Artefacts marine

CoUier, J. L., Brahamsha, B., and Palenik, B. (1999). The marine cyanobacterium Synechococcus sp. WH7805 requires urease (urea amidohydrolase, EC 3.5.1.5) to utilize urea as a nitrogen source molecular-genetic and biochemical analysis of the enzyme. Microbiol.—U.K. 145, 447—459. CoUos, Y. (1998). Covariation of ammonium and nitrate uptake in several marine areas Calculation artefact or indication of bacterial uptake Preliminary results from a review of 76 studies. In Integrated Marine System Analysis. Dehairs, F., Elskens, M., and Goeyens, L. (eds.). Vrije Universiteit, Brussel, pp. 121—138. 

Research in dimensional stabilization over the last 50 years has achieved limited but specific goals it has yet to fulfil its broader objectives. Niche markets, sueh as the use of polyethylene glycol (PEG) in the treatment of marine and waterlogged artefacts are well established. New technologies for commercial scale treatment of timber by acetylation, and for various heat treatments, have reached the market in the last decade, and may increase in importance in future. 

The minerals from which the metals are extracted, existed for millions of years in the earth s crust and are the most stable form of the metal. A considerable amount of energy is required to convert this mineral into the metal. Once this pure metal comes into contact with the natural environment such as sea-water or soils, the metal slowly converts back to its original starting material. Iron, for example, is obtained from the mineral, haematite, an oxide of iron. Once the pure iron comes into contact with water and air (oxygen), it slowly converts back to the oxide. This is called corrosion and the product is familiar to everyone as red rust. Nearly all metals will corrode in natural environments although the rates of corrosion will vary from metal to metal and alloy to alloy. In addition, the rates of corrosion will vary from one natural environment to another. Iron will corrode at approximately 50 pun per year in freshwater but at 120 pm per year in seawater. The reason for this is due to the difference in chemical composition between freshwater and seawater. The latter contains salt (sodium chloride) and this is very deleterious to the corrosion behaviour of the metal. Silver artefacts may be excavated after several hundred years buried in soils with only minimal amounts of corrosion. Those recovered from marine sites after a similar period of burial, have completely corroded and have reverted back to 100% mineral. This is entirely due to the presence of chlorides in seawater. 

In order to fully appreciate the reasons for carrying out the conservation method selected, it is important to understand in the first instance how the metal or alloy was manufactured. From modern theories of corrosion of metals in marine environments, it is possible to predict the mode of corrosive attack that the artefact may have experienced while being buried or laying on the bottom of the ocean floor. Any adverse effect on the rate of corrosion on exposure to the atmosphere can possibly be predicted. From this knowledge, the most efficient methods of field treatments, storage conditions and conservation can be recommended. 

Silver items recovered from marine sites are often completely mineralised due to the non-protective nature of the corrosion products formed in both aerobic and anaerobic sites. The corrosion product is either silver chloride (AgCl) or silver sulfide (Ag2S). All silver artefacts recovered from the Mary Rose were found to be in very poor condition. 

A relatively simple method is to dissolve out the chloride ions by immersion in a suitable solvent. Water has been used with the water being changed every month until no further chlorides are detected. This can take up to 5 years for marine artefacts with high levels of chloride buried within deep rust layers. Moreover, the metal will continue to corrode, while the artefact is immersed in the water for this length of time. By altering the pH of the solution it may be possible to dissolve out the chlorides without corroding the metal. This is achieved by forming a thin, passive film approximately 10 nm ( 10 9m) thick... 

For artefacts recovered from marine sites, they are often covered in concretions. These are hard layers of calcareous deposit derived from decaying shells of aquatic animals (e.g. barnacles, mussels, etc.) or hardness salts present in seawater. The latter is present as soluble bicarbonate ions (HC( )3 ) in sea or fresh water. At cathodic sites on the metal surface, there is a rise in the local pH due to the production of OH- during the reduction of dissolved oxygen gas (see Equation (10) in corrosion section). This results in the precipitation of solid calcium carbonate (CaC03) scale on the cathodic sites according to the following reaction ... 

The principle of this method of conservation is to immerse the artefact in a tank containing a suitable solution. The chloride ion dissolves from the rust him into the solution that is changed, initially, every week and subsequently every month. The chloride content of the solution is analyzed at the end of each changeover. The process is continued until there is no more chloride detected. At this point, the artefact is deemed to be conserved. This can take up to 5 years for marine artefacts with high levels of chlorides buried within deep rust layers. Even after this length of time, one is not absolutely certain that all the deleterious ions have been removed from the rust/metal interface. 

The marine environment presents a hostile and seemingly unlikely situation for the survival of archaeological wood, yet it does survive. Normally, wood does not survive long enough in marine environments to enter the archaeological record because of the activities of wood-boring animals and aerobic microbes. However, studies have shown that rapid burial in the anoxic sediments of the seabed will protect ships timbers and wooden artefacts from the physical, chemical and biological processes that influence the deterioration of exposed wood. 

As a rule, when the iron artefacts emerge from long periods in sea water, they are covered with concretions or gangue (a mixture of calcite, quartz and marine organisms) which hide their original surface. The first step in treatment is to remove these concretions, which can be done using electrochemistry. 

When artefacts emerge from marine environments, the ojuantity of chlorides that need to be extracted can be very high it can reach up to several kilograms of chlorides for one ton of cast iron. The baths must be regularly renewed as soon as the solution becomes saturated with chlorides. 

Fig. 5. Corrosion rate data from natural and archaeological analogue studies (after Miller et al 1994). The corrosion rates for the archaeological artefacts range from 0.1 to 10/xm a (note that samples 1 and 2 are from oxidizing marine conditions details of all other samples included in Johnson Francis 1980). The Nagra base case corrosion rate for steel canisters from Projekt Gewahr is also shown for comparison.

Holst, P.B., Anthoni, U., Christophersen, C., Nielsen, P.H., and Bock, K. (1994a) A racemic diterpene from the marine bryozoan Flustra foliacea, natural product or artefact Acta Chem. Scand., 48, 765-768. 

The major polyunsaturated fatty acids all contain cis methylene interrupted sequences and for years it was thought that most conjugated systems were artefacts of isolation. However, many such acids have now been firmly identified and are found in sources as diverse as seed oils, some microorganisms and some marine lipids (especially sponges). An example of one such acid would be a-eleostearic acid (Table 3.3). 

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