- if these artifacts have to be stored for weeks, before conservation commences it is much easier and safer to separate the wide variety of concreted artifacts which are made from different materials. Very often different metals in an electrolyte solution such as sea water cause a chemical-electrical reaction, which corrodes the metal.
- Secondly it is also much cheaper to transport smaller items than a large concreted block.
On the other hand concretions can also serve as a protection to the artifact, in the case of cast iron cannons the concretion protects it from physical and chemical damage during transportation from the wreck to the conservation lab.
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The conservator has to determine what constitutes the concretion, whether it is stronger metals such as copper alloy objects The concretion can be safely removed by striking with a geo pick which has a blunt end or hammer. But for weaker metals such as cast iron where this has to be treated with extreme care.
In the Former case: The flat end of the hammer is used and the blows are made perpendicular to the surface of the artifact. Care should be taken for the blows to be made to the concretion not the artifact.
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When objects made from silver bronze or cast iron are being physically deconcreted the outlines of the inscriptions physically appear in the corrosion product layer. If these are not legible there is a great temptation to rub, pick or wash the artifact. One should refrain from doing so because this would damage the corrosion layer and this would make the inscription more difficult to recover after consolidation.
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Acid is sometimes used to deconcrete an object but it is not recommended because it could attack the metal artifact. Though in cases of silver, the composition of the concretion is sufficiently different enabling the uses of acids. Also it is extremely recommended to physically remove the bulk of the concretion before acid treatment.
Once the concretion has been removed from the bulk and the artifacts are separated into material types the chemical threatment stage commences.
IRON
The Principle aim of all marine iron treatment is to remove the chlorides from the corrosion products,if this is achieved the rate of corrosion processes by chlorides ceases and the rate of corrosion is reduced to a reasonable level where conventional anti corrosion methods can operate effectively.
The two most common ways to do so are : Washing Method where the artifacts are immersed into an appropriate wash solution, and by heat treatment methods where the chloride is extracted from the artifact by heating it and chlorides removed by volatilization
- In Simple Washing : the chloride is removed simply by immersing the artifact in a solution of Sodium Hydroxide. Simple washing is easy to carry out but it gives inconsistent results sometimes succeeding and others not.
- Heat TreatmentMethods: The volatalisation of chlorides at high temperatures:Two types of heat treatment have been proposed, heating in air and heating in an oxidizing environment, in addition , with reducing athmospheres, some of the iron corrosion is converted to metallic iron which mechanically strengthens the artifact. When heated in the presence of air the iron is changed into iron oxide and iron chloride gas thus eliminating the chlorine in the artifact. This system was originally proposed by Eriksen & Thegel thus this process hardly results in success in these conditions the iron should be heated to a temperature of over 670Deg. Resulting in the damaging of the artifact.
Therefore if occurring in reducing athmospheres that is in the presence of hydrogen or nitrogen mixtures at temperatures of 300-400 deg. Celcius for periods of upto 60 hrs. Thus an important side effect to be considered is the change in the metallurgical structure of the artifact which is extremely important because it enables the archaeologist to find out the way the item was produced losing valid arcgaeological information.
After removing the Chlorides the Iron is dewaterised. These are taken from te final wash solution and washed briefly in flowing deionised waterthen wiped dry and allowed to drip for 20 mins. The artifacts are then completely immersed in acetone and left there for an hour. When the artifacts appear to have dried they are then immersed in methylated spirits containing 3% tanning acid for approximately an hour then allowed to dry again.
The artifacts are finally ready for impregnation with micro christalline wax which should have a melting point of 70-90 Deg. Celcius. The artifact is immersed in the wax and the process takes approximately 12 hours.
Copper & Copper Alloys
When Copper is recovered from the sea it generally has a layer of Chlorides in the form of Cuprus Chloride & Cupric hydroxy chloride. These compounds occur both as surface corrosion products and deep down in the crevices of the residual metal. In the presence of humidity and oxygen active bronze disease occurs. This is generally stripped by dipping the artifact in citric acid solution about 10% citric acid to 90% water. This is then immersedin a sodium sesquicarbonate solution to eliminate the chloride and residual citric acid.
Silver
In the case of silver to remove encrustations the artifact can be immersed in 10% hydrochloric acid this not only dissolves the encrustations but also removes the copper corrosion products. This is generally removed in 24hours.
Gold
This metal does not corrode in marine environments or in storage conditions, the removal of surface deposits is all the needs to be done. This is usually done by washing in warm soapy water and sometimes when more adherentit is soaked in a dilute hydrochloric solution.
Galvanic Electrolisic can be used but is very dangerous because it an strip layers of the metal itself. It has to be carried out “by ear” therefore very risky.