Home Conservation projects Conservation and Restoration of the Gnalić Wreck Iron Anchors

Conservation and Restoration of the Gnalić Wreck Iron Anchors

One of the most important post-medieval shipwrecks in the Mediterranean Sea was found in the 1960s near the small island of Gnalić, not far from the coastal town of Biograd na Moru. The sunken vessel carried a diverse cargo consisting of the products of artisans and craftspeople, manufactured in the late sixteenth century in workshops from the Mediterranean to the north of Europe. Among the many recovered finds were two large iron anchors, recovered from the sea and displayed in the open without prior conservation. Anchor No. 1 is exhibited at the front of the building of the Biograd na Moru heritage museum, while Anchor No. 2 was installed on an elevation on the square facing the building of the National Museum in Zadar.


Slika 1 Sidro br.1 na pročelju zgrade Zavičajnog muzeja Biograd na Moru

Anchor No. 1 at the front of the building of the Biograd na Moru heritage museum

The anchors experienced rapid deterioration in these conditions and were a danger to the visitors that climbed on them to take photographs. In order to prevent the further degradation of this valuable cultural property both anchors required urgent conservation and restoration treatment. Work was launched on both iron anchors using funding provided through the culture ministry. They were transported to the workshop of the ICUA Zadar conservation and restoration department where, in controlled conditions, the conservation and restoration process was initiated. Joining ICUA specialists in working on these anchors were Jean Bernard Memet and Philippe de Vivies, restorers with France's A-Corros Expertise and specialists in the field of electrolysis procedures involving metal archaeological artefacts, to whom we are grateful for their assistance and advice.


Slika 2 Sidro br.2 na platou ispred zgrade Narodnog muzeja u Zadru

Anchor No. 2 on the elevation on the square facing the building of the National Museum in Zadar


Step one involved preliminary cleaning and testing of the level of degradation on the surface of the anchors. Visible on the surface were corrosion processes caused, for the most part, by the presence of chloride ions. The plan was to have the treatment process focus first on removing these ions applying the electrolytic reduction method.
The removal of chloride using the electrolytic reduction method yields good results when working with large metal objects with a preserved metal core because there is, in the course of the electrolytic reduction, along with diffusion, also an electromotive force at play that directs the chloride ions towards the anode—that is to say it actively extracts chloride ions from the cathode, i.e. the corroded metal object that is saturated with these ions.
As our anchors met all the prerequisites for the success of this method the decision to move forward in its application was the logical choice. Two basins were fabricated to size for the desalination of the anchors.
Once the basins had been made, all large corrosion deposits on the surfaces of the anchors were removed in order to free up space for the electrical contact between the anchors and the source of electrical power, and the anchors were laid into the insulated basins. The electrolytic reduction process was conducted in the same manner for both anchors.


Slika 3a  Priprema sidra za postupak elektrolitičke redukcije   

Slika 3b Priprema sidra za postupak elektrolitičke redukcije

Slika 3c Priprema sidra za postupak elektrolitičke redukcije
Preparation of the anchors for the electrolytic reduction process

Stainless steel screws were fixed at five evenly distributed contact points in the metal core and connected by insulated wires to the negative pole of the power source. Stainless steel netting was then wrapped around the entire surface of the anchors and also connected with insulated wire to the positive pole of the power source. A five per cent solution of sodium hydroxide was then added to the basins to serve as the electrolyte.


Slika 4 Punjenje bazena otopinom elektrolita

Filling the basin with the electrolyte solution

The corrosion potential of the anchor was measured with a voltmeter and a reference electrode and a sample of the solution was taken to measure the chloride concentration prior to the activation of the electric circuit. A saturated calomel electrode was used as the reference electrode, with the first sample of the solution serving as an initial point of reference in monitoring the leaching out of chloride.
Once these parameters were established the power was turned on, beginning the process of stabilising the anchors by electrolytic reduction.
Our anchors are made of wrought iron and the potential value (based on a saturated calomel electrode) required for the electrolytic reduction stabilisation procedure is from -0.79V do -0.99V (≈ -800mV to -1000mV).


Slika 5 Monitoring elektrolitičke redukcije sidara

Monitoring electrolytic reduction of the anchors

Daily monitoring was required to keep the anchor potential constantly within the cited range, checking both the potential of the artefact and the voltage and current, given their proportional relationship. If the potential of the artefact increases, the current must be reduced and, conversely, if the potential drops, the current must be increased.


Slika 6a Radovi na  sidrima nakon elektrolitičke redukcije Slika 6b Radovi na  sidrima nakon elektrolitičke redukcije Slika 6c Radovi na  sidrima nakon elektrolitičke redukcije
  Work on the anchors following electrolytic reduction  


The potential of the anchor was measured at all five contact points at which the stainless steel screws had been affixed (using the reference electrode and a voltmeter). Samples of the electrolyte were also taken every seven days in order to measure the concentration of leached chlorides. When chloride measurements in the electrolyte samples no longer indicated an increase in the concentration, the electrolytic reduction process was completed and the anchors were neutralized with demineralised water.


Slika 7 Sidro br.2 nakon radova na platou domicilnog muzeja

Anchor No. 2 restored to the elevation facing the museum post-intervention


Once neutralisation had been achieved the basins were taken apart and the anchors dried. Drying was followed by the mechanical cleaning of the anchors using a combination of manual, electric and pneumatic tools.
Corrosion inhibitors and final protection appropriate to the outdoor presentation of the anchors were applied following completion of the mechanical cleaning of the anchors.


Slika 8 Detalj sidra br.1 nakon konzervatorsko restauratorskih radova

Detail of Anchor No. 1 following the conservation/restoration intervention


Antonija Jozić