Construction of the Trans-Atlantic Slave Trade Database: Sources and Methods

Classification as a Trans-Atlantic Slaving Voyage

One important problem slave-trade researchers need to address is whether vessels bound for “Africa” in the sources are slaving vessels. As late as the end of the seventeenth century, slaves formed less than half of trade by value between the Atlantic world and Africa. Many captains sailed to Africa to purchase gold, ivory, dyewoods, or spices., Numerous naval vessels, troop transports or storeships sailed from Lisbon to the Portuguese forts at Benguela or Luanda, and other European powers needed to supply their trading forts as well. For most of the French, Portuguese and Dutch voyages to Africa, researchers other than ourselves have made the decision on whether or not a ship was a slaver, though we have uncovered a few additional voyages from these nations where the object of the voyage remains unclear. It might be noted that records of ship departures have typically survived better in the historical record than records of ship arrivals.

For many British and Portuguese voyages, however, we have had to make some hard decisions in determining whether vessels were slavers or non-slavers. Many British and North American voyages returned to the port of origin after an interval of time during which a slave voyage could have taken place, but no information survives of the places of trade in Africa or the Americas. For most of these ships, clearance was for "Africa and the Americas" and many of the remainder in this group are ships leaving British American ports for Africa. Before the nineteenth century, ships rarely went from the Americas to Africa for anything but slaves. In all these cases the ship is assumed to have been a slaver. For several hundred more voyages from Brazil, even less is known. In Bahia, the main source of information on these is licenses of ship departures which specify "Elmina" (in West Africa) as the permitted destination. Large rolls of local tobacco were the trade good for African-bound Bahian ships, and in the eighteenth century slave traders of all nations depended on this tobacco. And as with North American Africa-bound ships, there is no evidence of a significant produce trade between Brazil and Africa. Gold was important in the first half of the eighteenth century and alcohol became more important later in the century, but return cargoes were always human. After 1680 many Portuguese voyages from Brazil show up in British records from Cape Coast Castle as well as the Dutch records for Elmina, and in the nineteenth century there is very good overlap between these licences and the observations of British observers on the movements of slave ships. We have made the decision to include these Bahia–Africa voyages in the data set. Nevertheless, we do have a file of 1,400 voyages of Atlantic voyages that might have carried slaves, but for which we are awaiting additional evidence. The majority of these certainly sailed from Europe to the Caribbean and then back again without sailing anywhere near Africa, but we cannot be absolutely certain and we retain information on them for future use as necessary. These "doubtfuls" are troublesome, but their numbers, compared to the voyages about which we are quite certain, are not great.

There remains the question of produce ships—defined as ships that sailed to Africa to trade for animal products, agricultural commodities or minerals. We have identified 1,450 voyages that departed Africa without obtaining slaves. In some cases they carried supplies for the European castles on the coast, but in the majority of instances they traded for African produce before returning directly to Europe. In addition, there were always a few "tenders" each year that went to the coast to supply slaves for a larger ship, but did not themselves carry slaves across the Atlantic. The great majority of these non-slaving ships were Dutch and British, the two nations that carried on the largest trades in African produce. We have identified produce (as opposed to slave) vessels sometimes on the basis of their voyage histories, sometimes on the known activities of their captains and sometimes on the basis of small crew-to-tonnage ratios, suggesting they were not vessels that required additional crew to control slaves.(9) Both the produce traders and the doubtful traders are held in a separate file and in the former case will be used as the basis for separate work on the African produce traffic.

Similarly, on the American side of the Atlantic, the editors often had to decide whether vessels carrying slaves were trans-Atlantic or inter-colonial slavers. For a few hundred ships arriving at ports in the Americas, doubts remain. Most of these voyages are to be found in Klein’s set of voyages to Havana, 1790 to 1820, taken from the Spanish archives. It is clear that many smaller vessels were inter-island slave traders. That is, they trans-shipped African slaves from colonies such as St. Croix or St. Thomas to major plantation frontiers, such as Cuba. To separate the inter-island from trans-Atlantic vessels arriving in Havana with slaves, we used a benchmark total of 140 slaves—the average number of slaves on vessels in the sample that can be identified as trans-Atlantic slavers. Other researchers will use different criteria for distinguishing inter-American from trans-Atlantic slave voyages.

Finally, not all voyages that crossed the Atlantic from Africa carried slaves. Generally we have assumed that all such voyages were slaving voyages, and have included them in the data set, though there is a slight possibility that a few of these vessels traded at produce markets on the coast. In summary, about 5 percent of the voyages included in the data set lack information about their activities after the voyages began. We nevertheless feel fairly confident that these were slaving voyages, and, as noted, those about which we feel less confident we retain in a separate file.