Development of tools and techniques to survey, assess, stabilise, monitor and preserve underwater...

WET ORGANIC ARCHAEOLOGICAL MATERIALS

ICOM-CC17th Triennial Conference2014 Melbourne

DAVID GREGORY*National Museum of DenmarkKongens Lyngby, [email protected]

ANNE MARIE ERIKSENNational Museum of DenmarkKongens Lyngby, Denmark

ROBERT PEDERSENAKUTHundested, [email protected]

POUL JENSENAKUTHundested, [email protected]

MIKKEL HOLMEN ANDERSENUnisenseAarhus, [email protected]

BARBARA DAVIDDEIstituto Superiore per la Conservazione ed il RestauroRome, [email protected]

MARTIJN MANDERSThe Dutch Cultural Heritage AgencyAmersfoort, The [email protected]

THIJS COENENThe Dutch Cultural Heritage AgencyAmersfoort, The Netherlands

JØRGEN DENCKERThe Viking Ship MuseumRoskilde, [email protected]

CHARLOTTE BJÖRDALThe University of GothenburgDepartment of ConservationGothenburg, [email protected]

BRIAN SMITHSeabed Scour Control SystemsGreat Yarmouth, [email protected]

*Author for correspondence

KEYWORDS: assessment, underwater cultural

heritage, in-situ preservation, deterioration, stabilisation,

environment, waterlogged

ABSTRACT

The SASMAP project seeks to develop tools and

techniques to enhance the management of under-

water cultural heritage. The paper discusses those

Development of tools and techniques to survey, assess, stabilise, monitor and preserve underwater archaeological sites: SASMAP, a European research project

INTRODUCTION

In-situ preservation is increasingly being seen as a means to manage marine archaeological sites which, for economic reasons and current international trends favouring in-situ preservation, are not excavated, raised and conserved. However, in-situ preservation should not be a case of leaving a site where it is and hoping that it will be there when archaeologists and conservators have the capacity, research questions and desire to investigate these finds in the future. The current author (Gregory 2009) has previously discussed that there are five fundamental steps to ensuring the successful and responsible in-situ preservation and management of archaeological sites in general. These are:

• the extent of the site to be preserved

• the most significant physical, chemical and biological threats to the site

• the types of materials present on the site and their state of preservation

• strategies to mitigate deterioration and stabilise the site from natural impacts

• subsequent monitoring of a site and implemented mitigation strategies.

The SASMAP project received funding from the European Union to investigate elements of this approach over a three-year period from 2012 to 2015 and has currently been running for just over a year. This article is a status report of the work carried out to date and focuses upon results that are most relevant to the Wet Organic Archaeological Materials (WOAM) working group. The project includes other elements of the five-point approach outlined and further information is available on the project home page at www.sasmap.eu .

SCOPE OF SASMAP

Due to space considerations, it is not possible to give a full introduction to the project and the reader is referred to Gregory (2012), which is available online at the project’s home page. The scope of the SASMAP project most pertinent to the WOAM working group includes themes investigating:

• assessment of the burial environment and deterioration of organic archaeological materials

mailto:[email protected]









http://www.sasmap.eu

DEVELOPMENT OF TOOLS AND TECHNIQUES TO SURVEY, ASSESS, STABILISE, MONITOR AND PRESERVE UNDERWATER ARCHAEOLOGICAL SITES: SASMAP, A EUROPEAN RESEARCH PROJECT


2 ICOM-CC17th Triennial Conference2014 Melbourne

aspects of the project concerning the assessment

of the site environment; assessment of the state of

preservation of waterlogged archaeological wood;

methods to raise fragile organic artefacts during ex-

cavation; and the use of artificial seagrass in order to

stabilise sites that are preserved in situ. The project

runs from 2012 to 2015 and the current paper serves

as a progress report after the first year of the project.

• assessment of the state of preservation of waterlogged archaeological wood

• tools and techniques to raise waterlogged organic archaeological artefacts

• in-situ stabilisation of underwater archaeological sites.

The various themes are briefly introduced, followed by the status of the results to date.

Assessment of the burial environment and deterioration of organic

archaeological remains

The project is developing tools and methods to characterise the burial environment and to use this information to assess its effects on the deterioration of organic materials. The main tools being developed are a diver-held coring device designed to make the taking of sediments easier and two data-logging systems which can: a) monitor the open-water environment; and b) assess pore water composition of sediments in situ to a maximum sediment depth of 50 cm.

The coring system in its current state (Figure 1) consists of a pneumatically driven vibrating unit that is supplied by compressed air from scuba-diving tanks. The coring tube is made of transparent polycarbonate and diameters of 50 and 80 mm have currently been trialled. The tube has a simple system for attachment to the vibrator head and allows water to escape the coring tube during penetration. The head also makes changing of coring tubes under water simple. The downward force on the tube is achieved by a weight with a diver-held ‘handle bar’ connected to the vibrator through a spring that both protects the diver from the vibrations and ensures that all vibration energy is transferred to the coring tube. The weight system is connected to a lifting bag, making it possible to adjust the downward force by regulating the amount of air in the bag. The lifting bag can be filled by exhaust air from the vibrator and provides upward force to ease the retrieving of the core from the seabed. Trials of the corer have been made on a shallow, submerged prehistoric site in Denmark and were successful in obtaining and retaining sediment cores (of gytjja) down to ca. 70 cm, although other trials in sandy sediments have obtained cores 2 m in length. The shallow water of the site (ca. 2 m) made working with the system slightly challenging as the weight of the device is designed to be supported by lifting bags. Nevertheless, the cores were quickly taken (ca. 10 minutes per core) and the experience of working on an actual site and in shallow water gave rise to numerous improvements to the system, which will be worked on in the coming months. The cores from the prehistoric site were taken back on land and profiles of dissolved oxygen, pH, redox potential and sulphide were taken using Unisense’s standard laboratory sensors (Figure 2). The sediment obtained is finally being used in connection with assessing the turnover of organic material, which will be examined in year two of the project.

Sediment profiling and data logging system

With this system, it was desired to measure the same parameters as measured in the cores, but in situ and down to a depth of 50 cm. An

Figure 1Vibracoring system (without coring tube)

Figure 2Measuring of environmental parameters in sediment cores in the field




existing diver-operated underwater meter, manufactured by the project partner Unisense, was redeveloped for this purpose. A hollow ‘spear’ that could accommodate a microsensor with an attached hypodermic needle was developed, the idea being that the spear could be pressed into the sediment and an influx of water or sediment prevented by a silicone septum in the end. At the required depth, the hypodermic needle of the microsensor is pressed through the septum and a measurement taken. The process is then repeated at the desired depths within the sediment. The in-situ profiler system was trialled in an area adjacent to where the sediment cores had been taken in order to compare and contrast the methods and, in this way, validate both. In this instance, only sulphide microsensors were used as the main aim of the trial was to test the use of the equipment. The first impressions of the system were that the meter in itself was extremely simple to use, with commands and data being logged using magnetic keys controlled by a ‘wand’ from outside the underwater housing. The spear itself was easy in this instance to simply press/hammer into the sediment, although it has also been pressed into more compacted sediments in another trial using the vibracoring device. It was important to ensure that sand did not get into the open end of the spear, as this collected above the rubber septum and could damage the microsensor as it was passed into the spear. In this way, measurements were taken every 5 cm down to a depth of ca. 50 cm; the results, along with core measurements, are currently undergoing post-processing. To supplement the environmental monitoring aspects of the project, wood samples were taken from the discrete layers of the test pit – wood overlying the natural clay substrate, wood within the gyttja, wood overlying the gyttja and below the mobile sand/gravel layer and also currently exposed lying on the seabed. In order to gain more information on the actual deterioration of wood, a series of modern pine samples were placed on site using a ‘kebab’ system (Gregory et al. 2008). This entailed threading small cylinders of fresh pine wood onto a carbon-fibre rod at different depths down to ca. 50 cm, with a spear mounted on the end and pressing them into the site using the vibracoring system. These samples will be examined over the lifetime of the project and it is hoped in the future to better understand the deterioration and preservation of wood in waterlogged/marine environments.

Open-water data-logging system

This system is again based on the lander technologies that Unisense has developed. The parameters it measures are standard conductivity (salinity), temperature and depth data (CTD data). However, new developments include the integration of a compact acoustic Doppler current profiler (ADCP water current meter). Testing of the open-water data logger was in connection with the trialling of the artificial seagrass mats (see section on in-situ stabilisation) on the submerged Danish prehistoric site. The idea was to log data over two days on the shoreward side of the mat and then move it to the offshore side. The hypothesis was that there may be more current on the offshore side due to the greater fetch in this area and in this way see if the seagrass was actually reducing any eventual currents and thus prevent any resultant scouring of sediment. The logger




was assembled in a local harbour and transported onto the site using a small vessel (6.2 m). Although relatively large and heavy, the logger was easily lowered by two people onto the seabed. A single diver was then able to position it in the correct orientation on the seabed and on the shoreward side of the mat. Following logging for two days, the logger was moved to the opposite side of the seagrass mat (offshore), using a diver on the seabed and support on the surface. CTD and current data were successfully collected and are undergoing post-processing. The system will be trialled further in the coming year of the project.

Assessment of the state of preservation of waterlogged

archaeological wood

A handheld tool for assessing the state of preservation of waterlogged archaeological wood both in situ on the seabed and in the laboratory is being developed based on the non-destructive determination of the density of the wood. The net effect of bacterial deterioration is that as cell-wall material is removed and replaced with water, the density of the wood decreases – the more degraded the wood, the lower the density.

In the first year of the project, a laboratory-based stand (Figure 3) has been developed to check the feasibility of the proposed device. This has been able to characterise archaeological and prepared laminates of fresh wood with differing densities, recording profiles down to a depth of 10 cm from the surface of the wood. The Lab stand is based on the concept of hammering a steel needle into the wood and determines the correlation between depth of penetration and the energy used. The corresponding data on depth and strength of the wood are transferred to a computer, where they are transformed into a density profile of the wood by means of physical computerised models and standard curves for wood. To mimic degraded archaeological wood, a laminated sample was made of balsa (70 kg/m3) and beech wood (650 kg/m3). Figure 4 shows the density profile obtained by the Lab stand. The needle started the penetration from the softer and less dense balsa side. Based on the laboratory model, the project partner AKUT will manufacture a handheld wood tester, which will be tested on archaeological wood samples and then developed for use underwater.

Figure 4Density profile of a laminated block (balsa and beech) wood

Figure 3Laboratory stand for measuring the density of wood




Tools and techniques to raise waterlogged organic

archaeological artefacts

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, to the crucial stage of lifting artefacts from the seabed to the surface, where mechanical damage can easily occur. To surmount this, artefacts are often raised on supporting materials, or in sediment blocks (block lifting) whereby the artefact is excavated with surrounding sediment and subsequently excavated under controlled conditions in the laboratory. The project is developing the use of polymer-based consolidants which can both encapsulate and consolidate sediments, as well as the freezing of sediments and the development of lifting systems in order to enable the safe lifting and transport of waterlogged organic archaeological objects.

The first product tested was 3M Scotchcast Plus casting tape. This is a lightweight, strong and durable casting tape that combines the benefits of a fiberglass casting tape with the handling ease of plaster. The tape (bandage) contains a synthetic polyurethane resin which hardens in contact with water or when simply exposed to moist air, enabling immobilisation of fragile artefacts while being extremely lightweight and durable. The tests were undertaken in the lab and then underwater, with the diving conservators reporting on the ease of use of the product to recover a fragile wooden archaeological object. The 3M Scotchcast Plus casting tape is environmentally friendly and it is easily removed post lifting (also if in direct contact with the archaeological find). The second product tested was a sheet of carbon fibre, previously treated with cured epoxy-time in a plastic bag vacuum. The use of the carbon fibre was as follows: a polyethylene vacuum waterproof bag was first shaped in the form of the artefact to be recovered. The vacuum and waterproofing are achieved through the use of perimetral strips. Before the closure of the strips, a multilayer of Peel Ply tissue, carbon sheet and Peel Ply tissue was placed in the bag. Inside the bag, an epoxy resin was applied over the multilayer and then the bag was closed. The two mats in carbon fabric were handled easily and taken to the artefact (a wooden pole) to be recovered. The upper mat was made to adhere to the pole with the aid of lead slings that had also been used to weight the mat so as to ensure it remained on the lake bottom and adhered to the artefact. After waiting for the resin to harden (about 12 hours), the upper mat and the lower mat were placed with plastic clamps and brought to the surface. The carbon-fibre fabric, impregnated with epoxy resin, is a protective shell that adheres to surfaces, protecting the artefact by rapid drying and preventing possible trauma due to the poor state of preservation of the material. Furthermore, the procedure and method may also act as an effective container for temporary storage of waterlogged organic objects. Future work to be conducted at the Istituto Superiore per la Conservazione ed il Restauro (ISCR) laboratories includes: 1) testing organic and inorganic products for consolidating sediments; 2) testing a system to ‘inject’ polymers into sediment in situ. Laboratory work is currently ongoing to examine the use of polymers such as neutralised




polyacrylic acid, sodium polyacrylate and other superabsorbent polymers (SAP, also called slush powder). These polymers can absorb and retain extremely large amounts of liquid and polysaccharides of high molecular weight, including xanthan, guar and agar. The results of these tests will be presented in the future.

At the National Museum of Denmark, studies on the effects of freezing sediments on organic artefacts and the feasibility of block-lifting sediments have been carried out. The aim was, firstly, to see if fragile wooden artefacts would be damaged by the shock of freezing and, secondly, if there was no damage, to freeze them in different sediment types (sand and gyttja) to see what effect this had on the process. These trials were carried out on extremely degraded samples of archaeological wood (density 81 kg/m3). The effects of freezing were assessed through monitoring weight change and through looking at the structure of the wood under the microscope. In both tests, only minor changes in weight were seen and there was no sign of damage to the wood cell wall or surface of the wood under the microscope. In year two of the project, and following an evaluation of the aforementioned methods, either by polymers or freezing, a lifting system to raise consolidated sediments as a block lift will be developed.

In-situ stabilisation of underwater archaeological sites

One of the major threats to archaeological remains preserved in situ is seabed scour, that is to say, the removal of sediment through currents in the water, which leave artefacts exposed and can ultimately lead to their complete deterioration. The majority of methods used to protect sites depend upon limiting the access of oxygen, and this can most simply be achieved by covering with sediment. However, simply covering with sediment may not be sufficient as it may itself be washed away. An innovative method using artificial seagrass mats is being trialled on different case-study sites within the project.

The company Seabed Scour Control Systems (SSCS) is a producer of artificial seagrass mats that seek to reduce the effects of scour. Commercially, these have been used to protect pipelines and cables on the seabed. The plastic fronds, which float in the water column, effectively slow any currents running through them, thus reducing turbulence and hence erosion or scour (Figure 5). In many cases, the fronds actually trap sediment from the water column, creating a sediment layer between the fronds and on top of the site that needs to be protected. Within the project, different designs of mats are being trialled. As their standard mat, which uses anchors to hold the mat in place, penetrates the seabed and could potentially damage any underlying archaeological material, two new systems are being developed: a so-called edge-weighted mat (Figure 6), which has a gravel-filled apron around the mat that sits on the seabed surface, and a system that includes a mesh apron which can be held in place by sand bags deployed by divers. A range of the different mats have been placed on wreck sites in the Netherlands, Denmark and Italy and their effectiveness is being monitored over a year (due for completion in August 2014).

Furthermore, the durability of materials that have typically been used to stabilise submerged archaeological sites in the marine environment,

Figure 5Fronds from an artificial seagrass mat

Figure 6Edge-weighted artificial seagrass mat being placed on site




including sandbags, plastic sheeting, geotextiles and debris netting, is being tested.

ACKNOWLEDGEMENTS

The project is supported by EU grant 38340 under FP7. The authors would like to thank the project officer Attilio Gambardella for his support.

REFERENCES

GREGORY, D., A.C. HELMS, and H. MATTHIESEN. 2008. 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. Conservation and Management of Archaeological Sites 10(3): 204–222.

GREGORY, D. 2009. In situ preservation of marine archaeological sites: Out of sight but not out of mind. In In situ conservation of cultural heritage: Public, professionals and preservation, eds. V. Richards and J. Mckinnon, 1–16. Adelaide: Flinders University.

GREGORY, D. 2012. Development of tools and techniques to survey, assess, stabilise, monitor and preserve underwater archaeological sites: SASMAP. In Conference Proceedings for EuroMed 2012: International Conference on Cultural Heritage, Cyprus. International Journal of Heritage in the Digital Era, volume 1, supplement 1, 367–371. Available at: http://www.sasmap.eu.

How to cite this article:

Gregory, D., A.M. Eriksen, R. Pedersen, P. Jensen, M.H. Andersen, B. Davidde, M. Manders, T. Coenen, J. Dencker, C. Björdal, and B. Smith. 2014. Development of tools and techniques to survey, assess, stabilise, monitor and preserve underwater archaeological sites: SASMAP, a European research project. In ICOM-CC 17th Triennial Conference Preprints, Melbourne, 15–19 September 2014, ed. J. Bridgland, art. 2002, 7 pp. Paris: International Council of Museums.(ISBN 978-92-9012-410-8)

http://www.sasmap.eu/

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