The sea, its environment and impact
The sea is a particularly aggressive environment for most archaeological materials. Organic materials – wood, leather, rope and textiles for example are subject to attack from marine borers such as limnoria, and more insidiously from micro-organisms particularly bacteria. The most aggressive of these organisms need oxygen to survive and fortunately the deeper objects are buried the less oxygen there is so we have recovered some fine wooden objects and ship fittings when they are brought nearer to the surface as the sand is shifted by the tides. Bacteria and marine borers are also inhibited by metal salts, particularly copper. Objects, even fragile ones such as powder flasks that are made from wood with copper fittings and ornamentation survive in good condition because they have become impregnated with copper corrosion products.
Water itself is a major agent of decay. Long molecules such as cellulose that are one of the main components of wood are broken down by water and this makes the wood more open to attack by bacteria. Over time much of the substance of wood can be lost leaving the surviving skeletal structure of lignin which is highly resistant to decay only being supported by the water within it. If wood is allowed to dry without proper conservation the surface tension of the evaporating water is sufficient to cause the collapse of the cells resulting in warping, cracking along and across the grain and the eventual disintegration of the object.
The salt – sodium chloride – in sea water is a further major cause of both physical and chemical deterioration. The physical effect is mainly on porous objects such as unglazed ceramics or bone which will have been infused with sea water. If allowed to dry without removing all the salt they will be severely damaged because as the salty water evaporates the salt crystallises in the pores of the ceramic body or other porous material and the growth of the salt crystals exerts sufficient force to break up the object.
Corrosion of metals
Corrosion of metals takes place when there is an anode, which is positive and a cathode that is negative with an electrolyte connects them and allows a current to flow just as happens in a battery. Sodium chloride dissolved in water is an excellent electrolyte when it is in contact with metals Anodes and cathodes can be created in objects made from single metals, for example when one part of the metal is more highly stressed than another – a good example is a nail where the head and the point which have been formed by hammering – or in modern nails by compression – corrode more rapidly that the shaft because they are anodes.
When there are two different metals in contact with each other corrosion can be even more vigorous. Metals are categorised according to their “nobility”, or reactivity. The most noble metals are platinum and gold which are almost entirely resistant to corrosion. Below these the metals become increasingly prone to corrosion with gold followed by silver, then copper, tin and lead with iron as the most reactive. Graphite which has metal like properties falls between gold and silver. More detailed lists have been compiled, such as the galvanic series which is primarily used by engineers and includes many modern alloys but for our purposes the simple list given here is adequate. Sometimes the less noble metal may be lost completely.
This sort of corrosion can be useful. People with iron hulled boats will be familiar with anodic protection which they use to reduce the risk of corrosion of their hulls. A sacrificial anode of a metal that is low in the galvanic series such as aluminium or more commonly magnesium is fixed to the hull below the water line so that the current flows from the anode to the cathode. Over time the anode will be lost but the iron hull will not be corroded. We have used this process to protect some of the cannon on the sea bed by electrically bonding anodes to them so that not only is the iron protected from further corrosion but the chloride of the salt (sodium chloride) is also removed from the iron, We anticipate that this will reduce the time needed to conserve the cannons when they are removed from the site.
While iron objects are on the sea bed they become encased in a material called concretion. Concretion is as the name suggests, a concrete like material composed mainly of calcium carbonate and iron salts. The formation of concretions is a complex process that is first started by algae and coral and then the corrosion processes described above by the diffusion of iron ions and hydrogen from the metal and chloride ions into the metal. As the iron ions move further from the source metal they are precipitated as iron oxides and oxyhydroxides, with some of the iron replacing the calcium carbonate. The developing concretion eventually seals the object from the oxygen in the seawater and the continued development of the concretion depends on the action of bacteria that do not need oxygen but which can get their energy from electrons released by the artefact. This process continues until there is no iron left at all. The concretion, now very thick, becomes an accurate mould of the original object from which it is possible to make an exact replica by filling the void with a plastic material.
Read more about the immediate actions required in conservation.