Home Conservation projects Conservation work on sixteenth century bronze cannons from the Sveti Pavao Shallows site
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Conservation work on sixteenth century bronze cannons from the Sveti Pavao Shallows site

Surveying the waters surrounding the island of Mljet underwater archaeologists of the Croatian Conservation Institute in 2006 found the remains of a post medieval shipwreck at the Sveti Pavao Shallows. Among the varied archaeological material found on this sixteenth century shipwreck were seven bronze cannons. With the continued research of the site in late 2007 the bronze cannons were extracted from the sea and subsequently transferred to the conservation workshop of the CCI's Department for Conservation of Underwater Archaeological Finds in Zadar, where conservation processing on them was initiated.

 

Priprema brončanih topova za desalinizaciju Priprema brončanih topova za desalinizaciju

The preparation of the bronze cannons for desalinisation

 

Upon their delivery to the workshop the cannons were immersed in pools containing tap water, from which they were then individually withdrawn for documentation and cleaning. Delivered to the conservation workshop from the archaeological site along with the cannons were six cannonballs, three made of stone, three of iron. A preliminary inspection established that all seven cannons were manufactured of bronze, with four sizes present. The surfaces of all of the cannons were covered in algae and deposits of calcareous growth and calcification. There were larger aggregations of iron oxide at the rear sections of some of the cannons. These corrosive aggregations are all that remains of the entirely decayed iron breeches – the cannon's loading mechanism. In the sixteenth century cannons were divided into two basic groups based on the type of projectiles they fired: perriers, constructed for firing stone shot and having a mascolo mechanism for breech loading, the group to which cannon no. 2 belongs; and those constructed for the firing of iron projectiles, without a breech, and loaded from the muzzle, such as cannon no. 6.

 

Top prije restauratorskih radova

Top prije restauratorskih radova

Cannon no. 6 and cannon no. 2 prior to conservation work

 

After a preliminary inspection of the cannons and the documentation of their condition, the cannons were cleaned of sand and easily removed deposits, and then stored in desalinisation pools. The desalinisation process lasted for nine months during which, with instruments used to monitor salinity, the water in the pools was changed on a monthly basis. For the first seven months the desalinisation process took place in tap water, while the last two months the process took place in deionised water. The cannons were removed from the pool and gradually air dried upon the completion of the desalinisation process, which was followed by the cleaning of cannons no. 6 and no. 2. The cleaning of these cannons was undertaken using mechanical methods. Rough deposits of calcification and calcareous growth were removed from the surface of the cannon using a chisel, while the remaining products of corrosion were carefully removed from the surface of the objects using precise instruments.

 

Restauratorski radovi

Cleaning a bronze cannon

 

During the cleaning of cannon no. 6 an iron ball was found inside the barrel. The cannonball was entirely corroded with no preserved iron core, and was structurally impregnated with acrylic resin in order to retain its form. To retard the development of further corrosive process on the metal, an active stabilisation chemical procedure was applied to the cannon whereby its surface was treated with a bronze corrosion inhibitor, the BTA solution.
Once the stabilisation of the cannon's surface had been completed, protective coatings of the Paraloid B-72 solution and microcrystalline wax was applied, which will protect the object from impurities and harmful atmospheric influences.

 

Top nakon izvedenih restauratorskih radova

Top nakon izvedenih restauratorskih radova

Cannon no. 6 and cannon no. 2 after conservation work

 

With the interventions that have been carried out, cannon no. 6 has been entirely restored and conserved, while conservation work on cannon no. 2 is in the final phase. The iron breeches on the remaining cannons have entirely decayed, and will be x-rayed to establish their shape and to determine further interventions, following which the conservation work on these cannons will continue.

 

Mladen Mustaček