Finisterre Shipwreck Survey

How to conserve three 16th century bronze breech blocks in plain English!

By October 4, 2013February 6th, 2017No Comments
Bronze Breech Block after conservation (Photo: V. Folgueira).

Bronze Breech Block after conservation (Photo: V. Folgueira).

In the summer of 2012 three bronze breech blocks were recovered from the Ribadeo ship, a likely 16thcentury vessel (https://nauticalarch.org/blogs/finisterre/2012/11/) The blocks presented a similar size and weight, 25-27 cm long and 12-15 kg. They were covered with a concretion caused by iron corrosion products and calcareous depositions from various marine organisms which had to be removed in order to perform a series of conservation treatments to stabilize them, and to prevent their deterioration.

Bronze Breech Block before conservation (Photo: V. Folgueira).

Bronze Breech Block before conservation (Photo: V. Folgueira).

Calcareous concretions (Photo: V. Folgueira).

Calcareous concretions (Photo: V. Folgueira).

The first step was to mechanically clean the surface to remove the concretion. Although it may sound a little extreme, this operation was carried out with a hammer and chisel. The breech blocks were not at risk in any moment because the concretion was much softer than the metal beneath it. During the cleaning phase, some marks etched on the surface were discovered. These are now under study.

Mechanical cleaning (Photo: V. Folgueira).

Mechanical cleaning (Photo: V. Folgueira).

Breech block after mechanical cleaning.

Breech block after mechanical cleaning.

The bore of one of the breech blocks was filled with a homogeneous black substance, wich lead the conservator Victoria Folgueria that it might be gunpowder remains. Some samples were analyzed by X-ray diffraction and X-ray fluorescence to determine the composition and mineral phase. The results were inconclusive, but since it would not affect the future conservation of the piece, Victoria decided not to remove the contents of the bore to allow further investigation.

Black substance remains (Photo: V. Folgueira).

Black substance remains (Photo: V. Folgueira).

Once cleaned, the main concern regarding conservation was the removal of the chlorides produced by the salts in the marine environment. The best removal rates are obtained with electrolytic techniques, so we applied a cathodic polarization treatment of several months. An electrolytic cell was built with the breech blocks as cathode and a stainless steel mesh as anode, and immersed in an electrolyte. A low density, rectified current was applied to start the reaction.

Electrolytic cell (Photo: V. Folgueira).

Electrolytic cell (Photo: V. Folgueira).

Detail of breech block after conservation (Photo: V. Folgueira).

Detail of breech block after conservation (Photo: V. Folgueira).

Once the chlorides were removed, the pieces were thoroughly rinsed and inhibited. Two protective layers were applied to isolate the metal from the atmosphere: a first layer of acrylic resin and a second one of microcrystalline wax to dull the shine.

Bronze breech block after conservation (Photo: V. Folgueira).

Bronze breech block after conservation (Photo: V. Folgueira).

Bronze breech block after conservation (Photo: V. Folgueira).

Bronze breech block after conservation (Photo: V. Folgueira).

I hope you enjoyed the post!

 

Victoria Folgueira

Conservator – Finisterre Team