Conservation of archaeological material is dependent largely upon one factor - enough public interest to pay the bills. Wetland and marine sites preserve artefacts that are lost on terrestrial sites, so there is a lot more for people to get interested in. Unfortunately, these artefacts are difficult to conserve, and this results in a high cost spread over a long period of time.

Much of the material we are trying to conserve has been preserved in its waterlogged condition for longer than museums have existed. If part of a conservator's role is to preserve this material, then we must ask whether or not it would be better left where it is...

In these pages we will be examining some of the alternatives to conservation. The two main possibilities are reburial of material after excavating (Figure 13), and preserving the site in situ. Ideally these programs would allow us to preserve cultural heritage, without the expense of conservation, or risk to the archaeological material. To do this effectively we must be able to measure how stable the archaeological material is within a site.

Figure 13. Glastonbury Lake Village - an early reburial experiment

The excavation of Glastonbury Lake Village by Arthur Bulleid between 1892 and 1907 was one of the earliest studies on an English wetland archaeological site. At the time Bulleid determined that there were no acceptable conservation methods for waterlogged wood. As a result this material was either reburied, or stored in water to await the development of better conservation techniques. The stored material was finally conserved in 1962. Reburial of material is very common on marine archaeological sites today, due to the high cost of conservation. The author's picture of the village shown above was possible through a reconstruction of its ground plan by J. Coles and S. Minnit, working from Bulleid's original field notebooks (J. Coles and S. Minnit, 'Industrious and Fairly Civilised', pub. Somerset County Council (1995)).

Further scientific interest in wreck sites arises because they represent human impact experiments that have been running for centuries. Understanding how stable these sites are, and how they interact with the marine environment may shed light on the ultimate fate of the material that has been dumped at sea over the last hundred years. Further pressure for dumping has recently come from the need to decommission old oil production platforms. Initial plans to dump the 'Brent Spar' in deep water off the continental shelf rightly met with a storm of opposition. As the original report on this proposal stated, a study of the deterioration of shipwrecks would have provided an understanding of the likely environmental impact of this proposal, but very little work has been done. In the absence of such an understanding the economically more expensive option of dismantling had to be adopted. More platforms await decommissioning, the pressure to dump cheaply grows. The requirement to comprehend human impact on the seas surrounding us remains. This is a heritage study, which has relevance to all of us...

Measuring Preservation Potential

There are a broad range of factors which can influence the preservation environment of a marine archaeological site, probably the most important is the stability of the sediment cover over the site. When an artefact is buried it is supported from collapse by the sediment, protected from damage by storms, from wood eating organisms like gribble and shipworm, and from looting by divers. Further, the sediment prevents oxygen reaching the artefacts, so bacterial and fungal decay processes are slow. As a result the artefact undergoes slow chemical changes over a period of millennia, but may remain quite recognisable (Figure 14). From the preservation of sub-fossil leaves, we could hypothesise that, under ideal conditions, the artefact could remain intact for several million years. It is unlikely that the human race will survive that long!

Figure 14. Sketch of a semi-covered artefact

Generally, however, we only find marine sites when they become exposed above the sediment. Exposed sites are subject to conditions under which rapid decay may be favourable. The questions are - how rapid, how much of the site is at risk, and what can we do about it...

Exposure is due to sediments moving in response to storms, tidal currents, waves and the wakes of ships. Currents can be localised, giving rise to scour, or only occasionally be strong enough to cause damage (during storms). In addition, some sediment movement is under the influence of gravity, for example, settlement and turbidity currents. Biological organisms, 'worms', squat lobsters and other creatures will also tunnel or burrow into the sediment about sites, and this activity will be independent of water currents.

Clearly current measurement alone cannot determine how stable a sediment is, and large numbers of meters would have to be deployed over a long time to look at this parameter. Such a study might be feasible on a single test site, but would not be a practical technique in the field for determining the risk of deterioration on any given site. As a result, we intend to look at the problem in a slightly different way...

Remember we are not interested in the physics of sediment movement, what is important is - how effectively is the sediment protecting the site?

Sediments not only physically protect a site, they also trap seawater within their porous structures. As this water is separated from open seawater its chemistry will change due to a range of diagenetic and biological processes. When the sediment is stirred up, exposing the site, the interstitial water will be replaced with seawater. A useful measure of how stable the sediment is about the site may, therefore, be how great the difference is between the seawater and the water trapped within the pores of the sediment.

If this is true, our problem to find an easily measured chemical variable, and determine how quickly it changes when it is trapped within the sediment.

As a result one of our goals will be to develop a sensor which can be inserted into the sediment and give direct readings of how long ago the seawater within that sediment was trapped there. In practice, clearly, there will be a lot of problems...

What types of parameter will change?

Dissolved oxygen concentration is the most obvious parameter, as bugs eat the artefact and other entrapped organic matter, the oxygen within the seawater will be depleted. As a result of this, the entire redox chemistry of the water within the sediment will change. Appropriate probes may be simple redox electrodes or more complex oxygen sensors.

Another important equilibrium process that will be upset by entrapment within the sediment is that of the CO₂/bicarbonate/carbonate system. Simplistically this is the inverse of the oxygen concentration gradient, CO₂ being produced by the metabolic processes of the bugs within the sediment as they use up the oxygen. Changes in this equilibrium will change the pH of the water in the sediment, and pH is more easily measured than dissolved oxygen.

Finally, there are chemicals released from the artefacts themselves, metal ions released as a result of corrosion, organic breakdown products as wood and other material is metabolised on the site.
It is our intention to look at all of these parameters, and pick out the most reliable indicators for determining the stability and preservation of the site.

Problems with iron wrecks

The above discussion is general to any marine site - wooden sailing ship, submerged landscape or super tanker. Steel vessels, or wooden vessels carrying large iron artefacts such as armaments, however, pose some special problems and opportunities. Wooden vessels, if not buried in sediment, will normally collapse as the iron fastenings rust away, leaving only a scattering of guns and perhaps some heavy keel timbers on the surface to mark the location of the site. Exposed smaller timbers and light artefacts may become scattered over quite large areas through the action of currents.

Iron and steel hulled vessels are more recent, so whilst the metal from which they are made is likely to disappear eventually due to corrosion, they are frequently in good shape at the present time. It is apparent that burial again helps to preserve these wrecks by limiting oxygen access to fuel corrosion, so again the generic tests above will aid our determining the rate of deterioration. In addition, however, there are well known techniques of cathodic protection that might be used to prevent further decay - at a cost.

Because corrosion is such a major economic problem, methods have been developed to estimate its rate and likely impact on modern structures. A direct measurement of the corrosion rate on archaeological iron could be compared to the known age of the vessel and or the depth of corrosion product on the item. This would tell us if corrosion has speeded up, indicating that the site has recently become less stable.

In short, it may be possible to 'ask' the artefact whether or not it is OK!

Preliminary studies on this topic are being carried out at Cellardyke pool, close to St Andrews. This pool has been set up by members of the Scottish Institute of Maritime Studies and the Archaeological Diving Unit (both at St Andrews) as a site for advanced training for the Nautical Archaeology Society

Equipment for Use in In-Situ Conservation Studies

Phase 1 - Equipment development

In order to examine the archaeological site we must be able to take accurate measurements of the important chemical variables. This process is very much more difficult underwater than it would be in the laboratory. The first stage of this project is to design or adapt equipment to allow measurements to be taken.

Voltages are the simplest parameters to measure and record. These measurements can be carried out using standard equipment - digital voltmeters (DVM's) or data loggers. Data loggers record data, and can be left on site to take measurements when diving is not possible due to tidal currents or storms.

There are a large number of chemical sensors that can be used in conjunction with voltmeters, pH probes, corrosion potential probes, and selective probes for a wide range of analytes. In addition, simple current and temperature sensors can also be added, allowing us to determine important physical parameters.

One of the greatest challenges is to armour this equipment to survive the marine environment, and yet be simple enough for a diver to use easily. Examples of equipment that we have modified for marine work are shown below.

Diver operated equipment

Some equipment, such as the in situ monitor (ISM - Figure 15) has been modified at the University of St Andrews to allow a diver to carry out measurements underwater during the course of the dive. The ISM1 features a small commercial voltmeter connected to a corrosion potential electrode. The voltmeter is encapsulated in a polycarbonate cylinder sealed at either end by brass plates into which 'o' rings have been inserted. The cable from the electrode is sealed to prevent water getting to the electronics. The equipment is switched on and off by magnetic reed switches, operated by magnets through the clear case, so there are fewer holes for switches through which leaks can occur. Polycarbonate is one of the toughest clear polymers available, and this equipment has been pressure tested to the equivalent of 100m depth.

Figure 15.  In Situ Monitor 1

Data logging Equipment

Figure 16.  Data Logger V2

A diver can use an ISM at any time while underwater, but it cannot record events that do not occur during the course of the dive. Hence the need for data logging equipment, like the V2 (Figure 16, shown out of its polycarbonate shell during calibration). This features two independent meters that can be attached to different sensors which are positioned by the diver, who can trigger the start of the record, again using a magnet, and then leave the V2 in place to record information over periods of up to one month.