One of the terms of the Treaty of Utrecht of 1713 transferred the asiento de negros from the Portuguese to the British, and the British government assigned the operation of the trade to the South Sea Company. The asiento, as it became called simply, was one of those regulations instituted by the Spanish absolutist regime through which it tried to maintain control over one particular aspect of the connection between the homeland and the colonies. By it the government in Madrid assigned the right (the asiento was the licence) to transport slaves from Africa to America and to sell them there to a succession of trading groups. It had been repeatedly switched from one concessionaire to another over the preceding century and more, Genoese, Dutch, Spanish, Portuguese, French. Admiral du Casse had negotiated with the new Spanish King Philip V in 1701 to have the asiento transferred to the French Guinea Company and Louis XIV. (This was one of the first fruits for the French of the accession of a French king to the Spanish throne.) The Portuguese, whose posts in West Africa were a prime source of the slaves, were also deeply involved. Now it was assigned to Britain, and then to the South Sea Company.

The value of the concession was not so much in the transport of slaves, though that in itself was normally profitable, as it was for the cover it provided for smuggling goods apart from slaves. Slaves were, of course, transported, but the Spaniards in the colonies had never restricted themselves to the asiento in purchasing slaves, nor did they in buying other goods. The asiento was therefore a profitable trade, partly because of its main commodity – there was a particular demand at Cartagena for slaves to work in the new gold mines in the interior, the sort of work which killed them quickly – but also since it allowed foreigners in non-Spanish ships to call legally at the destination ports in America, where the slaves would be legally sold, while other goods being carried were illegally smuggled and sold, an activity in which both Spaniards and foreigners cooperated in defrauding the king of Spain of his tax monies. It was the legal trading which formed the foundation of the presumed wealth expected to be gained by the South Sea Company, and which would lead in turn to the extraordinary financial antics summed up in the phrase ‘South Sea Sea Bubble’.