The Maryland Chemical Works of Baltimore used a mix of free labor, hired slave labor, and enslaved people held by the corporation to work in its factory. Since chemicals needed constant attention, the rapid turnover of free white labor encouraged the owner to use enslaved workers. While slave labor was about 20 percent cheaper, the company began to reduce its dependence on enslaved labor in 1829 when two slaves ran away and one died. The location of Baltimore in a border state created opportunities for enslaved people in the city to run away and find freedom in the north—as Frederick Douglass did. Therefore, slaveholders in Baltimore frequently turned to gradual manumission as a means to secure dependable and productive labor from slaves. In promising freedom after a fixed period of years, slaveholders intended to reduce the costs associated with lifetime servitude while providing slaves incentive for cooperation. Enslaved people tried to negotiate terms of manumission that were more advantageous, and the implicit threat of flight weighed significantly in slaveholders' calculations. The dramatic decrease in the enslaved population during 1850-60 indicates that slavery was no longer profitable in the city. Slaves were still used as expensive house servants: it was cheaper to hire a free worker by the day, with the option of dropping him or replacing him with a better worker, rather than run the expense of maintaining a slave month in and month out with little flexibility. On the eve of the Civil War, Baltimore had the largest free black community in the nation. About 15 schools for black people were operating, including Sabbath schools operated by Methodists, Presbyterians, and Quakers, along with several private academies. All black schools were self-sustaining, receiving no state or local government funds, and whites in Baltimore generally opposed educating the black population, continuing to tax black property holders to maintain schools from which black children were excluded by law. Baltimore's black community, nevertheless, was one of the largest and most divided in America due to this experience. Maryland was not subject to Reconstruction, but the end of slavery meant heightened racial tensions as free black people flocked to the city and many armed confrontations erupted between black people and whites. Rural black people who flocked to Baltimore created increased competition for skilled jobs and upset the prewar relationship between free black people and whites. As black migrants were relegated to unskilled work or no work at all, violent strikes erupted. Denied entry into the regular state militia, armed black people formed militias of their own. In the midst of this change, white Baltimoreans interpreted black discontent as disrespect for law and order, which justified police repression.