The planters attributed the violence to recently imported Africans and agreed to a 10-year moratorium on slave importation through Charlestown. They relied for labor upon the slave communities they already held. The 1740 Negro Act also tightened controls, requiring a ratio of one white for every ten blacks on any plantation (which was often not achieved), and banning slaves from assembling together, growing their own food, earning money, or learning to read. Drums were banned because Africans used them for signaling; slaves were allowed to use string and other instruments. When the moratorium expired and Charlestown reopened to the slave trade in 1750, the memory of the Stono Rebellion resulted in traders avoiding buying slaves from the Congo and Angola, whose populations had a reputation for independence.