Gradual emancipation of slaves in New York began with a law of 1799, which freed the children of slave mothers at 28 year if a male child and 25 years if a female child. These children of slaves were not legally slaves, but were free persons, bound for a certain number of years to the service of the person who held their mother in slavery. This children, however, were often tallied as slaves in the census of 1810 and 1820. Only with a law of 1817 did New York actually declared that enslaved persons would be freed. In 1827, all slaves born in New York were legally free.
Yet, as other historians have noted, slavery in New York continued after 1827. Slaves in New York after that date belonged to a few categories. First, there were travelers in New York who brought their slaves with them, and could stay up to nine months. Other slaves in New York after 1827 were those had been born outside of New York, so the laws affected them differently. According to the law of 1817, slaves born outside of the state after 1799 and then brought to New York remained slaves until 28 years of age for men and 25 years of age for women. Slaves born before 1799 and imported into New York were not affected and apparently could have remained enslaved indefinitely. The census of 1830 shows that such situations were rare. Modern sources often give the number of 75 slaves in New York in 1830, but the Abstract of the Returns of the Fifth Census actually gives 76, divided among the following counties: New York city and county (17), Putnam (4), Albany (2), Chenango (3), Montgomery (26), Oneida (15), Washington (8).  Of these, 64 of were female and 12 were male.
But there appear to a number of problems with the original census enumerations for slaves on the New York census of 1830, which has resulted in a large overcount. First of all, the township-level records for Washington County and Albany County indicate no slaves in the county. Even the records for New York City, at the ward level, don’t indicate any slaves. Here, the error can be blamed directly on the enumerator, who appears to have consistently tallied free black females as slaves only to move them to the free colored persons category when adding them up. In one case, he even crossed them out and wrote “free” with a circle around it. 
A similar enumerator error may have occurred in Montgomery County, but here, I suspect, another error is responsible for a substantial overcount. In Montgomery County at least four persons tallied as slaves in 1830 were under 10 years of age, as was one in Oneida County. Since New York had made the legal importation of slaves quite difficult and the sale of imported slaves illegal after 1801, and because any children born in the state after 1799 were not slaves, it is difficult to imagine how there could any such young slaves at all, especially by residents of Montgomery County who tended not to have come from slave states. This suggests that at least some of these slaves were not slaves at all, but children still bound to service, technically free, but bound to service as incorrectly counted as slaves.
At least four of those recorded as slaves in Montgomery in 1830, two in Putnam County, and two in Tioga County lived in households with free blacks only. In all of these situations in which slaves lived with free blacks, it was female slaves and free blacks males, which may be just a coincidence, or it could mean that the enumerator again entered these black women in the wrong category.
Although each county and each case requires independent investigation and involves some speculation, it appears that the slave count for 1830 in New York was less than half of what it is traditional said to have been. The original count may have been based largely on enumerators placing black women in the wrong category and children of slaves incorrectly entered as slaves. Another possible source of error is ghost tallies, essentially ink marks that have bled over from other pages, which can be confirmed or denied by the column counts at the bottom of the page. These errors affected Carter Woodson’s 1924 count of slaves living with free blacks in 1830 New York. Based on the above considerations, the real slave count for New York in 1830 was between 38 and 51.
By 1830, New York’s slaveholders had almost completely relieved themselves of the legal responsibility of maintaining superannuated slaves. In 1840, some four slaves remained in New York.
 Abstract of the Returns of the Fifth Census (Washington: Duff Green, 1832), 7-10. Compendium of the Enumeration of the Inhabitants and Statistics of the United States. In 1840, three slaves in King’s County, one was under 10, and two between 10 and 24, while one slave in Putnam county was in the 55 to 100 age category
 My count for Chenango County is four male slaves, not three.
 The 1830 New York Census is available in its best form on ancestry.com, where one can search for households with slaves by county. The search for slaves by counties does not yield 76 results – which of course doesn’t mean that these tallies might not be found elsewhere.
 For example, in Montgomery County, Elizabeth Cockburn, listed with only two members in the household, a free black male between 10 and 24 years, and an enslaved female under 10 years of age and no white persons. Harry Cockburn, a enslaved male between 55 and 100, living on his own, nearby. Canajoharie, Montgomery, the household of William Hawn, 3 free black males (1 between 10 and 24, 1 between 24 and 35, and one between 36 and 55) and one female between 24 and 35. Dian Livingston, a household of only blacks, two of whom were female slaves between 24 and 35 years of age, living with four free black men. Thomas Wilson household, Two females under 10, and one between 24 and 36, with also one free black male between 24 and 36. No whites. Abraham Jackson, his own household, one enslaved female under 10, one free black male between 10 and 24. Similarly, two female slaves in Putnam County lived in the household of a free black man, Joseph Townsend. In Tioga County, similarly, two enslaved females (24 to 36 years of age) are listed in the household of free blacks, Tobias Rice and James Hall (both in the 10 to 24 year category), yet no slaves from Tioga County were reported in the 1830 census abstract report.
 Carter G. Woodson, Free Negro owners of slaves in the United States in 1830, together with Absentee ownership of slaves in the United States in 1830 (Washington, D.C.: The Association for the Study of Negro life and history, 1924), 23. Woodson has incorrectly read the census source regarding a slave in Washington County, and many of those listed for New York City have certainly been entered in the wrong category, as is evident from the tabulations at the bottom of the page of the original census form.
 Chenango – 4, Albany – 0 , NYC – 0 , Putnam – 2 to 4, Tioga – 0 to 2 , Montgomery – 18 to 26, Oneida – 14 to 15.