British Historic Cannonballs 5

[dik]

Any suggestions on how to keep the cannonballs from corroding on exposure to air?

From the BBC:

The Mary Rose sank in 1545 and was raised from the sea in 1982. The cannonballs on board are difficult to conserve because chlorine has got into them from being in the sea. The chemical has got in all the way to its core. This means that they start to corrode if they are exposed to the air.

Dr Schofield's team has tried to remove the chlorine by soaking some cannonballs in solution. But the researchers found that although they had extracted some they hadn't removed it all. They discovered this when the cannonballs began to disintegrate when they were put on display.

http://www.bbc.com/news/science-environment-43334700

Dik

 

[dik]

Maybe placing them in a partial vacuum and using nitrogen to fill the vacuum... don't know what the reaction is, but, the cannon balls they have under wraps are reasonably good and those exposed to air are cracked and terribly deteriorated.

Dik

 

[bimr]

If this corrosion requires oxygen, then would blanketing the air tight display container with nitrogen do the job? You could purge out the old N2 every few days or so and replace with fresh N2 for about a month or so, just in case there is some residual air in the pores?

People seem to be skipping over the Hunley article which answers these questions:

"Phase Three: Soaking the Salts out of the Hunley

Still underway, this phase is the most time consuming and important: removing the salts that threaten the Hunley’s existence. The process of cleansing the salts (or chlorides) embedded in the iron hull will be attempted in a matter of years to undo the harm the ocean took over a century to cause.

To do this, the vessel is being submersed in a chemical solution designed to soak the salts out. In May, 2014, the 75,000-gallon holding tank the Hunley rests in was filled for the first time with a bath of Sodium Hydroxide. This was the first of many long soaks and the goal of this initial bath was mainly to loosen up the rock-like layer of concretion. Scientists are carefully removing this layer of debris because it inhibits the effectiveness of the conservation treatment from leaching out the salts.

There are often higher levels of corrosion along the seams holding a structure together. Because of this tendency, sometimes artifacts are dismantled for conservation. This approach would be a tragedy in the Hunley’s case. “We worked hard to develop a plan that would allow us to preserve the original fabric of the submarine as much as possible. The Hunley’s structural integrity is important to us, and important to history.” Mardikian said.

During treatment, the Hunley will be constantly monitored and once the chemical bath is saturated with the salts it has leached from the submarine, it will be drained from the tank, neutralized, and replaced with a fresh solution. This process, which is estimated to take approximately 5-7 years, will be repeated until the level of salt in the iron is low enough to allow the Hunley to be rescued from its delicate and dangerous state."


"Iron artifacts from the sea in particular are susceptible to rapid deterioration when exposed to an oxygen rich environment like air. Chlorides from the seawater penetrate the iron on the molecular level. When in solution, these chlorides present no real problem. However, when exposed to air, they slowly dry in crystal form. As the crystals grow, the surface of the metal will expand and flake away, eventually reducing the artifact to a pile of iron oxide dust. Similarly, organic materials will literally collapse upon drying in a matter of days. These reactions are due to a sudden break in the equilibrium reached by the artifact after years of submersion."

 

[dik]

Thanks bimr... the hunley is a classic restoration example and worthy of noting the manner in which restoration was approached.

dik