David Gregory

Environmental Monitoring at Nydam, a Waterlogged Site with Weapon Sacrifices from the Danish Iron Age. I: A Comparison of Methods Used and Results from Undisturbed Conditions

Abstract In situ preservation has become a common practice in the management of archaeological sites during the last decade. With this practice follows the need to have good, reliable methods for monitoring the environmental conditions at the sites. This paper describes the results from seven years of monitoring at the waterlogged site Nydam in Denmark. Results from a range of different monitoring methods are compared, and the advantages and disadvantages of each method are discussed. Measurements of water level, chemical composition, and redox potential are covered, with comparison of results from dipwells, filter probes and measurements directly in the soil. The results indicate that, with regard to methods 1) in situ measurements directly in the soil give the most reliable results, 2) analysis of water samples from dipwells and filter probes can give reasonable results except for a few species, 3) purging of dipwells before sampling improves the results when combined with a fast sample handling. With regard to the site 4) the burial environment at Nydam is permanently waterlogged, anoxic, slightly acidic, dominated by iron reduction and with a low concentration of dissolved sulphide, 5) there is a significant spatial and temporal variation of the concentrations of most dissolved species, 6) the redox potential as measured with permanently buried gold electrodes is very stable.

The Importance of Analysing Waterlogged Wooden Artefacts and Environmental Conditions when Considering their In Situ Preservation

Abstract The importance of analysing the state of preservation of waterlogged wooden artefacts as part of the in situ monitoring programme at the Iron Age site of Nydam is discussed. Determination of density, wood cell wall components (cellulose and lignin), coupled with macroscopic and microscopic analysis showed that artefacts were completely degraded, and that it was only the lignin rich compound middle lamella and water in the cell wall and lumen which retained the shape of the artefacts. Erosion bacteria (cellulose degraders) were identified as the main cause of deterioration of the wood. With no cellulose remaining, and under the prevailing conditions, it is unlikely that the wood can act as a substrate for further microbial deterioration. One of the major threats to the future preservation of wooden artefacts is drainage. Experiments to simulate the effect of desiccation on artefacts showed that there was the possibility of fungal deterioration by white rot species, yet more seriously, artefacts would suffer immediate collapse with loss of archaeological information.

In situ corrosion studies on the submarine Resurgam: A preliminary assessment of her state of preservation

Abstract The submarine Resurgam was the worlds first mechanically powered submarine, built in 1879 and lost in 1880 off the north coast of Wales. She was discovered 116 years later in 1996 and as part of an initial survey the corrosion characteristics and state of preservation of the hull were investigated. The results showed that the hull was actively corroding, although the corrosion mechanism was such that the corrosion products formed were likely to be of a protective nature. The present corrosion rate of the wrought-iron hull of the submarine was hard to quantify based on established means, as the profile of 116 years of corrosion products had not been retained over the entire hull due to spalling of the corrosion products. Where the concretion profile was retained it was possible to calculate an average corrosion rate of 0.1 mm per year. This, coupled with the knowledge of the corrosion rate of freshly exposed wrought iron in sea water, enabled a ‘worst case’ corrosion rate of between 0.1–0.2 mm per year to be estimated. Two arrays of sacrificial anodes were attached to the hull to test the feasibility of using this method to slow the corrosion rate and stabilize the submarine In situ. It is too early to say how successful these will be in the long term, yet initial results were encouraging as the corrosion potential was seen to shift to a more cathodic and thus less corrosive potential after their attachment.

The Use and Deployment of Modern Wood Samples as a Proxy Indicator for Biogeochemical Processes on Archaeological Sites Preserved in situ in a Variety of Environments of Differing Saturation Level

Abstract In situ preservation of archaeological sites is becoming an ever increasing trend as a means of preserving our cultural heritage. In connection with this the environmental conditions, such as water level, dissolved oxygen, temperature, pH and water chemistry, of a site are often monitored. It is generally agreed that a waterlogged and anoxic environment is essential for optimal preservation conditions but the set-up and maintenance of an environmental monitoring programme can be costly. This paper discusses the design and use of a system whereby modern samples of wood can, with a minimal disturbance of the soil, easily be deployed and retrieved from archaeological sites. The system was deployed in an unsaturated environment, an environment with fluctuating water levels and a fully saturated peat bog. The samples were assessed after two years using microscopic, physical and molecular biological methods, and the types of wood-degrading organisms seen were compared with the results of environmental measurements. Modern wood samples were used, as the microbial ecology of wood-degrading organisms in these different types of environments is relatively well documented. Preliminary conclusions show that the deterioration processes of modern wood samples in these environments act as a good proxy indicator of the environmental conditions and biogeochemical processes ongoing at a site.

The Correlation between Bulk Density and Shock Resistance of Waterlogged Archaeological Wood using the Pilodyn

Abstract The correlation (R2 = 0.87) between bulk density and the shock resistance of homogeneously and heterogeneously degraded waterlogged archaeological wood, with densities ranging from 100 to 700 kg·m-3, was determined using the Pilodyn wood tester. The results showed that the energy required to fracture a given mass of wood substance was constant (R2 = 0.83) and independent of wood species. However, measurements in the three main orientations (longitudinal, radial and tangential) showed a significant variation and therefore further measurements were preferentially taken in the radial axis. A model to enable the determination of the average bulk density of waterlogged wood from the depth of penetration of the Pilodyn showed that the average bulk density of homogeneously and heterogeneously degraded waterlogged wood could be determined with an average accuracy of ± 9%. A high correlation coefficient (R2 = 0.995) for measurements taken in water and air enable the Pilodyn to assess waterlogged wood under water as well as in air.

Strategies for Protection of Wooden Underwater Cultural Heritage in the Baltic Sea Against Marine Borers. The EU Project ‘WreckProtect’

Abstract Marine borers constitute a great danger to historical shipwreck in marine environments as they are able to decompose wood material in just a few years. Recently, there have been indications that the marine borer Teredo navalis is spreading into the brackish Baltic sea, where thousands of invaluable historical wrecks for centuries have had unique preservation conditions. The WreckProtect project was a coordination and support action funded by the European Commission within the 7th Framework Program. The main objective of the project was to develop tools for predicting the spread of marine borers into the Baltic and to evaluate methods for in situ protection of the historical wreck and submerged settlements. This paper gives a summary final report of the project and an overview of results.

Nydam Mose: In Situ Preservation at Work

Abstract The site of Nydam Mose saw the beginning of systematic research into in situ preservation of waterlogged archaeological sites on land at the National Museum of Denmark. In the past fifteen years a generic approach to in situ preservation of archaeological sites has been developed based on this research. This article is primarily a review of this generic approach, summarizing the methods and results with particular reference to the published results from the investigations in Nydam Mose.

Degradation of Archaeological Remains by Shipworm

The marine borer, known as shipworm, has long been known for their degradation of wooden constructions in the marine environment. This review presents the results from a variety of experiments conducted at the National Museum of Denmark throughout the past five years dealing with the in situ preservation of maritime archaeological artefacts in light of the threat from shipworm. The aim of the research has been to improve methods and analyses for conducting experiments with shipworm, and to better our understanding of their attack pattern and how to stop an active attack in situ.

Recovery of Fragile Objects from Underwater Archaeological Excavations: New Materials and Techniques by SASMAP Project

Underwater archaeological excavation represents a traumatic and essentially destructive event in the history of artefacts, especially organic, and its underwater context. Indeed, due to their fragility, organic archaeological materials from underwater sites can be challenging to excavate, support, raise and transport to conservation facilities. This is due to the inherent difficulties of working underwater (limited time and potentially harsh conditions) and in particular the crucial stage of lifting artefacts from the seabed to the surface where mechanical damage can easily occur. Block lifting of fragile archaeological materials is a useful procedure often adopted on land excavations and allows the collection of information which could be irretrievably lost during more rapid excavation. This procedure could be used with success to recover fragile objects on an archaeological underwater excavation thanks to the new materials and techniques tested and adopted in the SASMAP project (http://sasmap.eu) as will be discussed.

Reburial and Analyses of Archaeological Remains in the Marine Environment — Investigations into the Effects on Metals

Abstract The treatment and long-term storage of recovered cultural material from underwater heritage sites is becoming less cost effective, and reburial of archaeological sites and the associated artefacts in the marine environment is becoming increasingly common practice in managing the submerged cultural resource. Following recent large-scale underwater archaeological excavations in Marstrand harbour, Sweden, the majority of recovered finds were reburied in defined trenches in the harbour sediment. Subsequently, the Studio of the Western Sweden Conservators in conjunction with the Bohus County Museum initiated a fifty-year research project to evaluate reburial as an appropriate method of preserving waterlogged archaeological artefacts in the long term. The research project, entitled ‘Reburial and Analyses of Archaeological Remains’, was launched in 2002 and consists of six sub-projects. The main aims of these sub-projects are to analyse the extent of deterioration of the most common material types found on underwater archaeological sites, assess the stability of packing and marking materials used in archaeological documentation, and monitor the reburial environment. The aim of the metals sub-project is to investigate the short- to long-term corrosion behaviour of metals buried in the marine environment by examining the deterioration of reburied and exposed modern metal coupons and eventually compare these results to the analysis of actual shipwreck artefacts. The environmental monitoring sub-project is designed to complement the other sub-projects by assessing the physico-chemical changes occurring in the reburial environment over time and the effect on the deterioration of the different reburied material types. In comparing the results obtained over the past seven years from both the metals and monitoring sub-projects, it should be possible to more accurately evaluate the effectiveness of reburial as a long-term in situ preservation strategy for metallic archaeological remains.