Although freshwater does contain dissolved minerals, we do not have to contend with the large amount of chloride ions that are in sea water. Chlorine is particularly troublesome with metals such as iron and copper because of its strong negative charge, which gives it a strong attraction to positively charged metals. When combined with an electrolyte solution such as sea water, this strong attraction sets up galvanic electrical currents that lead to corrosion and metal loss.
In freshwater, metals combine with oxygen and other dissolved anions (electrically charged particles) such as sulphates and phosphates. The biological activity in freshwater is different from that in seawater and there is no precipitation of the calcium carbonate that leads to the massive concretions that occur on marine objects. One corrosion problem that does occur with metals in freshwater is increased galvanic corrosion, which happens when two dissimilar metals are indirect contact. (See the picture of the copper kettle.) When copper and iron are in direct contact, the iron corrodes preferentially, becoming what is termed a sacrificial anode. This kettle is from an inland submerged riverine site, at Horsetail Falls of the Granite River, in the Gunflint Trail region of Minnesota.
The iron handle of this copper kettle has completely corroded, but has left the iron minerals from the corrosion products behind on the copper surfaces.
Water-logged organic materials such as wood, leather, and other fibrous materials can be very fragile, while appearing to be in good condition. Do not be fooled by appearances! If allowed to dry out completely without special treatment, these objects will be destroyed with little hope for restoration. Valuable historical and archaeological information will be lost forever.
The conservation of water-logged organic materials requires extensive knowledge of chemistry and currently available practices, and often requires specialized equipment and supplies. Treatment is often slow, and can be expensive. It is for these reasons, and others, that it is best to leave these objects where they are. If that is not possible, then contact a qualified conservator to help with detailed recovery and conservation plans and specifications.
“Put it back”
An option that is somewhat akin to the conservation philosophy is the current “put it back” movement among well-meaning avocational and professional underwater archaeologists. Although it sounds simple and effective enough at first look, this is not necessarily the correct option in all cases.
Particularly with organic objects, re-submerging them after they have dried or been treated can spell disaster. Water-logging them again will cause stresses in and between fibers and structures that have already been weakened first by the previous water-logging,
then by the equilibration to the air. Treatments may have introduced chemicals into the materials that make them water-resistant and have changed their density. Metal objects will be less affected by putting them back into water, but there are still risks and questions to be answered. These include: Is the object structurally stable? Will the increased pressure affect it adversely? Is the environment to which it will be returning the same over all as when the object came out, or have certain parameters changed?
It would help answer the questions we have about putting objects back on sites if we had a body of data on objects that have been put back so far. Written and photographic documentation of the bjects, with as much recovery and treatment history as is possible, will help conservators assess and analyze the effects of resubmergence on objects.
This article may have raised more questions than it has answered. Wet and water-logged objects pose complicated problems, but the stories that they can tell us are invaluable historical resources. Cooperation among all interested and involved parties can go a long way in solving some of these problems.
General Guidelines will have to wait for another post...