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Zanzibar's emergence as an entrepôt for international trade was remarkably rapid. Omani merchants, so successful along the maritime routes linking the Persian Gulf and India, were well-equipped to carry more and more ivory to Bombay. An increasing proportion of this ivory was destined for transshipment to Europe, especially London. Since Oman's rulers were also involved in this trade, they supported policies promoting it. Zanzibar itself was well-situated with respect to the monsoon winds, and its deep, wide harbor easily attracted European and American ships. Zanzibar's trade increased tremendously—nearly five-fold—during the first half of the century (Sheriff: 1987:109).

Seyyid Said, the sultan of Oman and Zanzibar (1806-1856), realized that control over mainland ports was critical. By 1822 he had managed to subordinate nearly all of the Swahili towns, with the notable exception of Mombasa, which he conquered in 1837. Omani governors exercised limited administrative control, but it was sufficient to keep foreigners out of mainland ports. This monopoly was most effective on the coast of Tanzania. Seyyid Said also strengthened Zanzibar's position by negotiating treaties with the United States, Britain, and France. These treaties made it illegal for nationals of those countries to obtain African goods anywhere other than Zanzibar. Kilwa's economy suffered when its direct trade in ivory and copal was prohibited. Another important tool was Zanzibar's discriminatory tariff structure that favored Arab traders because others, like the Nyamwezi of central Tanzania, had to pay substantially higher duties. Zanzibari garrisons (posted along the coast) used force to reduce and then eliminate tolls paid to coastal peoples by passing caravans. Nevertheless, neither Seyyid Said nor his successors ever aspired to control politically any territory beyond the coast strip. As the tentacles of Zanzibar's commercial empire spread deep into the interior, there was no concomitant political expansion. In the 1870s, when Sultan Barghash intervened militarily in central Tanzania, where Mirambo, a Nyamwezi chief, posed a threat to a coalition of Arab and Nyamwezi traders, he failed.

Zanzibar's major exports were ivory, slaves, and eventually cloves, but the real "engine" of its transformation was the continuous growth of ivory exports. A very rapid rise in the British demand for ivory in the Bombay market boosted trade between India and the East African ports. The ivory trade was stimulated throughout the century as demand in Europe and America continued to grow. Eastern Africa responded, but supply could not keep up with increasing demand, so that the price of ivory rose throughout the nineteenth century (for the evidence, see Sheriff 1987). In Europe and America ivory was used for piano keys, billiard balls, umbrella and knife handles, and combs.

Of special interest to Americans is the role of American merchants, mostly from New England, who came seeking ivory and copal, and later cloves. Americans usually obtained ivory in direct trade with Zanzibar, while their British competitors continued to buy African ivory in Bombay for several decades. In the years before the Civil War the United States was Zanzibar's most important Western trading partner. A treaty signed in 1835—the first between Oman and a foreign power—regulated America's commercial relations with Zanzibar and remained in force until the 1890 Anglo-German partition of East Africa. Americans brought textiles, especially unbleached cotton (amerikani), manufactured from cotton grown on the South's slave plantations. By the 1830s, they were bringing muskets and gunpowder. Salem traders also visited Madagascar, buying hides for Salem's tanneries and cured beef for Cuba's sugar plantations, where it was consumed by slaves (Sheriff 1987: 91).

The profits of the ivory trade enriched the coffers of Zanzibar's customs house (revenues doubled between 1804 and 1819) and augmented the capital available to Zanzibar's merchants. Swahili merchants found their niche in Zanzibar's local markets and coastal trade, but rarely in the export sector. In contrast, Arabs from Oman and Indian merchants settled in Zanzibar to participate more directly the entrepôt's foreign trade. The role of Indians as providers of credit and capital to Zanzibar's Arabs was a critical resource, enabling the latter to expand shipping and to provision large caravans. However, as British subjects, Indians were not supposed to trade or hold slaves. Overall, Arab capital accumulation lagged behind that of Indians; those who were most successful soon began thinking about how to invest it. Seyyid Said involved himself in experiments with cash crops, such as cloves, coconuts, sugar, and coffee, although only the first two became major exports. He planted cloves, urging other Arabs to follow his example. For those with capital or the ability to borrow from Indian lenders, cloves became an attractive option. Even poor Arabs, if successful in the caravan trade, could invest their savings in clove plantations.

The islands of Zanzibar and Pemba were well-suited to cloves, a tree crop that required very specific soil and climatic conditions and soil. There was also much uncultivated land on Zanzibar. Although cloves were introduced about 1812, apparently from Reunion, the clearing of land for large plantations correlates closely with the greater availability of cheaper slave labor after the Moresby Treaty of 1822. High prices led to a rapidly expanding plantation sector during the years 1835 to 1845. Since it takes more than six years for trees to come into full production, landowners did not realize that they were overplanting until it was too late—that Zanzibar's cloves were saturating the world market. When the price dropped sharply in the late 1840s, they were in a bind. They needed to buy more slaves, to handle larger harvests, as more trees came into production. When the Hamerton Treaty came into effect in 1847, the price of slaves plummeted and eased their predicament. Cheaper slaves provided labor that increased the production of grain and also coconuts on the mainland coast. The number of slaves being absorbed by clove plantations and coastal agricultural was transforming the slave sector of Zanzibar's commercial empire. As the internal slave trade increased, the relative importance of the export slave trade declined significantly (Sheriff 1987: 59 to 60; 223 to 231)