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The Tallinn cog : smart conservation

Eero Ehanti, Heikki Häyhä et Priit Lätti
p. 355-356


Cet article présente le projet de conservation d’une épave médiévale découverte à Tallinn, en Estonie. L’objet, l’épave bien conservée d’une cogue hanséatique, est brièvement présenté, mais l’accent est mis principalement sur les mesures de conservation. Le processus décisionnel est discuté et le plan de conservation résumé. A l’issue de ce projet, on peut conclure que les choix opérés et les actions de conservation menées se sont avérés fructueux.

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1A very well preserved, early 14th century wreck of a cog, the ship type used by the Hanseatic League, is undergoing conservation in Tallinn in order to be exhibited inside the old town city walls. The 18m-long wreck with a beam of 6 m was found in 2015 during construction works in a park area near Tallinn shoreline. The ship is made of oak, which dates after 1296 (terminus post quem) according to dendrochronology. The bottom of the hull is carvel-built while the strakes are clinker-­built. The planking is fastened to frames with wooden pegs, and strakes are connected to each other with iron nails. Sintels are used to hold the wooden laths in place on the plank seams. Moss is used for caulking.

Fig. 1: Priit Lätti and Heikki Häyhä cleaning the hull in December 2018

Fig. 1: Priit Lätti and Heikki Häyhä cleaning the hull in December 2018

(photography Eero Ehanti)

2The ship and numerous artefacts found on board were carefully excavated and brought to the Estonian Maritime Museum for conservation. The cog was placed inside a spacious and sturdy tent on gravel. Air circulation was improved by an industrial fan. It became evident that the ship is the best-preserved medieval vessel found in Estonia and will thus become the star exhibit in the new permanent exhibition in the Estonian Maritime Museum’s renovated Fat Margaret premises.

3Before any kind of display, the complex wet object had to be conserved in order to stabilise it and enhance its exhibition value. Plenty of cleaning was required, and reinforcements were needed to keep the pieces together. The major question was whether to PEG treat or not, whether to use polyethylene glycol, the trustworthy polymer utilised successfully in such shipwreck finds elsewhere. This led to a subsidiary question : should we first dismantle the entire wreck for proper impregnation, or keep the wreck together and spray-treat, as has been done with Vasa for instance ? There was also the consideration as to whether the oak was in good enough condition to withstand controlled drying without the introduction of stabilisation agents inside the wood matrix. Some cracking and damage caused by too fast drying upon excavation were obvious on the upper part of the hull. The fear was that this might continue without PEG treatment.

4Eventually, we took the somewhat bold decision to air-dry without any stabilising agent. A very relevant fact affecting our decision-making was that the wreck had already dried considerably when the conservation process was initiated : it was no longer waterlogged, merely wet. We believed that it was too late for PEG treatment unless the entire wreck was returned to a waterlogged state, which would have caused unnecessary stress to the wood. Another cornerstone of our evaluation was the observed and researched fact that the oak wood is for the most part quite sound, although the upper part of the ship has been damaged by direct sunlight. Wooden pegs attaching the hull planking to the frames were still holding the structure together. Hence, we estimated that the wood would withstand controlled drying and the structure would remain together given that suffi­cient support was added by replacing corroded iron nails and missing treenails as needed. External and internal supporting structures following the ship’s form were also needed for safe transportation and meaningful exhibition.

5Needless to say, a very laborious cleaning process had to be completed in order to make the whole presentable and minimise the possibility of biological growths. Cleaning was mostly done mechanically, using brushes, scalpels, vacuum cleaners and so on, but we are also very proud to report that dry ice blasting was used on outer surfaces of the hull as well as parts of the inside, with great results. This method might sound a bit harsh for shipwreck wood, but after thorough testing on loose pieces we learned that when gentle enough parameters have been defined, the results are very good in terms of sensitivity, efficiency and aesthetics.

6Everything was minutely documented by traditional measu­ring and photography methods, but also by laser scanning. The latter was carried out twice during the conservation project. A local coordinate system was established, and the measured point clouds were compared together. We were able to see that the changes in the hull were minimal. Deformation monitoring of the hull in the final exhibition will be based on the same technique. The exhibition hall will, of course, be climate controlled.

7The cog is far from complete. A lot of the structure is missing, but the shape of the hull and the information as to how it was built has been beautifully preserved. The keelson is complete and the mast step made out of a single tree trunk is still there. Moss caulking between the hull planking has been preserved in many places. Even some sintels holding the caulking together were preserved. With internal steel structures, new wooden pegs and acid-proof bolts, the cog will hold together and will represent a cognitive anchor to medieval Tallinn.

8The cog will be ready for display within the previously established schedule, which was indeed quite tight for the large-scale conservation of such a precious item. The schedule affected our decision-making. Insisting on a PEG treatment process of, for example, ten years’ duration, would have had significant repercussions on the entire project, including finance. As it is, the cog exhibition will be opened in November 2019. At the time of writing, late May 2019, everything appears fine. The wood has dried nicely without dramatic reactions.

9This brings us to the notion of smart conservation’ as mentioned in the title of our article. We think that conservation is not always about elaborate treatments involving special chemicals after very scientific analyses. It is rather about understanding the object at hand and doing only what is needed to preserve the tangible object while enhancing the intangible. By the latter, we refer to the wealth of significance that historical objects possess. Why are we preserving this and how are we telling its stories ?

10Sometimes very drastic, slow and costly conservation processes are needed, but they should not be resorted to when not necessary. Here we have a fine example of one of the ideals of conservation : minimum intervention. Or smart conservation, as we like to call it1.

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1  For more information about the cog and a blog about its conservation: ;;

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Table des illustrations

Titre Fig. 1: Priit Lätti and Heikki Häyhä cleaning the hull in December 2018
Crédits (photography Eero Ehanti)
Fichier image/jpeg, 1,2M
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Eero Ehanti, Heikki Häyhä et Priit Lätti, « The Tallinn cog : smart conservation »Archaeonautica, 21 | 2021, 355-356.

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Eero Ehanti, Heikki Häyhä et Priit Lätti, « The Tallinn cog : smart conservation »Archaeonautica [En ligne], 21 | 2021, mis en ligne le 01 juillet 2022, consulté le 03 décembre 2023. URL : ; DOI :

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Eero Ehanti

For Our Generation OU, Pikaliiva tn 90/2-32, Tallinn, harju maakond, 13516, Estonia

Heikki Häyhä

For Our Generation OU, Pikaliiva tn 90/2-32, Tallinn, harju maakond, 13516, Estonia

Priit Lätti

Estonian Maritime Museum, Vesilennuki 1, 10415 Tallinn, Estonia

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Le texte seul est utilisable sous licence CC BY 4.0. Les autres éléments (illustrations, fichiers annexes importés) sont « Tous droits réservés », sauf mention contraire.

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