A slave who actually had contact with owner was significantly more likely to be freed for good service than one who didn’t. Given that most urban households had more male slaves than female slaves working in important positions, option was not as available to women, though it was not uncommon, especially for personal attendants, wet-nurses, midwives, and physicians. After manumission, there was an expectation of continued loyalty to one’s former owner, especially when a woman was freed for as a reward for good service.
Some women were freed for marriage or concubinage, or as a result of a sexual relationship with her owner that may have ended with her manumission. The Julian Marriage Laws forbid marriage between a man of senatorial rank and a freedwoman, but other men were known to have married their former slaves. By the 2nd century, however, this was publically discouraged and many held that if one wished an official relationship with one’s freedwoman, it should be one of concubinage. Other women were freed in order to marry other freedmen, as a reward to both the man and the woman.* Often in cases like this, the couple were already in a de facto marriage,** and the wife sometimes unofficially belonged to her husband (contubernalis) as a slave prior to her manumission.
A few writers held that one should free a slave who had four children, since she had produced more slaves for the household, though this was a less common attitude. Some women managed to buy their own freedom, though less frequently than men since they were less likely to have access to funds, and when they did, they often had help from a third party, who they later had to repay. How common the manumission of slaves was is uncertain, but it was central to the Roman conception of slavery.
*Often more for the man though. Or at least, it was thought of that way, rewarding the man with his wife’s freedom.
**Known as contubernium.
Perry, Matthew J. Gender, Manumission, and the Roman Freedwoman. New York: Cambridge University Press, 2014.
Pomeroy, Sarah B. Goddesses, Whores, Wives, and Slaves. New York: Schocken, 1995.
Treggiari, Susan. Roman Freedmen During the Late Republic. Oxford University Press, 1969.