Re-Conservation of the Early Croatian Boats from Nin – Part 1

Funding provided through the culture ministry has seen the International Centre for Underwater Archaeology's conservation and restoration department now in its third year of work on the process of the re-conservation of the Condura Croatica—two late eleventh century early Croatian boats recovered at Nin. The condura type boats were discovered in the 1960s in Nin Cove and were excavated and then extracted from the marine environment under supervision of late prof. dr. sc. Zdenko Brusić. The years that followed saw the conservation and restoration of the boats using the PEG method at the Archaeological Museum in Zadar conducted by Božidar Vilhar. Since the late 1980s they have been on display at the Museum of Nin Antiquities. With the highly vulnerable wooden structure exposed to the unfavourable microclimatic conditions in which the boats were housed, it was not long after the conservation procedure that degradation was observed in the wooden structure. An analysis of the cause of the decay in 2010 pointed to the presence of iron sulphate, i.e. pyrite, in the wooden structure of the boats. The long-term exposure of the boat structure to these unfavourable microclimatic conditions, in combination with the large quantities of polyethylene glycol (PEG) used in its conservation, had led to accelerated oxidation. The oxidation of pyrite produces hydrous iron sulphates (melanterite, rozenite) visible as large quantities of powder formed on the boat structures, and sulphuric acid that penetrates into the structure of the wood and acidifies it, which in the end leads to the accelerated chemical and physical degradation of the wood.

Re-conservation was launched in 2016 with the drawing up of documentation detailing the earlier archaeological and conservation/restoration interventions, the current condition of the boats, and new analyses conducted with the objective of ruling out the presence of new causes of deterioration. Drawn documentation and a 3D model of the boats were created. The report also put forward a plan for future re-conservation work, to be done in phases in the period from January of 2017 to December of 2021, with an option to extend work into 2023 in the event of unforeseen developments.

Phase one of the re-conservation work, successfully carried out in the course of 2017 on both boats, included the preliminary cleaning of the boats and the removal of excess impregnant. The preliminary cleaning included the removal of all visible dirt, i.e. accumulated surface impurities such as particles of dust, sand and impurities created by insects (cobwebs, eggs) and the formed powder (white, yellow and grey), i.e. the visible product of the oxidation of pyrite (melanterite, rozenite) using a vacuum cleaner and various brushes and scrubbers. The removal of the polyethylene glycol impregnant, used in the earlier conservation/restoration process as surface protection, from the surface of the wooden structure of the boats was achieved by melting the coating with a hot-air gun and then removing the excess PEG with spatulas and absorbent paper. This phase of the intervention also saw analysis of the wooden structure for the possible presence of acids in collaboration with restorer Sophie Fierro-Mircovich of the ARC-Nucléart conservation and restoration workshop of Grenoble in France. The analysis established that the wood was acidic in places at which there had once been iron elements, i.e. the nails used to join the planking to the frames when the boats were first built. These acidic areas are visible on the planking as circular openings (holes) with a diameter of about one centimetre, and as wide and deep fissures in the frames.

The removal of the polyethylene glycol from the surface of the boats revealed details of their structure, i.e. the method by which the wooden structure was constructed, the appearance of the frames and planking, the positions of iron elements and wooden pegs, metal ties used to bind wooden sections, and the method used to fill in gaps. The newly discovered details were documented in drawings in 2016, which was also the start of work in 2018. The 2018 phase of the re-conservation work included the cleaning and removal of acidic sections of the wood and other products of the oxidation of pyrite. All visible circular openings in the planking and all fissures in the frames were tested for the presence of acids using demineralised water and universal litmus paper, with the paper turning from dark yellow to red in the presence of acid. Removal of acidic areas of the planking and frames was done mechanically using power chisels, hand-held chisels of semi-circular profile and scalpels of various sizes. The detailed cleaning phase was completed with the cleaning of the surface of each wooden element with ethyl alcohol to remove undesirable glossiness and oily surface texture and to restore a more natural wood appearance. This effort saw one of the boats completely cleaned, as well as most of the planking of the other boat. The cleaning of the frames and the remainder of the planking of the second boat will continue in 2019.

Given that every part of the boat's structure was cleaned individually we concurrently removed metal ties and the mixture of polyethylene glycol, hemp, sand and sawdust used to fill in gaps in the previous conservation/restoration procedure—the re-conservation plan anticipates the replacement of this filler with other suitable material (araldite, balsite, balsa wood). This procedure made it impossible to simply restore small parts of the boat structure to their actual positions and they were therefore temporarily fixed to a thin substrate of polystyrene (Styrofoam) using thin malleable stainless steel wire and in this manner placed back in their original position in the boat's structure ahead of the phase that will see the parts of the boat structure re-joined in their entirety. Top view and side view drawn documentation was made of the frames on a 1:1 scale on transparent polypropylene substrate in order to have the best possible documentation. They were additionally protected and fixed in place by wrapping them in transparent wrapping foil and as such temporarily returned to the boat planking.

In the coming years we plan to carry out the remaining phases of the re-conservation plan, including joining all the parts of the boat structure, integrating the gaps created in the cleaning process, the continued removal of acidic parts of the wood, and the toning of integrations on both boats.

Carrying out the re-conservation intervention to completion is only one segment of the overall protection of this unique cultural property. The expected results of the re-conservation are the improvement of the structural integrity of the boats and their renewed visual identity, while suitable and stable microclimatic conditions are critical to maximising the retardation of the degradation of the wood. Establishing stable microclimatic conditions will minimalise the speed of the pyrite oxidation process and thereby achieve the greatest possible retardation of wood degradation in the Nin boats. A complete overhaul of the exhibition space is, therefore, absolutely essential, otherwise the same issues will crop up in the very near future.

Anita Jelić