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After being captured and held in the factories, slaves entered the infamous Middle Passage.
The Middle Passage took place from one to six months depending on weather conditions.
Ships contained up to several hundred slaves aboard one slave ship with a crew of 30. The male captives were normally chained together in pairs to save space; right leg to the next man's left leg — while the women and children may have had somewhat more room. The captives were fed beans, corn, yams, rice, and palm oil. Slaves were fed one meal a day with water, but if food was scarce, slaveholders would get priority over meals. Sometimes captives were allowed to move around during the day, but many ships kept the shackles on throughout the arduous journey.
Most contemporary historians estimate that between 9.4 and 12 million Africans arrived in the New World. Disease and starvation due to the length of the passage were the main contributors to the death toll with amoebic dysentery and scurvy causing the majority of deaths. Additionally, outbreaks of smallpox, syphilis, malaria, measles, and other diseases spread rapidly in the close-quarter compartments. The number of dead increased with the length of voyage, since the incidence of dysentery and of scurvy increased with longer stints at sea as the quality and amount of food and water diminished with every passing day. In addition to physical sickness, many slaves became too depressed to eat or function efficiently because of the loss of freedom, family, security, and their own humanity. This often led to worse treatment like force-feeding or lashings. Some even committed suicide by jumping over board before they arrived in the New World.
For two hundred years, 1440–1640, Portugal had a quasi-monopoly on the export of slaves from Africa.
Meltzer's research puts this phase of the slave trade's overall mortality at 12.5%. Around 2.2 million Africans died during these voyages where they were packed into tight, unsanitary spaces on ships for months at a time. Measures were taken to stem the onboard mortality rate such as mandatory dancing above deck and the practice of force-feeding any slaves that attempted to starve themselves. The conditions on board also resulted in the spread of fatal diseases. Other fatalities were the result of suicides by jumping over board by slaves who could no longer endure the conditions. Before the shipping of slaves was completely outlawed in 1853, 15.3 million "immigrants" had arrived in the Americas.
Raymond L. Cohn, an economics professor whose research has focused on economic history and international migration, has researched the mortality rates among Africans during the voyages of the Atlantic slave trade. He found that mortality rates decreased over the history of the slave trade, primarily because the length of time necessary for the voyage was declining. "In the eighteenth century many slave voyages took at least 2-1/2 months. In the nineteenth century, 2 months appears to have been the maximum length of the voyage, and many voyages were far shorter. Fewer slaves died in the Middle Passage over time mainly because the passage was shorter."
Once across the Atlantic, the slaves then entered "seasoning camps" where the slaves were tortured for the purpose of "breaking" them (like the practice of breaking horses) and conditioning them to their new lot in life. Jamaica held one of the most notorious of these camps. Milton Meltzer also states that 33% of Africans would have died in the first year at seasoning camps found throughout the Caribbean. Many slaves shipped directly to North America bypassed this process; however most slaves (destined for island or South American plantations) were likely to be put through this ordeal. All in all, 5 million Africans died in these camps reducing the final number of Africans to about 10 million.