Building Bricks as Shipboard Ballast

Our research on two suspected slave ships bound for St. Thomas suggests that "commercial ballast" found at shipwreck sites has unique value for historical and archaeological interpretation.

This post relates to a paper I co-authored with Dr. Lynn Harris on commercial ballast bricks. In 2015, I assisted in documenting two shipwreck sites located in Cahuita National Park, Costa Rica. The results of this fieldwork and subsequent examinations of the wrecks can be found here.

During that study, we found ourselves wondering about all the shipwreck sites we find that only consist of ballast remains. Beyond moving the stones to see what is underneath or intermingled with the rocks, there isn't a lot to learn from that, right? But what about ballast that isn't composed of just stones? One of the two shipwreck sites in Costa Rica at first glance looked like it had only a few bricks wedged in the sand. As you swam along the site, however, you kept seeing the bricks... It was only when you finally took a step (fin kick) back, and looked at the site in its entirety that you could see the nearby reef was in fact sitting atop a large mound of bricks - ones that were still neatly stacked together! What we were looking at was the very bottom of a ship that gently ran aground and settled on the sea floor. As the wood of the hull eventually broke away, the neatly stacked ballast bricks at the lower section of the hold were exposed. Over the years they became enveloped by the reef system, which now uses the site as its foundation.

Archaeologist Sean McGrail (1989) developed an analytical classification for ballast, in which material fell into two groupings: commercial ballast and non-commercial ballast. In order to explain his grouping, McGrail (1989:356-357) also outlined the concepts of cargo density and stowage factors. Examples of low stowage factor cargo are high density, heavy items that take up less comparative space per individual unit, such as bricks, lead, iron, or marble. Think big, dense, heavy things that take up less space while still weighing the ship down so that it sails properly. High stowage factor goods were objects of typically greater quantity and bulk, but lesser weight and cargo density such as wine, firearms, or fabrics. These are items that even when packed together in barrels or boxes, still took up more space in the hold compared to the weight on the hull. Ship owners and captains could put high stowage factor goods at the very bottom of their ship, while loading lower stowage factor items on it to maximize the capital output of a single voyage. One of the most well documented types of commercial, high stowage factor ballast, was brick.

Yellow bricks like the ones we found in Costa Rica were used in colonial settings for aesthetic and practical purposes. After careful documentation and comparison with known types found in other archaeological contexts, the Costa Rican bricks most closely resembled Danish brick types made near the Flensborg fjord and on Sjælland, Denmark. This, along with other historical and archaeological evidence led to the preliminary identification of these sites as the wrecks of Christianus Quintus and Fredericus Quartus - two Danish slave ships that got lost on their way to St. Thomas in 1718 and after a mutiny, the ships were scuttled in modern-day Costa Rica. Many historic structures in St. Thomas were made using the same yellow-bodied brick type found on the two shipwreck sites, and it was likely that the ships utilized this high stowage factor commercial ballast with the intention of selling it for the construction of new buildings or fortifications on the Caribbean island.

Research into the stowage of ship's holds can also yield insights into the intentions of the captain and crew. In the example of the two Danish slave ships, enslaved Africans were viewed as human chattel cargo by the slave ship captains. To optimize the profitability of a single voyage, slave ship captains packed their ships with as many enslaved individuals as possible. Ships were only so big, however, and the bodies of enslaved people (no matter how tightly they are packed) took up a lot of space compared to the weight borne on the hull. Therefore, captains were more likely to use high stowage factor ballast, like bricks, in the lower portion of the hull to offset the weight/space ratio lost by packing the rest of their hold with "human cargo." This likely explains the preponderance of bricks associated with the two Danish vessels. The Danish captains were trying to not only capitalize on the sale of other humans, but also the seemingly insignificant ballast bricks deep down in the hold that truly highlights the twisted efficiency of the "trade."

A 1788 diagram depicting the stowage of enslaved Africans on the British slave ship Brookes (Wikimedia Commons).

Study into commercial ballast, when coupled with historical research, is a useful tool to analyze behaviors of past mariners in the context of a significantly deteriorated shipwreck site. There are numerous other sites in the Caribbean and elsewhere that have similar archaeological signatures associated with the shipwreck site (for example, site PA1.144, "Yellow Brick Site," on Buck Island, St Croix, and documented by the National Park Service had similarly stacked, yellow-bodied bricks likely associated with commercial ballast intended for the island). The research continues!

References

McGrail, Sean

1989 The Shipment of Traded Goods and Ballast in Antiquity. The Oxford Journal of Archaeology 8(3):353-358.