Conservation Science: Heritage Materialsby B Des Barker
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Conservation of artefacts and heritage materials is an increasingly popular and fascinating area, spanning both historical and scientific disciplines. Materials come in many forms ranging from sunken ships to tapestries, from buildings to books. With this wide range of matrices and materials to analyse and preserve, an interdisciplinary approach is needed drawing upon skills from many areas of knowledge. Conservation Science: Heritage Materials links these fields of research together forming a comprehensive text book that discusses analytical aspects, wall paintings, organic and inorganic materials. It provides up to date information on subjects including research on decay and degradation and an understanding of the deterioration mechanisms of historic and artistic works. Also included are a number of case studies of particularly important finds including the upkeep of the Mary Rose and the preservation of the sail on Nelsons ship HMS Victory. This book provides an essential guide and reference source for those working in all areas of heritage conservation.
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By Eric May, Mark Jones
The Royal Society of ChemistryCopyright © 2006 The Royal Society of Chemistry
All rights reserved.
Maritime Archaeologist; Archaeological Director, Mary Rose Project 1967–1994; Archaeological Consultant 1995–2003
1 CULTURAL HERITAGE: THE ARCHAEOLOGIST, THE CONSERVATOR AND THE PUBLIC – AN ESSENTIAL COALITION
The recent growth of public interest in archaeology and their enthusiasm for the past is reflected by the astonishing success of 'heritage' programmes on television. A stranger to British television is often surprised at the number of cookery, gardening and archaeology programmes offered for entertainment every day and their success may reflect a growing wish to escape from the stresses of modern living and a desire to understand our past. All three subjects can be related to our heritage and the wonderful legacy left by our ancestors.
The better archaeological programmes often demonstrate the application of technology to solve the problems of dating, identifying and conserving cultural material, and the range of periods, sites and cultures investigated is wide and diverse. Usually, these techniques are not new as many were originally developed for commercial or military use, but their application to archaeology is new, and the public, through good television presentation, is able to understand both the technique and the problem.
One evening, we joined the architectural historians as they solve the problems of how a building developed as its use changed over the years, and the consultants discussed how best to interpret and preserve it for the future. The next evening, we joined a group of students of garden history who explained the importance of soil analysis and remote prospection using ground-penetrating radar or a magnetometer to enable the researcher to present an accurate recreation of a lost 18th-century garden. In the realm of the Field Archaeologist, we are offered programmes several times a week showing desperate attempts to solve all the problems of a complex excavation in 72 hours.
The use of modern computer techniques and the abilities of the forensic scientist skilfully displayed within these programmes have familiarised the general public with technology in a way that has never happened before. The result is that they cannot be fobbed off with conjecture about archaeological material – they expect facts. They also expect that the objects and any evidence found by modern archaeologists will be preserved to be reassessed and displayed for education and pleasure. In parallel with the growth in public interest there has been a growing investment of public money and with this comes accountability.
1.1 The Archaeologist
The task that is faced by the archaeologists, whether their work is publicly funded or not, is enormous and the skills required are diverse. The recognition that any excavation of buried material is inevitably destructive means that the intrusion into a site has to be justified and well planned. Pre-disturbance appraisal including soil analysis, geo-prospection, topographical survey and aerial photography will provide the outline for a strategic plan and a wise archaeologist would include the scientists in planning the programme of work at this early stage.
Archaeology has moved away from the collection of individual 'things' to the collection of information about the people who made and used the objects. This change in attitude requires a forensic approach and commitment far removed from treasure-hunting and early barrow digging. Even an important and highly glamourous site like the 7th century tomb discovered at Prittlewell, during the rescue excavations in advance of road development, was part of a well-designed programme founded on examination of the material discovered in the 1920s. The importance of the site, with its mixture of Christian and pagan ritual material, cannot be over emphasised and although the public may be dazzled by the gold crosses, the presence of wooden objects and small bone dice suggest that soil analysis may yield more information. The lost burial and vegetable materials buried with the body may throw new light on the environment in Essex in the early 7th century; only time and rigorous analysis of samples will tell.
1.2 The Conservator
The skills of the conservator and the high standards of diagnostic analysis and documentation required are an essential part of any programme to investigate and preserve evidence of our past. Whenever possible the monument or building should be preserved in situ in the context and the landscape where it was created. Interest in stately homes and public buildings exceeds interest shown in a sequence of objects displayed as art objects or collector's items in a museum gallery. The kitchens and the bathing arrangements in a house like Longleat, Wiltshire, UK are at least as interesting as the paintings and ceramics. However, the collections in a stately home need constant vigilance to ensure that climate changes, insects and dust do not destroy the legacy. While the house itself, like any building, needs constant repair to ensure that it will survive.
Any homeowner visiting a house like Longleat will recognise the problem but the scale of the maintenance and repair programme on any building whether it is a Cathedral, a Parish Church or a tollbooth is large and the needs are constant and unavoidable. This cannot be entirely funded from public funds and good interpretation and user-friendly display are essential if the site is to earn enough income to support the project. Both commercial sponsors and the public need to know that the site is well managed and that the future of the building and its contents is secure.
2 FROM DISCOVERY TO DISPLAY
So the archaeologist has to have a plan that will ensure good management from discovery to dissemination of information by popular and scientific publications and by public display. He or she also needs good scientific advisors. Certain constraints remain as tried and tested dogma:
1. Sites should not be excavated unless they are threatened with destruction or they fit into an agreed programme of research.
2. All excavation should be preceded by non-destructive survey.
3. A full programme of investigation, excavation, conservation, scientific analysis and publication must be agreed before work starts.
4. Funding should be in place or guaranteed from the start.
5. All stabilisation and preservation treatments should be reversible.
While all these constraints are desirable it is rare for them all to be possible before work begins and any programme must be monitored and reassessed as work progresses. The importance of involving a multi-disciplined team in the process of establishing and monitoring any programme cannot be overemphasised. Usually the archaeologist has to divide a programme into phases and progress from one phase to another depends on the results of one phase and the availability of funds.
Often a site is partially destroyed before the archaeologist is involved. This occasionally happens on land sites if there is poor liaison between the archaeologists, the planners and the developers. This happens more often underwater where a site may be unexpectedly exposed by unusual storms or a change in currents caused by the construction of sea defences or offshore dredging.
2.1 Special Problems of Underwater Recovery
The usual scenario is that the exposed wreck site is discovered by divers, either accidentally or as a result of research. The team then try to identify and record the wreck and some objects are recovered to aid the process of identification and brought ashore. At some stage the finds and the site are reported; in the UK this would be to the Receiver of Wreck and eventually to English Heritage. If the site is thought to be of importance a team of professional diving archaeologists will visit the site, eventually the site may be designated as being of importance under the Protection of Wrecks Act 1973.
If the original team of divers is lucky they may have the unpaid assistance of professional archaeologists and conservators who will advise them from an early stage. They may also decide to apply for their 'wreck' to be designated with the approval of their team as licensees. A programme of work has to be submitted before the application for a licence will be approved and among other requirements there is the requirement for a nominated archaeologist and a conservator. A difficulty arises if the team (or the Ministry) requires the archaeologist to work full-time alongside the amateur team whenever the amateur team is on site. That would mean that the archaeologist would have to be paid and, as they would then be a diver at work, they would need professional diving qualifications, insurance and the support of professional divers as backup. The responsibilities and the costs rise beyond the budget of a team of vocational diving archaeologists. In the face of this it is inevitable that some sites will not be reported.
2.2 The Raising of the Mary Rose: A Case Study
The Mary Rose was discovered (see Figures 1 and 2), surveyed, excavated and recovered using a pre-designed floating programme of work that could be adjusted and cut if circumstances changed. It was recognised that any disturbance of the seabed would threaten the physical integrity of the wreck and plans were made accordingly. From the beginning of the search phase in 1967, there was an archaeologist in the team and advice on conservation was always available on request from Portsmouth City Museum and scientists at Portsmouth Polytechnic (now the University of Portsmouth).
Objects, including large guns, were recovered from the sea prior to the formation of the Mary Rose Trust in 1979, and they were taken to the conservation laboratories of the City Museum the same day. Close planning and liaison with the people providing equipment to lift the guns from the wreck site and take them ashore, the road transport and the museum staff were essential. Once the operation became full-time in 1979, the Mary Rose Trust, a charity and a company limited by guarantee, was formed to excavate and, if feasible and desirable, recover the wreck of the Mary Rose and all its contents. There were a series of cut-off points, and these were essential because there were no funds available from the local government. The whole operation depended on commercial support for equipment and personnel and the generosity of the public.
A full-time team of divers, professional diving archaeologists and conservators were recruited to do the fieldwork and a small team including a press officer, scientists, illustrators and conservators worked on the finds once they were ashore.
A programme of sampling the sediments that had begun in 1977 was enhanced and an environmental scientist was employed by the Trust in 1979 to analyse the material and select samples for appraisal by consultants. The strategy of the sampling programme was two pronged:
First – To identify environmental material derived from containers including barrels and boxes or as contaminants on clothing, cordage and dunnage.
Second – To identify a series of microenvironments within and around the ship.
This was necessary in order to understand the complex processes of sedimentation, preservation and collapse, which occurred immediately after the ship sank and the series of events that occurred after the wreck was buried and localised intrusions occurred (see Figure 3).
For the Mary Rose, the cut-off points were designed to present the least possible threat to the ship if the project had to be aborted. In the early years, the site was backfilled at the end of each season. As work progressed, the Mary Rose site became too large (roughly 40 × 25 × 3 m) and the work involved in backfilling and re-excavating the site would have been counterproductive. Therefore the exposed timbers of the hull were draped in Terram, an inert geo-textile, held closely in position by carefully chosen exotic gravel that could be easily recognised and removed if work continued in the following year. This cover protected the hull timbers from mechanical attrition by current-borne detritus and inhibited colonisation of the timbers by larger marine animals and plants.
Only after the decision to lift the hull was made in January 1982, was the final excavation begun to undercut the hull and leave the empty hull for recovery the following autumn. Work to define the engineering strategy to recover the ship had begun two years before, and in the spring of 1982, work began to prepare the ship for recovery and conservation.
A conservation laboratory equipped to deal with finds had functioned at the Trust headquarters at Portsmouth since 1979, and the external Advisory Committees met regularly to review progress and monitor the work as it progressed. In 1982 a suitable dock in the Royal Naval Base was equipped to receive the hull while it was cleaned, restored and conserved. The salvage plans and a detailed research programme to define passive and active conservation techniques went on concurrently with the excavation offshore between 1980 and 1982.
2.3 Preservation of the Evidence: A Multi-discipline Task
Organic material recovered from the sea, where it has been buried in a relatively stable anaerobic environment, is often well-preserved with fine detail. The obligation to preserve and record the evidence during excavation is paramount. Even on land sites, archaeologists have to contend with natural and man-made threats during excavation, and temporary shelters from rain, wind and frost are often necessary. Trying to excavate and record the remains of a timber building during a typical English summer is sometimes heartbreaking. The evidence is usually preserved as a series of changes in colour and texture and if the natural subsoil is clay or brick-earth, then it is often difficult to record and there is little to distinguish between the remains of a clay and daub structure and the natural subsoil. Summer weather is unpredictable and one day the archaeologist will be using a garden syringe to spray the site and reveal the features for photography while next day he will be erecting scaffold supports and tarpaulins to protect the site from a deluge of rain. In England it is often bad enough but in tropical regions a site that has been a desert for weeks may be destroyed and washed away completely by heavy rain.
However, underwater the problems are magnified. Tidal changes, storms and contamination by anchorage debris have to be contended with while an excavation is in progress. Decisions to leave an object in situ until it has been surveyed and photographed properly have to be resisted. The archaeologist, whether professional or amateur, should be capable of fixing the location of his find by reference to pre-determined datum points at the time of discovery and he should always carry a camera and a scale. This is not to advocate a policy of snatch and run, but an object or a sample swept away by a spring tide is of no use to anyone. Working in the turbulent shallow waters of the reefs of the Cape Verde Islands, I learnt that everyone, professional or amateur, had to be trained to record as they worked. There was often no going back; at least until the weather changed.
A supply of small sandbags stored close to the excavation are useful to support or protect an area while seeking additional help or advice but leaving an object exposed to tidal changes is a matter of good judgment and experience. Because there is no rule of thumb about when to leave and when to lift, it is essential that an experienced member of the conservation team should be a diver. The senior conservator should be aware of the situation underwater and be prepared to evaluate the natural threats to the underwater deposits, ensuring that each diver recognised the problems and was properly briefed and equipped before each dive. Preservation and documentation begin at the moment of discovery.
Whenever possible, objects should be lifted in a rigid container (not in a bag) and packed with a representative sample of the surrounding matrix to limit movement and damage during recovery. A supply of numbered plastic boxes with attached lids should, like the sandbags, be stored close to the excavation. If surface conditions are severe, the containers, suitably numbered and recorded, can be stored on the bottom until calmer conditions occur.
Backfilling, to protect a site between working periods requires judgement, experience and a thorough knowledge of seasonal changes in the area around the wreck. Once again there is no rule that applies to all sites. Knowledge of the site and an evaluation of any potential hazards, which might threaten preservation, are essential. Discussion with local fishermen whose lives depend on their knowledge of the local area can be very rewarding, particularly in parts of the world where meteorological data is difficult to obtain. Tide Tables are a useful guide, but it is more useful to calibrate any variations for a specific site and produce modified and reliable data. In many parts of the world, tables do not exist or they were drawn up in the dreamtime. It is easy to construct a tide gauge on a nearby beach and monitor the changes through a lunar month. The golden rule is that preservation, i.e. protection, begins at the moment of discovery and not when the object is sent ashore to the laboratory.
In Guernsey, Channel Islands, the remains of a 3rd century merchant ship lying in shallow water (4.8 m below chart datum) were covered with 16 tons of sandbags to protect the timbers. They were swept away in one storm leaving the site dangerously exposed (Rule and Monaghan, 1993). Thankfully, volunteer divers braved the winter weather to replace them, building the mound carefully like a Flemish bond brick wall. Their care saved the timbers and they are now awaiting display in St Peter Port.
Excerpted from Conservation Science by Eric May, Mark Jones. Copyright © 2006 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Meet the Author
Eric May is Reader in Microbiology in the School of Biological Sciences at the University of Portsmouth. He has worked on stone deterioration for 20 years and recently coordinated a EU study to assess the value of biotechnology for remediation of altered stone in buildings. He organised (with Mark Jones at the Mary Rose Trust) an international heritage meeting HMS 2005 in Portsmouth in June 2005.
Mark Jones is Head of Conservation at the Mary Rose Trust, responsible for the treatment of the Tudor warship, The Mary Rose, and her artefacts. Part-time Lecturer in conservation at the Universities of Portsmouth and Southampton, he has an MSc in the biodeterioration of materials and a PhD in marine fouling.
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