Conservation (Rule 24)

Pistol originally made of wrought iron, brass and nut wood, from the Cygne site. From top to bottom: concretions, x-ray and reconstruction of the artefact. The iron that had disappeared has been reconstructed on the basis of the imprints in the concretion. Raising objects from the sea causes drying, which accelerates degradation. This causes cracking and corrosion of metals. The worst effects on metals are seen on cast-iron and wrought-iron objects. In the short or medium term, this will cause a partial deterioration of the object’s surface and ornamentation, culminating in the long run in their complete loss. While in the sea environment, dissolved oxygen in the water is known as the driving force behind corrosion for metals. Amounts can vary from one geographical site to another. On a single site, the quantity of dissolved oxygen decreases with depth, temperature (according to the seasons) and the nature of the sediment (sand, mud or rock). Before any intervention can take place, restorers must stabilize the corrosive elements on metal objects and remove the salts from mineral and organic materials (this process is refered to as preventive conservation). Material-specific treatments are then carried out in the conservation-restoration laboratory, using both traditional techniques and the most advanced technologies. The aim is to uncover the object’s original surface and the attributes – ornamentation, manufacturing marks and traces of use – that will disclose its origins, how it was used, and the techniques employed in its creation. The laboratory procedure for

The term ‘conservation’ in Rule 24 refers to the whole subject of care and treatment of movable and immovable underwater cultural heritage. Rule 24 is closely linked to the excavation techniques and objectives mentioned in Rule 16.

  • Definitions

Archaeological finds have often only survived under water by reaching a physical and chemical equilibrium with the surrounding context. These artefacts are particularly vulnerable and their removal from their burial environment speeds up the processes of corrosion and decay, potentially leading to the destruction of archaeological evidence. Conservation and restoration aim at halting these processes, thereby preserving the heritage. They are the essential link between excavation and exhibition for underwater cultural heritage, from the sunken site to the museum. Conservation is, however, distinct from restoration.

Conservation encompasses all measures and actions aimed at preserving cultural sites and artefacts in view of stabilizing their existing state while ensuring their accessibility to present and future generations. Conservation actions can be divided chronologically into preventive conservation and curative conservation:

  • Preventive conservation includes all indirect measures and actions aimed at avoiding and minimizing future deterioration or loss of materials or artefacts. It is carried out in situ within the context and surroundings of an object or a group of objects, or in the excavation laboratory. It should be undertaken regardless of the age and condition of the artefacts concerned.
  • Curative conservation includes all actions directly applied to an object or group of objects and is aimed at arresting damaging processes and, when possible, stabilizing their condition against further deterioration.

Restoration is the continuation of the conservation process, when the latter is insufficient to re-discover the original surface of the artefact (without falsification), aiming at returning to the original appearance of an archaeological item as closely as possible and thereby providing a condition in which the artefact can be exhibited.

The conservation and restoration of underwater cultural heritage call for comprehensive knowledge of the environment in which a shipwreck or submerged site and its artefacts are found, as well as an awareness of the juxtaposition of artefacts and structures throughout a site. Consideration should also be given to the significance of the artefacts according to the research objectives. A familiarity with the materials from which these objects were constructed or which are likely to be found, is also necessary, as is an understanding of the degradation processes they have most likely undergone. Their potential for future analysis should also be envisaged, along with their ultimate use in display or research.


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The need for conservation


The aim of conservation is to preserve or rediscover the object’s original surface according to current professional standards. The main objective is to “make the artefact talk”, via its ornamentation, manufacturing marks, surface treatments, preserved organics and traces of use, about where it came from and how it was made and used.

The environment and its impact on the artefacts

As soon as a land site, a vessel or an object is submerged, it is subject to the impact of the new environment by the infiltration of water in the porosities, corrosion, colonization by fungi and algae, deposition of calcareous species, sand erosion, hydrolysis, etc. A process of degradation begins which is directly linked with the immediate environment and dictated by physico-chemical, biological or geological parameters. These parameters are related to the nature of water, to living organisms (microscopic and macroscopic), and to the type of substrate and silt/sand upon which the site is located respectively. After a few years, equilibrium is achieved between the surrounding water and the artefacts leading to a relative stabilization of degradation processes. Burial in underwater environments may thus have several effects: structures are weakened though they may still appear solid while on the seabed, and layers incorporating sediments and concretions (thick surface overgrowth) may develop.

Recovery and its impact on the artefacts

Raising objects from underwater inevitably results in them drying, which in turn accelerates degradation. This is due to the presence of soluble salts dissolved in the surrounding solutions on the seabed. In the new environment they dissolve or crystallize depending on relative humidity. Damages to the artefacts are likely to occur due to these potentially destructive physical pressures applied onto very fragile objects. Exposure to continuous fluctuation in relative humidity may even lead to the complete destruction of an object. In this way all activities related to recovery weaken artefacts’ structures and surfaces resulting in the cracking of pottery and ceramics, delaminating and crumbling of glass, shrinkage of organic materials such as wood, hemp, leather and fabric and corrosion and cracking of metals. In the short- or medium-term, this will bring about partial deterioration of the objects’ original surface, culminating in the long run in the global loss of all historical, epistemological or technical information, which could otherwise be derived from the object.

As part of “preventive conservation” it is crucial to ensure that from the minute it leaves the water, any object is kept in an environment identical or close to that in which it was found.

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Principal threats to artefacts during and after recovery

  • Drying may result in the cracking and delaminating of surfaces, irreversible shrinkage, salt crystallization and mould growth;
  • Increases in temperature and oxygen may result in increased speed of decay, biodegradation (algae and mould), corrosion, differential expansion and contraction;
  • Increases in light exposure may result in photo oxidation, fading, accelerated decay rates, growth of green algae;
  • Storing different metals together in one solution may result in galvanic corrosion; 
  • Insufficient physical support and poor handling may result in fractures and cracks of the structures;
  • Negligence in labelling, recording and documentation may result in the loss of context;
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Current professional standards


Rule 24 states that conservation shall be carried out in accordance with current professional standards. The conservation standards and ethical approaches that need to be respected in the conservation laboratories are best described as follows:

Registered interventions: all actions which are taken concerning an artefact must be registered in a reference book or database to ensure traceability of each artefact from the site to the museum, and to allow for the understanding of the long-term behaviour of materials. As far as possible, every picture or drawing should be linked to the file and all this information should be retrievable for future research.

Minimalist interventions: The conservator should first establish the necessity of each intervention and measure the degree of intervention necessary to minimize impact on the artefact also in the long-term, and to intervene to the least possible degree.

Reversibility of the interventions: As far as possible, every intervention should be reversible, i.e. any modification made to an artefact should be able to be undone or removed without adverse affect.

Visibility of the interventions: The goal of the interventions is not to create a “new” artefact but to reveal its shape and the archaeological information without losing the history engraved on it by the degradation process. All the interventions undertaken on the artefact must seek to restore the original surface of the object, so that in a glance, the public can easily understand its function.

Fundamental to the notion of archaeological study, the original surface of the artefact corresponds to the surface of the object at the time of its immersion. This surface is not only the area carrying all the ornamentation, manufacturing marks and traces of use relative to where the artefact came from and how it was made and used. It is also that which has been highly exposed to seawater aggression and, later, to excavation operations, removal and studies.

The conservation programme

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