In chapter 12, Calpurnia takes Jem and Scout to a Sunday service at First Purchase African M. E. Church, which is Maycomb's segregated black church that was purchased by freed slaves. During the service, Scout notices that there are no hymn books and finds it interesting that the congregation sings in unison using a technique called lining. After the service, Calpurnia explains that she is one of the few black citizens who can read and write.
Calpurnia goes on to tell the children that Miss Maudie Atkinson's aunt, old Miss Buford, taught her how to read. When Scout asks Calpurnia her age, Cal responds by saying, "I just have it on Christmas, it's easier to remember that way—I don’t have a real birthday" (Lee, 126). Calpurnia also mentions that she grew up on Finch's Landing. The reader is aware that Finch's Landing used to be a prosperous plantation before the Civil War and that Scout's ancestor, Simon Finch, owned slaves.
Since Calpurnia hails from Finch's Landing, it is reasonable to assume that her ancestors may have been owned by Simon Finch. Calpurnia also attends a church that was purchased by freed slaves and does not know her official birthday. These minor clues also suggest that her ancestors may have been enslaved.