There was a thriving trade in human flesh. By the twelfth century the slave trade in Venice far surpassed that of other cities and other countries. The Venetians were incorrigible slave traders, and the markets of the Rialto and S. Giorgio were centres of slavery. They were eager for this particular source of income, since the profit on each item was said to be 1,000 per cent. They sold Russians and even Greek Christians to the Saracens. Men and women and children were bought or captured in the region of the Black Sea — Armenians and Georgians among them — before being despatched to Venice where they were in turn sold on to Egypt and Morocco and Crete and Cyprus. They sold boys and young women as concubines. One doge, Pietro Mocenigo, had in his seventies two young Turkish men in his entourage.
Many of them were consigned to Venetian households. No patrician family was complete without a retinue or three or four slaves; even Venetian artisans owned slaves, and used them in their shops or workshops. Venetian convents possessed slaves for domestic service. The galleys were stocked with slaves. But the city always needed a fresh supply; servile status was not inheritable. Many slaves were freed in the wills of their masters or mistresses. Marco Polo manumitted one of his slaves, Peter the Tartar, before his own death in 1324. In 1580 there were three thousand slaves in the capital. The black gondoliers in Carpaccio’s paintings of Venice are all slaves.
That is from Peter Ackroyd’s Venice: Pure City.