Conservation Methods

Predicting Preservation

Preliminary evaluation of a site will facilitate planning of excavation and conservation strategies. In some

PH 7

Figure 2 Potential/pH diagram: solid parallelogram illustrates naturally occurring conditions in burial contexts. This is bounded by pH 3.5-9.5 and the limits of oxidation and reduction of water.

Countries this evaluation is considered to be a part of environmental impact assessments and is used to clarify the archaeological nature of the site, rather than to assess preservation of archaeological evidence and identify the nature of the ground environment. This is unfortunate because without any record of how well preserved the archaeological evidence was, it will not be possible to assess how effective any mitigation strategies might have been. Evaluation of sites in England’s Humber Wetlands and in the Netherlands made extensive use of wood, bone, and pollen samples to determine the possibility of long-term site preservation. These can be used for future comparison with new samples taken from the sites and any changes to the environment can be detected. As part of site conservation procedures, both excavation records and scientific samples should be properly curated, ready for future use in reference contexts.

Conservation of Un-excavated Sites

Left undisturbed and with their environment unaltered, archaeological sites can be expected to survive with the evidence they contain in a relatively stable state. Disturbing a site will affect the ground environment and potentially impact on still buried evidence. Construction work may allow greater oxygen ingress, which will raise redox potential and make metals more susceptible to corrosion (Figure 2). Decreasing soil pH, perhaps by acid rain or industrial processes, will also cause more corrosion to metals by dissolving corrosion layers that may otherwise slow corrosion rate.

The increasing emphasis on site preservation and mitigation of construction impacts or land use changes requires close collaboration with both developers and their architects or with land managers. For developments the priority is to minimize its impact on the site. In practice, a figure of 5% loss of evidence has become the norm, as this was deemed acceptable in a study of the redevelopment of the City of York.

By working with developers archaeology may be protected in matters like redesigning building foundations to ensure that piles avoid sensitive areas or reuse existing piles. Even with good collaboration this is not a straightforward process, as there is limited study of piling effects, and distortion of deposits adjacent to piles has been observed. Also piles may open pathways for water movement through otherwise impermeable layers, leading to dewatering of perched water tables or movement of contaminated water into uncontaminated layers. Reactions may occur between the alkaline cements of piles and archaeological deposits of lower pH. This is thought to have occurred between a concrete slab foundation and organic rich deposits in York. Increased soil compaction caused by the weight of a construction may damage objects and structures, as well as cause changes in soil moisture content by reducing voids between soil particles. Knowledge of the soil properties may allow for some prediction regarding these effects.

Excavation

Sites with fragile environments and those under extreme threats from basements, underground garages, mineral extraction, and cuttings cannot be preserved in situ and must be excavated. Site conservation here is neither physical reburial nor the creation of an open access monument, but curation and conservation of the resulting archaeological record.

In contrast, excavation driven either by research or by a curiosity designed to extend the boundaries of knowledge, will involve preservation of both the site and its record. Such excavations should be planned without risk to parts that will remain unexcavated. Excavation strategy will detail either reinstatement methodology or consolidation of structures intended for permanent display. Good site conservation must come via a clear excavation framework which progresses from initial proposal, through evaluation to full excavation with analysis of discoveries and publication of results. Ideally, excavation approval should only be given once funding and resources are in place for this progression, as set out in English Heritage’s Manual for Archaeological Practice (MAP2) (Figure 3). For each stage of the MAP2 process results are reviewed and progression only occurs if the information gathered warrants it. If the process is not completed, the information and finds recovered up to that point are archived.

Reburial of Excavated Sites

Reburial of sites can be a complex procedure. It involves reinstating an environment the same or similar to the original and guarding against climatic and other influences. Structures to be buried may be repaired with soft mortars and clays and secured with geotextiles. It is essential to design the water flow down through the soil so as to avoid physical damage to the archaeology, to avoid feeding oxygenated water into anoxic deposits, and to inhibit worm and insect action. Site reburial is a task that requires professional skills and, often, hi-tec materials. Uninformed and careless use of plastic sheeting to ‘protect’ buried surfaces may lead to disaster from factors such as worm action between the plastic and the archaeology that it covers, as well as from the disruption of groundwater flows.

Reburial of boats is a specialized task and has received considerable study in the Netherlands, where hundreds of vessels have been discovered and reburial is the only option. Working toward successful strategies for reburial of wood relies on scientific research into wood decay in the laboratory, experimental burials, and study of existing burial sites. Research underpins good reburial practice. Collaborative research between the Getty Conservation Institute, US National Parks Service and ICCROM aims to identify optimum methods for reburying Pueblo Indian preColumbian sites that have been exposed for many years.

Reburied sites may need proper management. The footprints of early hominids preserved in volcanic rock at Laitoli in Tanzania were reburied following their excavation in 1978-79. In 1985 the site was found to have been colonized by acacia trees leading to concern that the roots may be damaging the footprints. Between 1993 and 1998 the Department of Antiquities of Tanzania and the Getty Conservation Institute undertook a program of assessment and conservation to develop a reburial strategy. This included the construction of a replica site so that the effectiveness of the actual site could be evaluated. Because of the isolated location of the site, it was necessary to involve the local community in its care and a ceremony was held by the elders of the local Maasai tribe to bless the site and adopt it as a sacred site. Consultation is important, as people and politics can be an important factor in successful conservation strategies.

Conservation of Exposed sites

Successful exposure of sites for tourism requires careful planning and an appropriate budget. Risk

Figure 3 Flowchart for archaeological projects. Information taken from Andrews, Gill (1991) Management of Archaeological Projects, 2nd edn. Historical Building and Monuments Commission for England, London, p. 13.

Assessment of exposed archaeological structures must be followed by solutions that protect from weather, visitors, theft, and vandalism. Shelters or cover buildings may be necessary if risks from the weather cannot be managed. Provision of long-term maintenance schedules will control progressive decay of a site.

Shelters

Even in the most benign climates of the Mediterranean region, shelters may be necessary: at Paphos in Cyprus shelters range from timber and masonry buildings to tent-like hexashelters. In northern climates more substantial cover buildings are generally needed. A novel cover building has been created at Vergina in Greece where a tumulus covering the supposed tombs of Philip II of Macedonia and his family has been entirely removed to expose the tombs then replaced with a concrete dome of similar proportions. Within the space created it has been possible to install auditoria and comprehensive exhibitions of the outstanding finds of the tombs. A similar solution has made it possible to display a Bronze Age wooden funerary chamber in situ at Sziizhalombatta in Hungary. At the Pueblo site of Aztec in New Mexico, the remains of a Great House have been preserved under a reconstruction of the roof structure.

In England possibly the earliest cover building at the Roman villa of Bignor in Sussex was built in 1813 in the form of thatched barns. Custom-built buildings, like the imaginative and stylish construction at Brading Roman villa on the Isle of Wight, use walkways to prevent physical access to the archaeology while offering a full visitor experience of the site. Brading is run by a charitable trust and entrance money is plowed into site welfare.

Cover buildings can protect or damage archaeology according to their design. Their construction should create an internal environment in support of conservation strategies for the covered structures, rather than for the comfort of visitors. Solar gain and its influence on humidity can be problematic in poor designs. Materials like corrugated steel roofs can create baking internal temperatures in hot climates and may cause dripping condensation with cooling at night. In cold humid climates they may form ice overnight which melts during the day and drips onto the site. At Piazza Armerina in Sicily a range of buildings with outstanding mosaics were covered with a lightweight steel framework glazed with plastic sheeting. The impact of the hot Sicilian climate was extreme, rising damp is a problem, and the visitors suffer from the very high temperatures. In the original 1950s design air conditioning was included but due to cost constraints was never installed.

Good management would have altered the design to account for the lack of air conditioning. Conversely the old, but massive construction present in the rustic buildings at Bignor has generally performed effectively as a climatic buffer and it is only in recent times that there has been serious rising damp. The water table has been a problem at the Roman Palace close to the sea at Fishbourne in Sussex. Following excavation in the 1960s, the mosaics were preserved in situ. Because provision had not been made in the construction of the cover building to manage the groundwater, it became necessary to lift the mosaics and re-lay them onto a concrete floor.

Repair and Consolidation In Situ

Exposed sites must have ongoing maintenance if they are not to deteriorate. Weather, visitor-generated damage and improper climates within cover buildings will damage fragile remains. In Tunisia a collaborative program with the Getty Conservation Institute, through its project ‘Conservation of Mosaics In Situ’, has as its aims the better understanding of the causes of deterioration of in situ mosaics and the training of professionals and technicians in their maintenance, conservation, and management. It is essential that these aspects of care for in situ sites should be undertaken by professionals who understand the risks that might result from the use of inappropriate materials and techniques.

The materials used to conserve archaeological sites require careful application. Modern materials are not necessarily the best choice. Although they may be more durable than natural materials, their properties may be very different from those of the original materials. This may result in loss of porosity to a monument, uneven strength distribution, discoloration, and difficulty in removing the repairs. The choice of repair and reconstruction materials must lie with professionals. When in doubt it is often better to use weaker and less durable materials that can be readily reversed in the future. Consolidating wall paintings with moisture impermeable waxes or synthetic resins can cause subfluorescence of salts that will lift and flake the plaster surface, where in the past the salts would have crystallized relatively harmlessly on the surface of the plaster. Some materials like adobe mud brick have received considerable research attention to determine the best way of consolidating them. There also remain ethical arguments regarding the visual appearance of conserved monuments and whether reconstruction is appropriate. Old restorations of wall paintings, such as those at Knossos, Crete, where only small fragments of the original survive, can be rather imaginative and it is debatable whether they offer an accurate record of the original.

Decisions have to be made about the degree of intervention that is acceptable. Should mosaic floors be treated as works of art and be re-laid onto beds of concrete, or worse still have their finest parts removed to museums? Alternatively should these actions be considered to be destroying the archaeological integrity of the site? If mosaics or other architectural components are re-laid onto iron-reinforced concrete, it is probable that at some time the metal will corrode and in doing so will break up the concrete. This was seen to be happening to an isolated fragment of gladiator mosaic at Kourion in Cyprus. At many sites visitors are permitted to walk over unconsolidated mosaics with the inevitable result that tesserae are misplaced or even stolen and the quality of the floor is degraded.

Enclosed spaces need special consideration if their contents are not to be damaged. At the Lascaux caves unlimited access following the discovery of the outstanding Palaeolithic paintings caused microbiological growths that were destroying the rich colors that had survived for millennia and the only solution was to close the caves and build a replica alongside. At the tomb of Queen Nefertari in Egypt, unrestricted visitor access was altering the internal humidity and causing salts to crystallize both on the surface of the paintings and within the supporting plaster. Here the solution has been to severely limit visitors to 150 per day.

Outstanding sites should not be seen as unrestrained sources of income to national coffers. Maintenance is essential and is a continuous process. Vegetation soon takes hold of a site and will disrupt floors and topple walls. How to deal with the vegetation may create issues of health and safety that require risk assessment if toxic chemicals are used to control flora growth in areas accessible to tourists. Visitor routes must be monitored for signs of wear, especially where these involve traversing original surfaces. New routes or raised walkways may reduce risks. At a time when the World’s climate is changing, a continuous watch must be kept for signs of its effects on exposed and unexcavated sites - water tables may rise, coastal sites may be eroded, higher levels of rainfall may exceed the capacity of gutters and drains, and increased temperatures may cause greater salt efflorescence.

Whenever excavation reveals outstanding structural remains, there is a temptation to press for the site to remain open. This is an unwise strategy unless there are the resources and expertise for initial conservation, ongoing maintenance, site management, and proper security. Reburial is often the safer and more predictable option.