From the 1820s there was a surge in American commerce with western Africa, slave and legitimate, many of the vessels sailing via Cabo Verde. Collaboration between legitimate traders and slave traders greatly increased following the 1835 Anglo-Spanish treaty incorporating an ‘equipment clause’ that conceded the British navy authority to capture Spanish vessels carrying slave irons, lumber to construct slave decks and provisions requisite for slave cargoes. These restrictions were imposed on Portugal in 1839 and Brazil in 1845. Slave traders responded by sailing to Africa without incriminating cargoes, to be supplied by American traders paid with Spanish and Latin American gold and silver coins and bills of exchange from merchants in Britain, Portugal, Brazil and Cuba. Ineluctably, slavers and their intermediaries dominated western Africa's commerce.
