Wet metals are generally much more reactive than their dry equivalent. Of the factors that affect the deterioration rate of wet materials three are probably more significant for metals than they are for other materials. These are the:
- development or otherwise of a layer of encrusting material;
- concentration of dissolved oxygen available to the metal surface; and
- location and relationship to other materials.
Some metal compositions encourage biological growth. Iron objects provide a good substrate for marine encrustation and often become encased in cement-like concretions. By providing a barrier to the inward diffusion of dissolved oxygen and salts, concretions serve to slow destructive corrosion processes (Figure 2). They also provide reinforcement for the weakened structure of encapsulated objects and provide protection against physical damage that may be caused by moving sand and wave action. Copper and its alloys tend to be toxic to marine organisms and encrustation is generally much less on objects made from these materials than on iron and its alloys.
Dissolved oxygen concentration is influenced by factors such as salinity, temperature and depth of burial in the sediments. The depth of the water at an archaeological site becomes significant as it will affect the ambient environmental conditions. As water movement and turbulence are greater in shallower sites more oxygen is supplied to the metal surfaces leading to increased corrosion rates.
The least reactive of two metals in contact or close proximity can be protected by the other. In this process the most reactive metal or alloy corrodes while protecting the least reactive metal at the same time. For example if copper and iron were in contact the iron would preferentially corrode and the copper would be protected.
A beneficial example of this process is demonstrated by the use of sacrificial anodes attached to the hulls of ships. Similar protection can exist with the metal of an artefact. Unfortunately it will frequently mean that another part of the artefact, or a connected metal artefact, will suffer more severe corrosion to provide this protection. Note that a sacrificial anode in electrical connection with an iron object that remains submerged in seawater will protect the iron object from further corrosion and also begin its desalination in–situ.
Metals and their alloys most commonly recovered from wet sites include gold, silver, copper, lead, tin, pewter, iron and aluminium. Of these pure gold is the most stable and aluminium the most reactive (see Appendix 8). The following sections contain information on the identification, handling and stabilisation of these more commonly recovered metals. The current recommendation concerning the initial storage of wet metals is to maintain them in a wet state.
The appearance of gold and its extensive use in jewellery manufacture usually allow this material to be easily identified. It is possible however, that corrosion products from any alloyed metals, commonly copper and silver, will disguise the gold appearance. Alternatively selective corrosion of these metals can cause surface enrichment of the gold component giving an impression that the artefact is very pure.
As pure gold is soft, mishandling can damage the surface and/or distort gold artefacts. Alloyed copper and silver impart strength to uncorroded gold artefacts but selective corrosion of these metals causes structural weakening.
- store in freshwater as most gold artefacts are alloys. Pure or uncorroded gold may be rinsed with fresh water and stored dry;
- store any gold which shows signs of copper corrosion either in freshwater or preferably in an aqueous benzotriazole solution (BTA, 10 g in 1 L of water); and
- store any gold which shows signs of silver corrosion in freshwater so that any potentially recoverable surface details contained within the corrosion products will be retained.
Silver corrosion products are usually grey or black. Silver salts react in the presence of light and in some instances blackening of freshly exposed silver corrosion products has been observed in strong light. Blue-green copper corrosion products may also be present on alloyed silver artefacts recovered from aerobic sites.
Silver artefacts from freshwater sites are more likely to retain structural strength. As more extensive corrosion is expected in artefacts recovered from saltwater sites, handle these materials carefully.
Retaining corrosion product layers is very important as archaeological information can be preserved and recovered from them.
To stabilise silver recovered from a wet site, observe the following guidelines:
- store objects in freshwater initially. If the silver is sound, clean and uncorroded then rinse the artefact with freshwater and allow it to dry;
- keep the silver in freshwater if the artefact is mineralised or possesses corrosion product layers; and
- store silver-copper alloys in a solution of benzotriazole (10 g in 1 L of water).
Copper and its Alloys
In aerobic and particularly alkaline (high levels of carbonate) conditions, the presence of copper is often indicated by green-blue corrosion products (copper hydroxycarbonates) which frequently stain the surrounding burial material (Figure 3). Under anaerobic conditions protective layers of black copper sulphide are formed instead.
Due to the toxicity of copper compounds towards marine organisms copper objects usually possess little encrustation or concretion.
Selective corrosion in copper alloys can result in the formation of a surface corrosion layer derived from the alloying metal. This complicates identification. For example, under low oxygen conditions the galvanic corrosion of bronze produces a grey-coloured, tin-rich corrosion layer. Weight loss, diminished strength and an increase in porosity result. Corroded bronze objects can appear to be made solely of copper if there is extensive destannification. Dezincification of brass has a similar result.
Although artefacts made of copper and its alloys are frequently recovered in very good condition, support will be needed if extensive corrosion of sheet metal has occurred or if thicker more robust objects have been severely eroded. Handle cast brass and bronze objects with caution as sometimes they will have soft and weakened surfaces due to selective corrosion of the more reactive component metals of the alloys.
To stabilise copper artefacts, follow the procedures outlined below:
- store recently recovered and structurally sound copper objects in freshwater;
- immerse badly corroded and structurally unsound copper objects in a solution of sodium sesquicarbonate (10 g of sodium carbonate and 10 g of sodium bicarbonate in 1 L of freshwater). This washes out the chlorides and stabilises the object.
Iron and its Alloys
The following features indicate the presence of iron:
- a large concreted mass or concretion layer (cement-like corrosion product combined with burial material); and
- orange-brown rust colours (in aerobic conditions).
Iron found in a wet site is usually weakened, with the loss of strength related directly to the extent of corrosion. It is important to distinguish between cast and wrought iron as the effect of corrosion processes is different.
Carbon, some in the form of graphite, is a constituent of cast iron. This inert substance does not corrode and often will help retain the original physical shape and dimensions of the object even when the iron content has decreased significantly. Corrosion is directly associated with weight loss and diminished structural strength. Concretion protects structurally weakened surfaces and reinforces corroded cast iron artefacts.
The corrosion pattern of wrought iron is determined by its manufacturing process. Since the metal is forged, beaten and folded into shape, the object acquires inclusions of impurities and different qualities of metal are laid down. In addition, manufacture sometimes imparts stresses to the metal which become a focus for corrosion activity. Consequently corrosion processes produce a furrowed surface not unlike the weathered grain of wood. The amount of uncorroded residual metal and the extent of interconnection of the remaining structure usually determines how well an object survives.
Handle all concretions carefully as the encapsulating medium, while providing protection for the encased iron object(s), may also contain other fragile components or materials. Provide support and do not lift concretions by protuberances.
To stabilise iron objects recovered from wet sites:
- store them initially in either the water from which they were excavated or in freshwater which contains an alkaline agent such as sodium carbonate (washing soda);
- for long-term storage, keep iron in a solution of sodium hydroxide (caustic soda, 20 g in 1 L of water) or sodium carbonate (50 g in 1 L of water);
- store iron from freshwater sites initially in distilled or deionised water; and
- in the short term, place small iron objects in an airtight container with a desiccant such as self-indicating silica gel.
Note that on safety grounds it is generally advisable to transport iron artefacts in water and not in caustic or other alkaline solutions.
Alkaline solutions will attack composite iron objects such as tinned wares, enamelled wares and any associated organic materials and should not be used to store composite objects which contain these components.
Indicators that an object may be made of lead include:
- a high weight-to-size ratio;
- grey appearance;
- cream-white encrustation (aerobic conditions); and
- areas of black corrosion products (anaerobic conditions).
In wet alkaline environments a combined layer of lead sulphate and calcium carbonate may form. This protective cream-white layer may have surrounding debris attached.
Under anaerobic conditions an inert layer of grey-black lead sulphide will form. Aerobic conditions can transform this layer to lead sulphate. Passivation of lead in aerobic and anaerobic conditions protects it from further rapid corrosion.
Lead is susceptible to attack by organic acids however, such as those found in peat and clay deposits and those formed when organic matter decays.
Handle lead artefacts with gloves as lead and its corrosion products are toxic. Wear a dust mask to avoid inhalation of dry corrosion products.
The passivating nature of lead corrosion products usually ensures that these objects are often found intact. The innate softness of lead means that it is highly susceptible to physical damage. Lead objects are often found deformed, torn, or abraded.
Handle lead objects carefully to prevent further damage to the surface of the lead or to its shape. Large objects, especially those made of sheet lead, will need support to prevent bending under their own weight. Corrosion products impart little reinforcement and easily fracture and detach if bending occurs.
To stabilise lead objects:
- store in seawater or freshwater containing sodium sulphate (1 g in 1 L of water).
- remove any marine growth to prevent attack from organic acids produced during the decay of these organisms; and
- after treatment and when dry, store away from potential sources of organic acid vapours such as acetate adhesives and unsealed wood including composite wood products such as chipboard.
Do not place lead in distilled, deionised or soft tap water as it will speed up corrosion. If untreated lead is stored dry it may make later removal of concretion more difficult.
Depending on environmental conditions, pewter recovered from the sea can be found in excellent condition. Pewter surfaces are usually grey to dark grey in colour. Cracks and damaged surfaces will sometimes be defined by contrasting lighter corrosion products. Surfaces may exhibit a uniform-sized, blistered appearance with occasional large, sporadically distributed blisters.
As pewter is a relatively soft alloy it is susceptible to physical damage. Handle it carefully. Do not force distorted pewter artefacts back into shape as the metal may have become brittle.
To stabilise pewter artefacts recovered from wet sites:
- keep wet and remove any marine growth to avoid potential corrosion problems associated with their decay; and
- store in freshwater containing sodium sulphate (1 g in 1 L of water) and BTA (10 g in 1 L of water).
As for lead, avoid storing pewter in deionised water and soft tap water as it increases corrosion rates.
The type of object (e.g. an ingot or a food can) can indicate the use of tin in its manufacture or construction. Aerobic tin corrosion products are usually crumbly and whitish in colour.
Handle carefully objects made from either tin or coated with tin plate so that the corroded surfaces are not damaged and the intact metal is not subject to additional stresses. Support thin objects.
Initially store tin artefacts in the water from which they were recovered.
Aluminium has a light grey or silvery grey, naturally oxidised surface.
Aluminium objects need support when perforated by corrosion. Avoid exposing aluminium to alkaline and acidic substances and solutions.
Store all aluminium and aluminium alloys in the water from which they were recovered.