The conservation programme (Rule 24)
The scheduling of a conservation programme is a priority in any underwater cultural heritage project. It must ensure long-term preservation of the site and the artefacts, whether the decision is taken to keep them in situ or to extract them. The programme needs to plan well-ahead of the start of the project for the following actions that occur throughout its duration.
The conservation programme thus sets out the guiding principles but also plans the following activities in every detail:
- Documentation - design of finds records, registration, condition report, monitoring systems and site inventory (see Rule 26);
- Recovery and transport of the artefacts from the archaeological site to the on-site workshop, if artefacts are not preserved in situ (see Rule 24), and the transport from the on-site workshop to the conservation laboratory.
- Artefact treatment, preventive conservation of artefacts and intermediary storage – design of the methodologies and techniques applied (see Rule 24);
- Long-term storage – long-term archival deposit of find archives (see Rules 32 - 34)
- Curative conservation and restoration treatment (see Rule 24)
- Transport from the laboratory to the exhibition site (museum)
The budget is established based on the conservation programme and the necessary equipment is acquired.
Conservation before intervention:
- Prospection and documentation: During prospection, underwater archaeologists generally undertake some preparatory dives and sampling to confirm the archaeological importance of the site. This first prospection allows archaeologists and conservators to obtain a sound understanding of the nature, number and type of artefacts that are likely to be discovered. At the same time, they also obtain a good understanding of the whole site and all of its parameters (type of substrate, seawater parameters, hydrodynamic currents, tides, etc). This data will allow them to prepare the excavation project under the best conditions and fully aware of all the likely circumstances.
- Preparative work: Documentation at the preliminary stage will allow conservators to organize the adequate material needed to take care of the artefacts that are expected to be excavated (including materials for marking, recovering, conditioning, transporting and storing the artefacts). Usually, conservators will propose an on-site intervention kit and a list of materials that underwater archaeologists might need during their work.
These two preparative steps are important in regard to the security of artefacts and those working on the site. They also provide information that is valuable for accurate budgeting of the excavation. Conservation can be time consuming and costly, and must be properly considered prior to the excavation and recovery of archaeological material from a site.
The conservation programme should be integrated into the budget and the costs related to the preventive conservation should be distinguished from those related to curative conservation and restoration.
An underwater excavation should not start until a storage place and the budget for conservation have been decided and secured.
Conservation during intervention:
- Preventive conservation on site
- Preserving in situ versus recovering objects
- Lifting, handling and transporting recovered objects
- Storing recovered objects
Preventive conservation on site
During archaeological work, as soon as sand and/or silt is removed from the site, the natural physical and chemical equilibrium between the artefacts and the bed in which they lie is changed and the degradation process (re)starts. Therefore, objects should not be removed until secure arrangements are made to conserve them properly, especially if objects are taken out of the water.
The first steps of preventive conservation are emergency measures, like the first aid that a soldier receives from a doctor on a battlefield. It is about insuring perpetuity and integrity to the collection, but also about guaranteeing accessibility. At this stage, all artefacts need to be given the same attention. Limitation to only valuable artefacts based on the quality of the material or the good condition of conservation, increases the serious risk of neglecting other items that do not originally seem important, but may later reveal essential information following proper conservation and restoration.
Preventive conservation work has to be framed within the same professional standards that are applied in full conservation and restoration. Interventions should be immediate and minimal. They should be stable, reversible and recorded.
Preserving in situ versus recovering objects
The reasons for the recovery of artefacts must be well defined prior to the commencement of any excavation project. The scientific reason for the project should outweigh the damages caused by the artefact extraction to the integrity of the site. In addition the financial support for conservation, storage or exposition has to be assured. Months and often even years can elapse between the discovery of an underwater site and initial probes, actual excavation and the raising of objects.
Leaving artefacts on site
Displacing an artefact from a site changes its integrity, as the site is no longer complete. It also extracts the object from its authentic context, so that a very comprehensive documentation is needed to avoid depriving it of its historic sense. It is therefore often wise to leave sites intact for the scrutiny of future generations, in abidance of the principle in Rule 1.
There is also the issue of the financial burden posed by excavation and the ensuing need for conservation and storage. These considerations have led to the gradual emergence of preventive in situ conservation. Nevertheless, if the decision is taken to leave artefacts on site, some precautions must be taken regarding the further degradation of the artefacts as well as the risk of looting.
Three major approaches for in situ treatment are viable, which may be classed according to the materials being protected:
- an organic heritage approach favouring reburial of a site and follow-up over time consisting of probing, studying and excavating followed by reburial and subsequent monitoring of the remaining site;
- the built heritage approach, which first engages in preventive conservation and restoration work, but ultimately focuses on the creation of underwater archaeological parks;
- the metallic heritage approach, whereby preventive conservation prepares for excavation (extraction of wrecks and artefacts) or long-term conservation, including, for instance, cathodic protection.
Recovering objects of underwater cultural heritage is a very difficult operation, which requires continuous attention and meticulous planning. It must enable a quick break in the equilibrium between the materials and the environment. Some specific precautions are therefore required to ensure a good recovery. During this operation, the security of divers is always paramount to the security of artefacts. When undertaking artefact recovery, it is important to keep in mind that water and silt offer a natural support for the artefacts. The artefacts can collapse during the recovery if they are not effectively and efficiently supported. Support is even more critical for organic materials or glass. It is thus of primary importance to create and bring an adapted support for each fragile artefact to be recovered.
Immediately after recovery, finds shall be kept:
- waterlogged : fragile objects shall preferably be kept in water from the original location while more robust objects can gradually undergo freshwater baths in order to start the desalination process
- in the dark
- in inert containers
- separated according to materials of composition
- with great safety : weapons and potentially explosive materials should be handled with considerable caution and according to safety regulations
Lifting, handling and transporting recovered objects
The lifting, handling and transportation of artefacts is very sensitive and it requires careful pre-planning and adaptation to suit the specific needs of individual projects, depending on the fragility, significance, location, size and mass of objects, as well as project objectives and available resources.
Lifting and handling artefacts
Prior to lifting artefacts from the seafloor, all finds should be completely uncovered from the context (unless the retention of the surrounding context is important). It is important to move the objects very slowly under the water in order to keep the physical pressure exerted on them to a minimum. There are a number of methods to support finds during lifting, such as flat sheets, self-seal plastic bags, bubble wrap, plastic strings, cotton ties, pallets, large trays, block lifts and purpose-built devices. In any case, it is advisable to allow for decompression stops during lifting. If decompression occurs too quickly, the object may explode or break. The transmission of objects from the divers to the platform/boat staff should occur slowly and gently. Storage bins or containers should be readily available. Particular attention should be paid to large and fragile objects. The exposure of all finds to air and light should be kept to a minimum.
Transport of artefacts
Underwater artefacts should benefit during their transport (in the water, from the site to the workshop or from the workshop to the conservation laboratories) from special protection measures.
- Artefacts must be maintained in a humid state during the duration of transport, as far as possible regarding their dimensions and weight. Artefacts do not always have to remain immerged during transport. However, if they do remain immerged, they have to be properly secured to avoid contact with each other, which can cause damage. Also, the movement of a splashing water mass inside a plastic sheet or a container can be very destructive and should be avoided.
- Artefacts must be correctly wrapped in order to avoid being subjected to shocks during transit. The receptacle or tank in which they are placed must be hermetic, airtight and rigid enough to support their weight. A thin layer of water in the bottom of the tank must ensure the global 100% humidity during the transit.
Storing recovered objects
Immediately after the transit, artefacts must be re-immerged in a receptacle or a tank in an environment identical or as close as possible to that in which they were discovered. If this is not possible, then storage in a 100% humidity atmosphere is acceptable. The aim is to anticipate, restrict or halt any acceleration in the degradation of objects after their discovery and excavation. An appropriate storage must be envisaged in the interest of long-term conservation: every storage action must be planned considering that the storage can last weeks, or even years. Moreover, the solutions adopted for individual objects must be simple and easily renewable.
Each artefact should be wrapped in a specific material (conditioning material and conservation-grade inert material), which avoids shocks while favouring the rinsing procedure. All finds should be stored separately and according to constitutive material as each particular archaeological material is subject to specific degradation. The subsequent work led by the conservator will usually allow the original surface to be ‘revealed’.
Conservation after intervention:
Every project manager must plan all actions to which finds are exposed, from the initial handling in the excavation all the way through to the conservation laboratory in order to ensure an accurate traceability for each artefact. Any loss of material constitutes a loss of information. That is why it is necessary to properly preserve and stabilize the objects on site, or in the laboratory, before any further physical intervention takes place.
It would be presumptuous to try to present the state-of-the-art of conservation and restoration in a few lines but the major stages in post-excavation procedures and methods can be highlighted.
The overall conservation-restoration procedure proposed by conservation laboratories for treating underwater archaeological artefacts can be viewed in four key stages which follow each other chronologically:
The four key stages of the conservation-restoration procedure after intervention are:
- Preventive conservation and storage
- Condition report and diagnosis
- Curative conservation
- Restoration (finishing and long-term conservation)
Preventive conservation and storage
- Preventive conservation and storage: begin as soon as the artefacts break the surface of the water. When the collection enters the conservation laboratory, it is usually stored preventively in the same tank that is used on site, in order to avoid another brutal change in the environment.
Condition report and diagnosis:
- Condition report and diagnosis: upon arrival at the conservation laboratory, every artefact must be precisely marked, identified and described in order to record it and its condition. The condition report, also containing a diagnosis, will ensure proper transmission from hand to hand in the conservation laboratory and allows conservators to decide if complementary diagnoses are necessary (material chemical analysis, radiography, tomography, endoscopy,...). The condition report and the complementary analyses will then allow conservators to decide what kind of treatment will be the most relevant for the materials and the conservation state of the artefact.
- Curative conservation: once in the conservation laboratory, the objects need to undergo a “curative conservation” procedure that comprises several stages of cleaning of concretions, stabilization, rinsing. Cleaning the concretions and stabilizing the degradation, two closely interrelated procedures, help to render the object more comprehensible and allow for later risk-free restoration work. Most objects, especially if they have been buried in a seawater environment for many years, emerge covered by calcareous concretions. Their hardness, thickness and porosity depend on the characteristics of the sedimentary environment. That and the nature of the object itself will determine which cleaning, stabilizing and rinsing methods are most relevant to utilize, whether mechanical (micro-sandblaster, micro-chisel, and scalpel), chemical (immersion), electrochemical (electrolysis) or a combination of methods.
Restoration (finishing and long-term conservation):
- Restoration (finishing and long-term conservation): following cleaning, stabilization is essential, especially when objects come from a marine environment. It is above all based on the swift extraction of salts, especially those based on chlorides and sulphate ions. Some new technologies have been developed to accelerate the salt extraction and reduce the stabilization time, which includes techniques such as subcritical and supercritical fluids and computer controlled electrolysis.
Once stabilized, the objects are submitted to a controlled drying process. The dual finishing phase then begins: restoring their original surface so as to make them “readable” at a glance, and long-term conservation. This step generally consists of a gentle cleaning with vegetable or mineral abrasive, with a view to revealing the original surface of an object with its ornamentation, its designs and/or its inscriptions. The choice of abrasive depends on the hardness of the material.
Sometimes, it is necessary to consolidate the original surface through a specific consolidation and/or filling treatment, using reversible varnish, resins and so on. Decisions on such treatment should be taken in coordination with the curator responsible for the collection. Finally, application of a protective coating (of wax, varnish or resin) suited to the future exhibition site – interior or exterior – will help preserve the objects for the foreseeable future.
Conservation and restoration treatments are carried out using both traditional and technically advanced technologies. They are often long-term, ranging from several months to several years. This is particularly true for the processes related to the stabilization.
Restoration techniques for specific materials
- Metals: Treatments focus mainly on stabilizing corrosion by removing the chloride ions. For larger or more chloride-contaminated objects, the most effective means of achieving that end is through electrochemical treatment by chemical solutions. Electrolysis is used to clean concretions on cannons, anchors and other large metallic objects. Electric current from the power supply will either help remove concretions by causing hydrogen micro bubbling on the object’s original surface, or will spark chemical changes in corrosion products (reduction) that speed up the removal of chloride ions. Electrolysis also helps remove chlorides and surface corrosion products from non-conductive organic materials, ceramics, etc.
- Mineral objects: Controlled air-drying or a consolidation treatment, depending on their conservation condition, follows salt removal which begins by simple immersion in fresh water. Consolidation treatments entail a number of immersions in specific chemicals followed by gradual, controlled drying.
- Organic materials: Stabilization treatments seek to prevent any sudden drying of the object or contact with air that might cause shrinkage or deformation. Two types of treatment are known to stabilize organic objects: gradually replacing water in the pores with various concentrations of polyethylene glycol (PEG) followed by natural smooth drying or freeze-drying, or the ARC-Nucléart method which consists of rinsing the objects in an acetone solution, impregnating wood with a polyester-styrene resin and polymerizing the resin by exposing it to gamma radiation.
- Lithic materials: Stabilization treatment mainly consists of simple rinsing procedures by immersing the artefacts in fresh water.