Returning a 300-year-old cannon to its former glory: an insight into our materials conservation department
Video | Updated 2 years ago
After a long period of time spent underwater, sometimes more than three centuries, metal artefacts undergo critical transformations. Over time they become covered in corrosion products and start rusting. It belongs to the Western Australian Museum to preserve this historic metalwork and, thus, preserve the State’s scientific and cultural heritage. In this video conservator Jon Carpenter is talking about the methods employed by the Materials Department of the Museum to clean up and stabilise cannons excavated off the Western Australian coast.
Hi, my name is Jon Carpenter, I am the metals conservator at the department of materials conservation, Western Australian Museum, and today I am going to be talking about a cannon and how we conserve(d) this cannon.
Conservators are mainly concerned with materials from which these objects are made and that’s our main interest, I’m interested in the fact that it’s a cannon but it is the materials that we are concerned about. This is a cast-iron cannon and essentially we have to remove chlorites or salt from this object otherwise it would just corrode. When it falls into the sea, when the ship sinks, eventually these sorts of things end up as the main materials on the surface of the shipwreck and they become covered in corrosion products and start rusting and then sediments in sea bed materials, shells, corals, other objects on the wreck can all become attached to it. It is called concretion; it’s a layer that forms on the outside. This layer obviously has to be removed. When we find the shipwreck, when we find the cannon, first of all we have to raise it with lift bags so we have to detach it from the sea bed if it has become attached to it, and we attach lift bags, inflate them with air and raise the cannon to the surface and then maybe tow it to the shore, put it onto a truck, we will carefully wrap it, we hope the concretions is still intact because that stops it drying out, we want to keep this wet, and we bring it back to our laboratory. Once it gets here we can remove the concretion, and that’s simply done with a hammer and chisel. The idea is to crack the concretion, it’s like cement, there is sort of a soft sludgy layer between the hard cement material and the cannon and that allows the concretion to be lifted off. So you crack it, lift off the pieces, we expose the cannon’s surface. Sometimes that concretion also forms inside the bore of the cannon and if we are lucky enough to have a plug or the tampion or pompion, which is a wooden plug in the end, and that survives then the concretion doesn’t form inside and that means it’s clear.
This particular cannon still had its cannonball inside, it had the tampion, or plug, at the end, it had the wadding and we were able to remove all of that successfully. If not, if it had been clogged up with concretion, we [would] have to core it with a coring machine and that actually cuts the concretion out in several stages so we work our way down to the back of the gun. Sometimes the cannonball is still there, we pick it up in the core. Also we have to drill out the vent or the touch hole, because that is usually plugged up as well, and then we put the cannon into a tank containing a solution of caustic soda. Caustic soda is an alkali and that inhibits corrosion, and essentially that gun, this cannon could sit in that solution for a number of years and not corrode. However, what we do is connect it up to a process called electrolysis, the cannon becomes the negative side of the cell and two sheets of mild steel which fit either side are the anodes, or the positive side of the cell. We pass a voltage through of about 2 Volts, that actually drives out the chlorites, also reduces the corrosion products, as you know when iron corrodes it forms rust and that expends, it actually reduces the volume of that material, makes the surface of this cannon porous, so the salt can actually go into the solution. This can take anything up to about three years. A lot of our cannons come from shipwrecks and have been under the sea for three centuries. If you look at about one year per century to stabilize the cannon, that’s been under the water, sometimes it takes longer but on average that is about right. For something like an anchor, which is made of wrought iron as opposed to cast iron it can take about two years to treat. After we have removed the salt we have to remove the cannon from the solution but we have to remove the caustic soda solution which forms carbonate if we just take it out and allow it to dry - causes efflorescence on the surface and that material is obviously undesirable so we have to soak it for a little while to make sure we get all the caustic residues and then we can take it out, we de-water it, we can use methylated spirits to dry it off, and then we cover it with a rust inhibitor. The cannon when it’s been treated is this black colour, the rust inhibitor also enhances that and gives it the appearance that we would accept normally for a cannon. Once that is done we have to put this cannon in molten wax, we have a big tank of molten wax, we place it in the wax, sit it there for about a couple of days, the air in the porous surface comes out and we turn the heating elements off, the wax begins to cool and then it’s drawn into that porous surface and consolidates and strengthens that surface. And then finally we cover it with shellac, and we use shellac because it is compatible with the wax. If you just have a waxy surface, although it is a very hard wax, it can attract dust and make it bit dirty from handling. Essentially that’s the process.
The cannon carriage which we have here is a replica because often nothing survives of the carriage, it does happen sometimes, it completely depends on the environment. All of the fittings usually are wrought iron; they don’t survive normally so this is just a replica so we can actually show the cannon as it probably would have been on the ship. And essentially that is the process.