Lifting of the cannon
After being lifted from sea, the cannon was immersed in tap water in a temporary wooden storage tank and immediately transported to Evtek.
Before beginning the conservation treatment, further corrosion was inhibited by increasing the pH of the water with potassium hydroxide (see Pourbaix Diagrams).
The conservation plan is to use cathodic polarization at constant potential to remove concretions from the surface (cleaning process) and to extract chlorides which have penetrated into the metal (stabilization process) (see Application for Conservation Purposes).
One cannon from the same wreck was treated previously with hydrogen reduction. This treatment was unsatisfactory as one of the trunnions fell apart and there was a noticeable decrease in the hardness of the cannon's surface. By using another conservation procedure, it will be interesting to compare the results of the two different methods.
The treatment began in January 2001 when the cannon and the new stainless steel tank were moved into the school's laboratory. Stainless steel was chosen for the treatment tank as it can be easily cleaned and reused. Because of the large size of the cannon and the tank, and the small size of the door to the marine conservation laboratory, the lifting operation had to be carefully planned in order to move them indoors safely.
Mechanical removal of large concretions to speed up the treatment process
The cannon was heavily concreted in some areas with several large stones attached. In order to speed up the treatment process, some of the concretions were mechanically removed. During the cleaning process, a piece of waterlogged wood was found in the concretion. The analysis of the wood is in progress, but it is thought that it may be part of the cannon's carriage.
A stainless steel mesh cage was constructed to enclose the cannon, this will act as the anode during the electrolytic process. The cage must not touch the cannon or the tank and is separated from the latter using sheets of high density polyethylene. V-shaped supports for the cannon were also constructed from high density polyethylene. The cannon was placed into the tank upside down to prevent damaging any markings on the top surface during the first polarization. The cannon will be turned over later in the treatment when all concretion will be removed.
Drilling through the graphitized layer
Checking the conductivity between the two contacts
Two holes were drilled through the corrosion layer (approx. 1 cm deep), to obtain good electrical contact with the remaining metal surface. The conductivity between the two contacts was checked with a multi-meter and the cage was then closed. Steel rods were placed into the holes drilled in the cannon and connected to the negative output of the power supply. The cage was connected to the positive output of the power supply, the contact point is kept outside the electrolyte to prevent it being oxidized by galvanic corrosion (the nuts and bolts are stainless steel but copper wire is used to make the connection to the power supply).
Schematic diagram of the electrolytic treatment
A 1% potassium hydroxide electrolyte was prepared and poured into the tank. Potassium hydroxide was only available in pellet form in Finland however, for practical reasons, it is preferable to purchase it as a 30% w/v solution in 30L drums. Safety protection is required when working with caustic solutions. The Ecorr of the cannon was measured before beginning the polarization to check the contact between the cannon and the steel rods.
A catholic potential (Ec ) of -0,95V/Ag-AgCl will be applied during the electrolytical stabilization of the cannon (see Electrolytic Processes). This value was chosen as it favors the extraction of chlorides and avoids hydrogen bubbling. Because most of the concretion was already removed, strong hydrogen bubbling must be avoided as it may fracture the graphitic corrosion layer holding the archaeological information (inscriptions). As the corrosion layers are not highly conductive, it was necessary to initially increase the current by adjusting the power supply to obtain the desired potential at both contact points. The current must be increased slowly to prevent strong bubbling of hydrogen. For the same reason, it was necessary to reduce the current once -0,95V/Ag-AgCl was obtained. The plot below indicates how the cathodic potential could be adjusted by changing the current:
Ec is monitored daily during the first week, then every other day the following week, and once a week after that. Ec is measured by connecting a reference electrode and the cannon to a multi-meter (see Measuring Ecorr ). Samples of the electrolyte solution are also taken weekly to measure the quantity of chlorides extracted using a titrator.