By Morgan Creed – Conservator
Object Background
The UC-42 was a German World War I mine-deploying submarine. It sank near to Cork Harbour, off Roche’s point, on 10th September 1917 when a mine it was carrying exploded. All 27 crew on board the UC-42 perished in the sinking.
The wreck site was later rediscovered in 2010 at a depth of 27 meters. With the propeller subsequently found by diver Timmy Carey (Blackwater SubAqua Club, Cork) while diving for leisure in 2022. The propeller laid approximately 60m from the wreck.
In June 2024 the propeller was recovered by Mizen Archaeology, supported by supported by the National Monuments Service of the Department of Housing, Local Government and Heritage (Ireland), the National Museum of Ireland and the Federal Republic of Germany through their Embassy.
Condition
The propeller is a cupreous alloy, most likely brass due to its reduced galvanic reactivity in marine conditions. It was covered in marine concretion, which obscured the metal surface. This is a hard, concrete-like crust made up of crushed shells, sand and corrosion: it forms on metal objects while they are on the sea bed. In this case, some large stones had also become embedded in the encrustation, as well as marine biological growths. Copper alloys usually have a much thinner layer of concretion than iron objects from marine sites, as they are less susceptible to corrosion in salt water. This makes removal difficult as hammers and small chisels can easily penetrate through the concretion and mark the object’s surface.
Removal of the concretion determined that de-zincification has occurred in some areas. This takes place when the zinc corrodes preferentially to the copper in the alloy, leaving a red coloured surface. Beneath the encrustation, a rich dark patina was discovered as well as what may be a serial number.
Treatment
Since recovery, the propeller had been stored in tap water, which has allowed for partial desalination. Conservation treatment was undertaken in Kinsale, Co. Cork, to carefully remove the marine concretion, while preserving the patina underneath.
As well as being disfiguring, concretion can be problematic once an object has been recovered. During burial it acts as a barrier to oxygen and slows degradation, protecting the encased object. Once an object is removed from the sea, it must be desalinated to prevent further corrosion and the concretion stops acting as a protective barrier and instead traps salts against the metal, so it must be removed to preserve the long-tern stability of the object.
Following this stage of initial conservation, the propeller is now undergoing a process called passive desalination. The salt from seawater reacts with the copper in the alloy to form cupreous chlorides. These do not readily dissolve in water, so cannot be washed out of the propeller using fresh water alone. The propeller is immersed in a chemical that converts the cupreous chlorides back to salts. These are soluble in water and can be removed by replacing the chemical bath that the propeller is stored in. The Conservation Laboratory at York Archaeology is monitoring the desalination process from York, by testing samples of the propeller solution and measuring the salts it contains.
Once desalination is complete and the propeller is dried, it will go on display at Spike Island, Co. Cork.
