J. Carpenter, I. M. Godfrey, I. D. MacLeod and V. L. Richards
Materials recovered from marine, freshwater or wet terrestrial sites pose quite different conservation problems to those excavated from dry sites. Both the extent and the type of deterioration that occur in wet environments differ significantly from those that take place under dry conditions.
Factors that affect the extent of deterioration in a wet environment include the following:
- degree of oxygenation;
- presence of bacteria, fungi and other organisms;
- period of immersion or burial;
- nature of the material surroundings;
- extent of burial/exposure cycles; and
- development of encrusting concretions.
Warm, moist conditions usually increase degradation processes whereas cool, dry conditions are less damaging. Damp surroundings encourage breakdown of materials through chemical and biological processes. Water is essential for many chemical reactions, the corrosion of iron and the hydrolytic breakdown of the cellulose fibres of wood and textiles being examples of two such reactions. Moisture also encourages fungal and bacterial attack on susceptible materials.
The aqueous environment from which an object has been recovered will give an indication of the type and extent of degradation to which an immersed object has been subjected. Preservation chances improve when the temperature is low and when there are low levels of dissolved oxygen and light. Wood buried in low oxygen surroundings for instance, has been known to survive in marine environments for thousands of years. On the other hand, timbers exposed on the seabed will be broken down rapidly by marine borers, microorganisms, chemical attack and the abrasive action of waterborne particles.
The nature of the material surroundings may affect deterioration significantly. The chemical composition of the burial environment (e.g. acidity, salt content, dissolved oxygen levels), the presence of a sacrificial metal in contact with a less reactive metal (leading to protection of the latter) and the diffusion of metal corrosion products into porous organic objects that then either stain the objects or in some cases protect the objects from biological deterioration are just some of the possible effects of differing burial environments.
For objects found in wet terrestrial sites additional factors such as soil acidity and the chemical characteristics of the ground water influence degradation processes.
Some objects are fortunate enough to be afforded protection from their surroundings by being buried, encapsulated in concretion (a cement-like corrosion product) or by being under the protective influence of another material that is present in a neighbouring object. The process of removing objects from these protective surroundings upsets these relatively stable states. Unless appropriate conservation steps are taken immediately accelerated degradation will result.
Although the emphasis in this section will be on materials recovered from marine sites, the same principles can be applied to objects recovered from freshwater and wet terrestrial sites. Changes are expected in the condition of materials from these different environments with respect to the nature and extent of degradation and obviously the degree of salt contamination.
As wet ceramics and glass have been discussed in separate chapters dealing with those particular material types, this chapter will focus on organic materials and metals. Materials that contain at least an organic component include wood, leather, bone, ivory, horn, textiles and any materials made from natural fibres such as rope and matting. Organic materials may be found as discrete objects or as parts of composite objects that also incorporate other materials such as metals or ceramics.
It is essential to identify the materials from which an artefact is made. This information, when combined with knowledge of the environmental conditions to which an artefact has been subjected, will assist in determining its likely condition before it is handled and treated. Once identified, specific recommendations can be made regarding storage and treatment procedures.
Often familiarity with the type of object is a reliable initial indicator of its usual material composition or likely material of manufacture. The types of corrosion products associated with metallic objects are also good indicators. The colours and formation of corrosion products can provide information about the material type and the burial conditions to which an object has been subjected. In aerobic conditions for example, iron corrodes to produce characteristic orange-brown rust colours whereas copper corrosion under similar conditions produces distinctive blue-green corrosion products. If the site is anaerobic however, metals are generally blackened as metal sulphide corrosion products predominate.
Following excavation and recovery artefacts may visibly react to the changes in environmental conditions and in doing so may provide information about the artefact’s composition. Silver corrosion products for example may blacken when exposed to light. Corrosion products also sometimes produce smells that assist in materials identification. Iron corrosion products for example, have a characteristic odour.
Note that a magnet can be used to detect the presence of residual iron or magnetite.
On-site storage considerations
Following excavation, an artefact is exposed to a set of conditions quite different from those of its former burial environment. It will be vulnerable to both physical damage and chemical changes. For example a corroded iron object recovered from a reducing, anaerobic environment will usually exhibit black or grey corrosion products. Subsequent exposure to aerobic conditions results in a transformation to orange-brown rust. With time, the expanding products of the oxidation process are capable of destroying these objects.
It is important to provide stabilising conditions as soon as possible following excavation and to be prompt when recording information about recovered objects lest changes occur in the meantime. If the documentation procedure is prolonged then keep artefacts wet with a fine water spray and/or dampened cloth covers.
As the extent of material degradation of recovered artefacts is not always immediately apparent, handle objects with great care to minimise further damage. Label all artefacts and storage containers and include precautionary comments if required.
As there are too many possible scenarios for excavated wet objects, only basic guidelines can be given.
The guidelines listed below will afford recovered wet metal objects some protection:
- store in water, with freshwater preferred to salt water. Do not use soft water that is low in mineral salts as it will promote corrosion of lead and lead alloys;
- only store like metals in the same container;
- retain adherent concretions;
- remove the majority of marine organisms and plant matter if possible;
- use common sense if putting more than one object in a container with smaller, lighter objects on top;
- store fragile objects separate from more robust objects; and
- do not store metals in the same container as organic materials unless they are part of an inseparable, composite object.
Further details are provided for individual metal types later in this chapter.
Adhere to the following guidelines for organic materials:
- keep the object wet;
- support large or fragile objects;
- wrap fibrous objects with smooth materials that are unlikely to mark or otherwise damage the surface;
- retain concretions;
- separate objects that are contaminated with iron corrosion or similarly coloured products from uncontaminated objects;
- do not expose objects to direct sunlight;
- store objects at low temperatures (4 - 5 °C) and in the dark where possible;
- change the storage water if biological growth is observed;
- use fungicides in solutions if storage is prolonged and biological growth is a problem. Do not add any fungicides to a storage solution if the object is to be used for subsequent analyses such as radiocarbon dating; and
- begin treatment as soon as possible after recovery.
Upon recovery, do not allow organic materials to dry out. In most cases irreversible damage will be done if this occurs. Wood for example, is prone to shrinking, cracking and warping upon drying (Figure 1). Many objects recovered from buried wet sites appear black and it is sometimes difficult to identify the object’s composition. If unsure of the nature of the object, keep it wet.
When lifting large objects such as wood, or fragile materials such as leather and rope:
- support them on planks, netting or similar materials. Place padding between large objects and the support. If object surfaces are very soft use support materials that will not leave an impression in the surface. Netting without padding may leave damaging marks;
- lift objects by gripping the supports, not the objects; and
- protect against accidental damage by cushioning any fastenings or retainers used to keep the object in place. Provide padding and additional support for any protuberances.
To prevent fibre loss and damage due to water movement when storing fibrous materials such as rope and textiles:
- wrap them in materials such as tulle which will not snag the fibres;
- store them in lidded containers completely filled with water to minimise damaging ‘slopping’ movements; and
- alternatively wrap the objects in tulle and then wrap them again in water-saturated paper towel or cloth and then place the wrapped object in a sealed polyethylene bag or container.
Horticultural gel media products used to retain moisture around cultivated plants are also useful for wet organic materials storage. A wrapping is still advised if the gel is likely to intermingle with fibrous or other similar materials.
Store small objects in polythene bags or containers. Initially water from which the object was recovered may be used to keep the object wet. Placing one bag inside another provides additional protection against both drying out and physical damage.
Use freshwater for longer term storage, in conjunction with an appropriate fungicide if biological growth cannot be eliminated by changing solutions. A range of fungicides is available including Kathon (used as a 0.1 – 2 % solution), Panacide (0.2 %), Shirlan Plus (1 %), Dowicide (1 %), Densil P (0.3 %) and thymol crystals (excess in tap water). The choice of fungicide depends on the nature of the material in storage. Dowicide and Panacide for instance, are not suitable for use with textiles or leather due to their alkaline character (Pearson 1987). Exercise care when handling fungicides as many are toxic. Use appropriate personal protective equipment and observe all precautions described in their respective material safety data sheets.
Begin treating wet materials as soon as possible after their recovery. Long-term storage is not recommended as deterioration not only continues but often accelerates during such storage. If long-term storage is unavoidable, attempt to maintain conditions which minimise deterioration. Keep temperature, light and oxygen levels as low as possible. The latter two factors may be assisted by the use of narrow, deep storage tanks that conform as closely as possible to the dimensions of the object. Use refrigeration to minimise biological degradation and to reduce the need for biocides but do not freeze objects. Do not expose objects to direct sunlight (Keene 1977).
Where possible, retain adherent concretions until treatment begins.
If polythene bags, tanks and other containers are not available, wrap materials in dampened, water-absorbent material such as cotton cloth or hessian (or Burlap in the USA), wrap them again in plastic and seal joins with adhesive tape. Self-sealing plastic cling wrap is also good for maintaining moisture. Inspect the wrapped materials regularly to check that the object is still moist and that there have been no outbreaks of mould.
Packing and transportation
As for on-site storage, care and common sense are needed when packing wet materials for transportation to a treatment laboratory. Find a balance between the maintenance of an appropriate storage environment, support for the object and the weight of the object and its storage medium. If 100 % relative humidity can be assured, actual immersion in water may not be necessary thereby reducing the overall weight. Avoid using biocides in solutions used to transport objects as the chemicals may have detrimental consequences should they leak. Pack objects according to the type of conditions that they are likely to encounter while in transit so that they will not be damaged. Packing to prevent damage to objects being transported by sea for example, is challenging if damage is to be prevented in rough sea conditions. An overview of packing and transportation is provided by Leskard (1987).
Containers used for wet packing must be waterproof with lids that form tight seals. Plastic containers, liners and bags, although still susceptible to physical damage by puncturing or fracturing are preferred to glass containers for transportation.
Avoid problems of excess weight or the possibility of damage being caused to an object by water movement by using damp wrapping and padding materials.
Do not use packing materials that will contribute to the further deterioration of the object or that are difficult to remove when the object is unpacked. Improvise if suitable padding materials such as bubble wrap are not available. Use materials such as the original supporting matrix (e.g. soil, concretion), sawdust, seaweed, undyed fabrics and even moss to maintain a damp supportive environment (Lucas 1982). Preferably wrap the object prior to padding especially if the improvised packing/padding materials could deteriorate and contaminate the object during prolonged transit.
When using improvised packing/padding materials be aware that most countries will not allow the importation of foreign soils and plant materials. Quarantine delays may compromise the preservation of artefact materials.
Do not expose objects to extreme conditions such as direct sunlight during transport. Use insulating materials around supportive packaging to avoid this problem.