Two shipwrecks discovered in shallow waters off Costa Rica in the 1970s have finally been identified as named Danish slave ships from the early 1700s – with the help of tree-ring dating and some distinctively coloured bricks.
The wrecks had long been assumed to be pirate ships, and this was questioned only after 2015, when US marine archaeologists found yellow bricks at one of the sites near Puerto Limón in Cahuita National Park.
In 2023 archaeological divers from Denmark’s National Museum and its Viking Ship Museum carried out excavations at a wreck-site lying about 4m deep. They took samples of timbers and bricks from the cargo, as well as picking up a number of clay pipes, but only now have their conclusions been published.
Analysed by experts at the National Museum and the University of Southern Denmark, the samples confirmed historical sources that describe the loss of the infamous Danish vessels Fridericus Quartus and Christianus Quintus in 1710.
These ships had left Copenhagen in December 1708, heading for the Danish West Indies and carrying cloth, metal goods and weapons intended to be traded in West Africa along the way for almost 700 enslaved people.
The two vessels came to grief on Costa Rica’s reefs in 1710 as the result of navigational errors. Fridericus Quartus was reported to have been set ablaze, while Christianus Quintus had its anchor-rope cut before being wrecked in surf.
According to historical records many of the slaves escaped, though 100 were recaptured and sent to work on cacao plantations. The wreck-sites were not recorded.
“The analyses are very convincing and we no longer have any doubts that these are the wrecks of the two Danish slave ships,” said marine archaeologist David Gregory, research professor at the National Museum where he heads its new Njord maritime research centre. He led the underwater excavations with fellow-museum marine archaeologist Andreas Kallmeyer Bloch.
“The bricks are Danish and the same goes for the timbers, which are additionally charred and sooty from a fire,” said Gregory. “This fits perfectly with the historical accounts stating that one of the ships burnt.”
“It’s been a long process and I’ve come close to giving up along the way, but this is undoubtedly the craziest archaeological excavation I‘ve yet been part of,” commented Bloch. “Not only because it matters greatly to the local population, but also because it’s one of the most dramatic shipwrecks in the history of Denmark, and now we know exactly where it happened.
“This provides two pieces that have been missing from the history of Denmark.”
Tree-ring dating of charred oak timbers from one of the wrecks showed that the tree had been felled between 1690 and 1695 in the western Baltic, an area encompassing the German province of Mecklenburg, Schleswig-Holstein, Denmark and Scania.
The bricks had the same dimensions as ‘Flensburg’ yellow bricks. Analyses of the clay showed that it came from either Iller Strand or Egernsund in the country’s Flensburg Fjord, where bricks were produced extensively for use in Denmark and its colonies in the 18th and 19th centuries. Such bricks were not used in buildings elsewhere in Europe at the time.
The clay pipes were standard Dutch-produced pipes that would have been used on Danish ships. Their size, shape and pattern indicated that they were produced in the period leading up to 1710. Clay pipes rarely lasted more than five years.
Also partnering in the wreck investigations were the National Museum of Costa Rica, the Archaeological Commission of Costa Rica and other Costa Rican bodies. The project marked the country's first underwater archaeological excavation.
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