Page 185 - Manual for Activities directed at the Underwater Cultural Heritage

 © Igor Miholjek, Mladen
Pešic´, Fotodocumentation of the Croatian Conservation Institute. Bronze cannon of the 16th century merchant ship, Sveti Pavao Shallows, Island of Mljet, Croatia.
Surveying the waters surrounding the island of Mljet underwater archaeologists of the Croatian Conservation Institute found
in 2006 the remains of a post medieval shipwreck at the Sveti Pavao Shallows. Among the varied archaeological material found
on this 16th century shipwreck were 7 bronze cannon. During the continued research of the site in late 2007, the bronze cannon 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.
Upon their delivery the cannon were immersed in pools containing tap water, from which they were then individually withdrawn for documentation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 cannon were 6 cannonballs, 3 made of stone, 3
of iron.A preliminary inspection established that all 7 cannon were manufactured of bronze, with 4 sizes present.The surfaces of all
of the cannon 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 cannon.These corrosive aggregations are all that remains of the entirely decayed iron breeches – the cannon's loading mechanism.
In the 16th century cannon 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.
After a preliminary
inspection of the
cannon and the
documentation
of their condition,
the cannon were cleaned of sand and easily removable deposits, and then stored in desalinisation pools.The desalinisation process lasted for 9 months during which, with instruments used to monitor salinity, the water in the pools was changed on a monthly basis. For the first 7 months the desalinisation process took place in tap water, while the last two months the process took place in deionised water.The cannon were removed from the pool and gradually air dried upon the completion of the desalinisation process, which was followed
by the cleaning of cannon no.
6 and no. 2.The cleaning of
these cannon 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.
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 chemical stabilisation 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 were applied, which will protect the object from impurities and harmful atmospheric influences.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 cannon have entirely decayed, and will be X-rayed to establish their shape and to determine further interventions, following which
the conservation work on these cannon will continue.
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Conservation and site management