The bell of the Hercules
Maritime
Lanka
 
Last modified:
23 Dec 2001		  

The bell of the Hercules: conservation
Adapted from the 1992 report of the Galle Harbour Project

The bronze bell of the Hercules was discovered in 1992, wedged in a crevice between three rock boulders. In this position it was almost entirely exposed to the sea water. The inner and outer surfaces were covered with an assortment of marine growth averaging less than 5mm thick (discounting the larger living forms such as sponges and bivalves, particularly those inside the bell, which averaged 30mm thick). The encrusting growth had caused the bell to become attached to the boulders at the points of contact. Part of the bell's crown was missing, including the loop for suspension; the iron clapper had also been lost. In some areas where the actual surface of the bell was visible, it appeared to have a green patina, which changed to a grey-green colour where there was surface loss due to corrosion. In the latter case this indicated that the surface had become at least partly mineralized and possessed a more porous structure, which is relatively soft compared to uncorroded bronze. After photographic recording and in situ examination, it was decided that the bell was sufficiently robust to withstand handling for recovery.

Despite a search in the immediate area, the missing part of the crown and the clapper from the bell were not found. It does not seem possible that the impact of landing in the underwater crevice was the cause of this damage to the bell. It is more likely that consequences of the ship being wrecked are to blame. The state of preservation of the bell would normally be enhanced if the iron clapper were present, as iron will corrode and provide galvanic protection. Should the missing bronze part of the bell be found, it is likely to be well preserved. The clapper itself is expected to be completely corroded, however, with its shape possibly surviving in the encapsulating concretion. If so, it may be cast and replicated. The bell was discovered less than one metre from three large iron cannon, which may have afforded it cathodic protection.

With photographic and video recording, the bell was carefully deconcreted using an assortment of flat-bladed hand tools including dental picks. Deconcretion of a decorative band revealed the inscription 'AMOR VINCIT OMNIA ANNO 1625'. Some of the letters and figures were worn, probably due to sand abrasion during rough sea conditions. The concretion comprised mainly sand grains and the shell-like structures which accommodate various marine organisms. The surface of the bell appeared to have a layer of calcium carbonate, but it was not totally or densely covered.

After most of the encrustation was removed, some localized areas of the bell exhibited a perfectly preserved bronze surface. These areas possessed bronze colouration and fine lines attributed to the finishing technique used during manufacture. Generally the surface of the bell was corroded to a depth of 1-2mm, and it retained sufficient hardness to permit the removal of the encrusting material without damage. The edge of the lip of the bell and areas around the hole in the crown exhibited slightly more corrosion and abrasion. Overall, the bell is very well preserved after more than three centuries in the sea.

Prior to treatment the bell was stored in sea water; afterwards it was soaked in a solution of sodium sesquicarbonate to remove salts. Samples of the marine encrustations which contained corrosion products from the bell were examined by Dr Ian MacLeod of the Western Australian Maritime Museum Conservation Laboratory, who reported as follows:

'The concretion consists of a mizture of calcareous materials that have been stained with the corrosion products derived from the surface of the bell. The oyster shell concretion has a series of layers of copper (II) hydroxy chlorides resulting from the diffusion of copper (I) chloride complexes. These have undergone a series of hydrolysis and oxidation reactions. As the copper chloride complexes hydrolyse, the acid that is released dissolves the aragonite in the organisms' shells. Changes in pH and the presence of oxygen cause the oxidation of copper (I) to copper (II), which precipitates out as a mixture of the hydroxy chlorides and the hydroxy carbonate malachite (to be confirmed by X-ray diffraction of the corrosion products). 'The other major colonizing species are Bryozoa and coralline algae. Amongst the corrosion matrix, which is typical of an aerobic environment, a small amount of redeposited copper is seen in the form of small lustrous specks approximately 0.2mm across. The redeposition of copper is the result of restricted oxygen access to the micro-environment. The reduction of the corrosion products to the metallic state would have facilitated continued corrosion of a part of the bell.'

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