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The so-called "Bremen Cog"

In the context of the simultaneous exhibition “8 Objects, 8 Museums” by the Leibniz research museums, the German Maritime Museum in Bremerhaven presents the salvage, preservation and presentation of the Bremen Cog.

The Bremen Cog is the best-preserved merchant ship from medieval Northern Europe. Its salvage and the decision to preserve and subsequently exhibit it led to the foundation of the German Maritime Museum, which was opened in Bremerhaven in 1975.

A sensational discovery
In 1962, the astonishingly well-preserved wreck of the Bremen Cog was found during dredging work to expand the harbour in the Weser River. The sensational discovery posed significant problems for the researchers: dredging activities for the harbour expansion and impending ice drift on the Weser threatened the site of the discovery. Human curiosity and illegal attempts to salvage parts of the wreck led to increased time pressure.   

Based on its appearance, the vessel was identified as a cog. As we know today, this was problematic, since ships of this type were only known from artistic renditions such as book illustrations and the medieval seals of the Hanseatic towns.

The term ‘cog’ was used for larger cargo ships that were rigged with a square sail and were only ocean-going to a limited extent. It is not clear whether the term referred to a specific type of ship with a round, spacious hull and a high aftercastle or to cargo ships in general.

Since the 19th century, the idealised cog has been considered the most successful type of ship of its time in the North and a symbol of the Hanseatic League. The Hanseatic League or Hansa (literally: troop) was formed in the 12th century initially as a loose association of merchants, and from the 14th century on, it also included cities and towns. Its purpose was to offer its members support, protection and political clout in foreign trading places.

The salvage – a wreck emerges
The wreck was secured in three separate salvage operations, including an emergency salvage. The cog could not be raised in one piece; without the supporting sand around it, the timbers would have fallen apart. Therefore, the ship was dismantled and the individual pieces were brought ashore. Yet, it proved highly challenging to mark the exact position of the ship’s individual timbers in a reconstruction drawing.

In order to salvage the widely scattered parts, the diving bell vessel “Carl Straat” was deployed in 1965.

The artefacts that were recovered along with the “Bremen Cog” included a single leather shoe.

Wooden blocks from the “Bremen Cog”, used to keep the shrouds under tension. Shrouds are ropes used for bracing the masts.

The Bremen wreck still contained the ship’s toilet (‘head’).

The reconstruction – seven years of puzzle work 
Based on the reconstruction drawing and with the aid of photographs, a team under the supervision of shipwright Werner Lahn undertook the precise reconstruction of the vessel. For the reconstruction, it was essential to preserve the original shape of the timbers by all means. However, wet wood shrinks when dried in the air and changes its shape. Therefore, the timbers had to be kept wet. 

The cog’s timbers were laid out in a large hall inside the newly established German Maritime Museum, where Werner Lahn and his team ensured they were continually kept wet by means of a sprinkler system.

Preservation – eighteen years in an artificial wax solution
For 600 years, the Bremen Cog was submerged in the Weser River. Its wood had become soft and spongy. Artificial wax was supposed to replace the water contained in the wood and to stabilise the cell structure. However, this would likely have caused the preserved timbers to lose their elasticity, rendering the ship’s reconstruction more difficult. Therefore, the ship was reassembled first and only then soaked in polyethylene glycol (PEG). It took eighteen years before the wood could be stabilised with minimal shrinking and under preservation of its natural appearance. 

Polyethylene glycol is one of the standard materials used for the preservation of wet organic archaeological objects. The artificial wax can occur in a solid or liquid state and is completely soluble in water.

The German Maritime Museum developed a two-stage preservation process using different PEG solutions for treating wood with various degrees of damage. To determine the preservation’s end point, wood samples were taken twice a year and the levels of the PEG in the wood were measured.

The image shows dried model boats made of wet archaeological wood. The model on the left was dried without preservation, the one on the right after being soaked in a tank of PEG.

Under observation – the cog is alive! 
Following the preservation process, the cog was suspended from the hall’s ceiling to great effect and presented to the public in 2000. However, even in its preserved state, the cog’s wood is a living material: it is flexible and reacts to temperature fluctuations and humidity. It soon became apparent that the hull had warped much more than initially expected. In part, this was the result of the spacious ship’s own weight, which was no longer counteracted by water pressure.

As a provisional measure, the cog is now kept in shape with the help of metal supports and is under observation using 3D measuring methods. In order to develop a reliable monitoring system and a long-term concept for the cog’s presentation, the German Maritime Museum compares the collected data by means of data fusion.

Photogrammetry uses a large number of photographic images to create a 3D model. The problem: In order for the images to be truly meaningful, they must be shot from exactly the same position and under comparable lighting conditions.

In laser scanning, the object is captured by laser beams. The data are used to compute a three-dimensional, rotatable image of the ship that enables observation from different perspectives. The problem: Depending on the scanner’s angle, the object’s dimensions are not captured correctly and are represented with distortions.

In the total station measuring method, a certain number of points on the ship are scanned. They form the framework for constructing a 3D object. Since it concentrates on fixed points, the method is very precise. However, it is also problematic: a dense network of permanently installed measuring points restricts the view of the object.

None of the possible observation methods is entirely reliable. Therefore, the German Maritime Museum combines the values from various measurements by means of data fusion.

The father of the Bremen Cog – Werner Lahn (1922-2003)
The cog was his life’s work: Beginning in 1966, Werner Lahn (left), a shipwright with many years of experience in wooden ship building, dedicated a quarter of a century to the cog’s reconstruction – first at the Focke Museum in Bremen, and then at the newly founded German Maritime Museum in Bremerhaven.
The German Maritime Museum 
The German Maritime Museum was opened in 1975 in Bremerhaven; it is the largest museum of its kind in Germany. Its collections, research and exhibitions are centred around the subject “Man and the Sea.” The collection comprises ten original ships in the museum harbour and a large inventory of model ships, signal guns, sea charts, personal testimonials and image sources.

The museum building from above

The museum harbour is located directly adjacent to the museum.

In the museum harbour, the visitors can explore the interior of a whaling ship, a salvage tug, a submarine and the barque “Seute Deern” and gain an impression of the austere living conditions at sea.

The German Maritime Museum is in possession of valuable original vessels and shipwrecks. However, maritime history is also reflected in the historically significant collection of approx. 3,000 model ships, originating from the 18th to 20th centuries.

The museum’s research focus is on historical questions - e.g., the history of sea ports and merchant shipping. A current research project explores the question how seafaring and trade were conducted between the North Atlantic islands and the Hanseatic cities when the battle for the stockfish trade errupted.

Credits: Exhibit

“8 Objects, 8 Museums” is a collaboration project between the Leibniz research museums and the Leibniz-Institut für Wissensmedien in Tübingen in the Leibniz Year 2016.

Research and exhibition project by the German Maritime Museum regarding the “Bremen Cog”

All documents and photos:
German Maritime Museum, Photos: Captair, Felix Clebowski, Per Hoffmann, Erik Hoops, Hans-Jörg Kröhnert, Egbert Laska, Günter Meierdierks, Frederic Theis, Annika Thöt, Daniela Wittenberg
British Museum
Focke-Museum, Bremen

Text and object selection: Ruth Schilling, Stephan Speicher, Frederic Theis
Collaboration: Marleen von Bargen, Amandine Colson, Sunhild Kleingärtner, Tobias Wulf

Translation: Hendrik Herlyn

Literature:

Mike Belasus, Tradition und Wandel im neuzeitlichen Klinkerschiffbau der Ostsee am Beispiel der Schiffsfunde Poel 11 und Hiddensee 12 aus Mecklenburg-Vorpommern (Rostock 2014).

Jörgen Bracker (Hrsg.), Die Hanse. Lebenswirklichkeit und Mythos (Hamburg 1989).

Amandine Colson – Julien Guery – Massimiliano Ditta, „Bremen Cog“ – Long term monitoring of deformation process, in: Condition, conservation and digitalisation. Conference proceedings, Gdansk National Maritime Museum (Danzig 2015), 107-111.

Gabriele Hoffmann – Uwe Schnall (Hrsg.), Die Kogge. Sternstunde der deutschen Schiffsarchäologie, Schriften des Deutschen Schiffahrtsmuseums 60 (Hamburg 2003).

Natascha Mehler – Mark Gardiner, On the Verge of Colonialism: English and Hanseatic Trade in the North Atlantic Islands, in: Peter Pope – Shannon Lewis-Simpson (Hrsg.), Exploring Atlantic Transitions: Archaeologies of Permanence and Transience in New Found Lands. Society of Post-Medieval Archaeology Monograph 8 (Woodbrige 2013), 1–15.

Credits: All media
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