1394 - 1460
One night as Prince Henry of Portugal lay in bed it was revealed to him that he would render a great service to our Lord by the discovery of the said Ethiopias...in these lands so much gold and rich merchandise would be found as would maintain the King and the people...of Portugal.
Duarte Pachece Pereira, Portuguese Explorer, 1506
Judging by his name, you might think that Henry the Navigator was a great explorer with extraordinary navigating skills. Truth is, Prince Henry of Portugal never set sail on voyages of discovery. A nobleman of English, French, and Spanish ancestry, Prince Henry gained his reputation by sponsoring many voyages of discovery along the western coast of Africa.
Prince Henry had several reasons for dispatching his expeditions. He hoped to find rumored Christian allies, add to geographic knowledge, and perhaps find a sea route to the Orient. But he also hoped to find gold. For centuries gold objects from sub-Saharan Africa had made their way to Europe. Some Portuguese even believed that the objects came from a "River of Gold." If only this gold supply could be found, Henry's costly expeditions could begin to pay for themselves and perhaps even strengthen Portugal's economy.
In 1441, two of Henry's captains, Antam Gonclaves and Nuno Tristao, set out, separately, to Cape Bianco on the western coast of Africa. To the south of the Cape they came across a market run by black Muslims dressed in white robes and turbans. There they received a small amount of gold dust. The Portuguese crew also seized twelve black Africans to take back to Portugal, not as slaves, but as exhibits to show Prince Henry. (These would not be Portugal's first African slaves.)
The new captives included a local chief who spoke Arabic. The chief negotiated his own release, the terms of which were that if he and a boy from his family were taken back to their homeland and released, they would provide other black slaves in exchange.
In 1442, Antam Goncalves sailed back to Cape Bianco, then returned with more gold dust and ten black Africans. The following year, Portuguese explorers returned from Africa with nearly thirty slaves.
Within ten years, thousands of slaves had been transported by sea to Portugal and the Portuguese Islands. http://www.pbs.org/wgbh/aia/part1/1p259.html