greater numbers than might be expected from a Poisson distribution, which suggested they occurred as infestations. Some insect species appeared in combination with other species at a higher frequency than expected, but the reasons for these associations were not always obvious. Pheromone traps collected more webbing and case bearing clothes moths (Tineola bisselliella, Tinea pellionella) than traps without attractants, much as expected. There are hints that covered traps may collect fewer insects than simple blunder traps.ConclusionsNo obvious drivers of high insect count were identified. Trends over time were not especially clear. The results provide clues to management of insect pests in historic properties. The presence and trapping of insects at a given property is individual in nature, and so mitigation strategies need to be site-specific. Although an insect might occur in low numbers in some periods, the possibility of infestation remains. Future work will examine the trends in catch more carefully and ascertain the importance of various environmental factors.

Air Exchange Rate – the Dominant Parameter for Preventive Conservation?

Abstract This article aims to demonstrate that air exchange rate is extremely important for the performance of display cases with regards to relative humidity buffering, dust and pollutant ingress and corrosion of metals displayed within them. The display cases built for English Heritage to house the Wernher Collection of small and precious objects at Rangers House, London are investigated. The displays were set up four years ago, and air exchange rates were measured and leakage rates reduced as far as possible at that time. The extended methods for measuring air exchange rates are discussed and the longer term performance of the cases is investigated. The importance of the capacity of the cases to maintain appropriate RH levels without using the hygroscopic objects to act as buffers is also highlighted. Maintenance of display case performance requires monitoring, actions and effort, as seals and mechanical parts age.

An Improved Firing Treatment for Cuneiform Tablets

Abstract Unfired clay tablets bearing cuneiform script form an unparalleled archive of the ancient Mesopotamian culture. The need to handle cuneiform tablets in order to read them has led several institutions to adopt firing-based treatments to protect the tablets. A combination of thermal analysis, physical testing, and trials with tablets has allowed the development of an improved firing schedule for tablets requiring treatment. The analysis has also indicated that a single firing schedule is suitable for all tablets within the collections of the British Museum.

Assessment of indoor air quality and the risk of damage to cultural heritage objects using MEMORI® dosimetry

Air pollution is one of the environmental influences that degrade cultural heritage objects situated indoors. Other essential influences, such as temperature, relative humidity, and light are often well monitored. The presence of air pollutants is less often measured or included in risk assessment. The MEMORI® technology presented in this paper was developed as a tool for easy measurement and assessment of the general risk of degradation of heritage objects situated indoors due to indoor exposure to air pollutants. MEMORI dosimetry was performed in locations belonging to English Heritage and Tate (both located in London) and the National Archives of Norway in Oslo, to assess air quality. The related damage risk for collection objects and the protection offered by display and storage designs was assessed. A high level of acidic effect was observed inside a number of showcases, and a high level of oxidizing effect was observed in some room locations. Relatively simple mitigation measures, such as constructing tightly sealed showcases using low emitting materials, installing active carbon absorbing media inside a ‘microclimate’ frame, and using cardboard storage boxes for paper, significantly improved air quality. Overall, implementation of such measures is likely to improve the preservation of objects and reduce conservation costs.