For 200 years, New England Yankees bought, worked and sold African slaves, with nary a second thought. For every slave the Yankee slave merchants sold to the South, they sold 20 more to Cuba, Brazil and the West Indies.
I have found two websites that explore and document the North's extensive involvement in slavery. One of them is "Slavery In The North." It is not a pro-South or pro-Confederate website, and there is no Southern bias that is obvious. It appears to be objective history.
This is from the website's Introduction:
African slavery is so much the outstanding feature of the South, in the unthinking view of it, that people often forget there had been slaves in all the old colonies. Slaves were auctioned openly in the Market House of Philadelphia; in the shadow of Congregational churches in Rhode Island; in Boston taverns and warehouses; and weekly, sometimes daily, in Merchant's Coffee House of New York. Such Northern heroes of the American Revolution as John Hancock and Benjamin Franklin bought, sold, and owned black people. William Henry Seward, Lincoln's anti-slavery Secretary of State during the Civil War, born in 1801, grew up in Orange County, New York, in a slave-owning family and amid neighbors who owned slaves if they could afford them. The family of Abraham Lincoln himself, when it lived in Pennsylvania in colonial times, owned slaves.
When the minutemen marched off to face the redcoats at Lexington in 1775, the wives, boys and old men they left behind in Framingham took up axes, clubs, and pitchforks and barred themselves in their homes because of a widespread, and widely credited, rumor that the local slaves planned to rise up and massacre the white inhabitants while the militia was away.
African bondage in the colonies north of the Mason-Dixon Line has left a legacy in the economics of modern America and in the racial attitudes of the U.S. working class. Yet comparatively little is written about the 200-year history of Northern slavery. Robert Steinfeld's deservedly praised "The Invention of Free Labor" (1991) states, "By 1804 slavery had been abolished throughout New England," ignoring the 1800 census, which shows 1,488 slaves in New England. Recent archaeological discoveries of slave quarters or cemeteries in Philadelphia and New York City sometimes are written up in newspaper headlines as though they were exhibits of evidence in a case not yet settled (cf. �African Burial Ground Proves Northern Slavery,� The City Sun, Feb. 24, 1993).
I had written one book on Pennsylvania history and was starting a second before I learned that William Penn had been a slaveowner. The historian Joanne Pope Melish, who has written a perceptive book on race relations in ante-bellum New England, recalls how it was possible to read American history textbooks at the high school level and never know that there was such a thing as a slave north of the Mason-Dixon Line:
"In Connecticut in the 1950s, when I was growing up, the only slavery discussed in my history textbook was southern; New Englanders had marched south to end slavery. It was in Rhode Island, where I lived after 1964, that I first stumbled across an obscure reference to local slavery, but almost no one I asked knew anything about it. Members of the historical society did, but they assured me that slavery in Rhode Island had been brief and benign, involving only the best families, who behaved with genteel kindness. They pointed me in the direction of several antiquarian histories, which said about the same thing. Some of the people of color I met knew more."
Slavery in the North never approached the numbers of the South. It was, numerically, a drop in the bucket compared to the South. But the South, comparatively, was itself a drop in the bucket of New World slavery. Roughly a million slaves were brought from Africa to the New World by the Spanish and Portuguese before the first handful reached Virginia. Some 500,000 slaves were brought to the United States (or the colonies it was built from) in the history of the slave trade, which is a mere fraction of the estimated 10 million Africans forced to the Americas during that period.
Every New World colony was, in some sense, a slave colony. French Canada, Massachusetts, Rhode Island, Pennsylvania, Virginia, Cuba, Brazil -- all of them made their start in an economic system built upon slavery based on race. In all of them, slavery enjoyed the service of the law and the sanction of religion. In all of them the master class had its moments of doubt, and the slaves plotted to escape or rebel.
Over time, slavery flourished in the Upper South and failed to do so in the North. But there were pockets of the North on the eve of the Revolution where slaves played key roles in the economic and social order: New York City and northern New Jersey, rural Pennsylvania, and the shipping towns of Connecticut and Rhode Island. Black populations in some places were much higher than they would be during the 19th century. More than 3,000 blacks lived in Rhode Island in 1748, amounting to 9.1 percent of the population; 4,600 blacks were in New Jersey in 1745, 7.5 percent of the population; and nearly 20,000 blacks lived in New York in 1771, 12.2 percent of the population.
The North failed to develop large-scale agrarian slavery, such as later arose in the Deep South, but that had little to do with morality and much to do with climate and economy.