The advantages of slavery by debt over "chattel" slavery – ownership of humans as a property right – were set out in an infamous document called the Hazard Circular, reportedly circulated by British banking interests among their American banking counterparts during the American Civil War. It read in part:
Slavery is likely to be abolished by the war power and chattel slavery destroyed. This, I and my European friends are glad of, for slavery is but the owning of labor and carries with it the care of the laborers, while the European plan, led by England, is that capital shall control labor by controlling wages.
Slaves had to be housed, fed and cared for. "Free" men housed and fed themselves. For the more dangerous jobs, such as mining, Irish immigrants were used rather than black slaves, because the Irish were expendable. Free men could be kept enslaved by debt, by paying them wages that were insufficient to meet their costs of living.