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

Inferring Places of Trade

Establishing the outcome of the voyage is an important prerequisite to inferring information about both places of trade and numbers of people purchased. We have a good basis for imputing locations of slave trading as well as estimating the numbers of slaves embarked and disembarked. We turn first to the geography of the traffic. For some voyages we know the intended ports of trade on the African coast and in the Americas. Private correspondence, newspaper reports, and official records of clearances from ports in Europe and the Americas frequently provide such information. Of the 34,941 voyages in the data set, at least 1,536 did not embark slaves, usually on account of capture or natural hazard. Of those that did, several hundred failed to complete the Middle Passage. The data set provides some information on African place of trade for 20,826 voyages or about half of those that are likely to have obtained slaves. While this information surpasses current knowledge of the geography of the slave trade, it is possible to glean yet more. For 4,084 voyages that left Africa with slaves, or could have done so in the sense that the ship was not wrecked or captured prior to trade beginning, we may not know the African place of embarkation, but we do know where the captain intended to buy slaves. If we assume that he did in fact do what was intended, then after eliminating those locations that are not easy to group into regions (for example the French designation Côte d’Or, which ranged from the Windward Coast to the Bight of Biafra), we are left with 25,010 voyages that contain useful information on place of African trade—or about 60 percent of those vessels in our sample that actually did or could have left Africa with slaves. Switching to the other side of the Atlantic, the data set yields some information on ports of arrival for 24,844 voyages. Once more we have additional information on where 5,444 voyages intended to trade their slaves even though we cannot be certain that they actually did so. If we assume that captains completed the voyage according to plan, then the sample for places of disembarkation increases from 24,844 to 30,288 voyages, or close to 75 percent of all those ventures disembarking captives.

How valid are these assumptions on imputing places of trade from information on intended place of trade? Most slave ships traded in the regions where owners declared they would trade. After eliminating captured ships that rarely completed their voyages as intended, as well as those ships with very broadly defined destinations ("Americas" or "British North America"), a Pearson product moment correlation run on ports of arrival in the Americas generated a coefficient of 0.83 (n=9,541). A similar procedure for region of trade in Africa and intended region of trade produced a Pearson product moment correlation of 0.714 (n=13,951). Ii should also be kept in mind that merchandise always had to be loaded in Europe and the Americas for a specific African region and was often impossible to sell in another region. It was unusual to find a specific manufactured good selling in more than one region.(10) Taken together, this evidence appears sufficiently strong to allow some modest inferences for those voyages that we know purchased slaves in Africa, or subsequently disembarked slaves in other parts of the Atlantic world, and for which the intended but not the actual region of trade is known.

In addition to these inferential issues, there are also known biases in the geographic data. The British signed three treaties with the Portuguese between 1811 and 1817 that contained clauses limiting Portuguese slave traders to regions of Africa south of the equator, and the last two of the treaties allowed British cruisers to capture Portuguese ships that did not adhere to these provisions. Brazil assumed these treaties when the country became independent in 1822. From 1815, slave ships arriving in Bahia, which had strong trading relations with the Bight of Benin or Slave Coast (north of the equator), usually reported their African port of departure as Cabinda or Malemba, ports just north of the Congo. British officials in Bahia, as well as naval officers patrolling the African coast, were convinced that all Bahian ships nevertheless continued to trade on the Slave Coast.(11)