The practice of slavery had been abolished in the British empire by the 1830s, and the trade on the Atlantic coast of west Africa was dying out. Although the sultan of Zanzibar had declared in 1845 that the export of slaves was forbidden, slavery within his dominions continued to be legal. British and French warships patrolled the coast in search of boats bringing slaves from the mainland, but this made little practical difference. Zanzibar's original Arab settlers had a tradition of slaving that was at least 2,000 years old; Arab caravans still raided the interior, and dhows, with their wretched cargoes close-packed below decks, were still running the naval blockade.

Cruel cargo
Prices varied according to the season of year, but in 1856, a dealer in Zanzibar could anticipate about £5 a head for an adult male slave and somewhat more for a female. Every year up to 40,000 slaves were imported into the island. Approximately one-third were put to work on the plantations and the rest re-exported to Arabia, Persia (where a child would fetch £20), Turkey and even farther afield. It was a brutal trade: in Zanzibar, some 30% of the slave population died every year through disease, neglect or malnutrition.

On mainland Africa, the economy of Khartoum in the Sudan was, in the 1860s, maintained by the trade in slaves. Any penniless adventurer could buy into the trade with a loan, mount an expedition south with 200 armed men and form an alliance with a local chieftain. Together, the tribesmen and the Khartoum slavers would fall upon villages, firing the huts and seizing the inhabitants. The women were particularly prized, and they were secured by a heavy forked pole on their shoulders. The head was locked in by a crossbar, the hands were tied to the pole in front and the children were bound to their mothers by a chain passed round their necks.

Traders' territory was strictly demarcated. In a good season, a small trader could reckon on obtaining up to 500 slaves at about £5 a head, plus ivory worth a small fortune. Slave masters on a big scale behaved like robber barons, controlling vast networks that disposed of thousands of slaves.

An ambivalent relationship
They had an ambivalent relationship with Western explorers. Men such as Tippu Tib (Mohammed bin Sayed) came to the aid of Livingstone and furnished Stanley with porters on his march across Africa. Burton was quite candid about his preference for the company of slave traders over that of native Africans, but Livingstone was appalled by the carnage that the trade in human beings left in its wake.