And he that is eight days old shall be circumcised among you, every man child in your generations, he that is born in the house, or bought with money of any stranger, which is not of thy seed. He that is born in thy house, and he that is bought with thy money, must needs be circumcised: and my covenant shall be in your flesh for an everlasting covenant. (Gen. 17: 12-13)
The Talmud discusses the circumcision of slaves who are born into or brought into the household. The Torah text makes clear that they are circumcised as a part of the Israelite family’s household.
there is [a slave] bought with money who is circumcised on the first [day], and there is [a slave] bought with money who is circumcised on the eighth day
That is – for a female slave who is purchased while pregnant and then gives birth to a male, the child is circumcised on the 8th day. But if a female slave is purchased with an infant male that child is circumcised on the first day of purchase.
But the text also describes a different case:
There is [a slave] born in his [master's] house who is circumcised on the first [day], and there is one born in his [master's] house who is circumcised on the eighth [day];
This one is much less clear. That a male born to a slave would be circumcised on the 8th day makes sense (according to the biblical text). But under what circumstance would he be circumcised on the first day?
If she gives birth and then has a ritual bath,(thereby becoming liable to all the responsibilities of a Jewish woman) that is [a slave] born in his [master's] house who is circumcised on the first day
There is controversy as the whether this is the agreed on circumstance. For example:
Said R. Mesharsheya: [It is possible] where one buys a female slave on condition that he will not subject her to a ritual bath.
Thus there is no claim on the mother, but the child is circumcised and the 8th day is not critical.
Of course, we are today very uncomfortable with the immoral institution of slavery – although we have to recognize that it was the reality of the ancient (and not so ancient!) world. It is interesting to note, though, that the slave of Jewish owners were ritually brought into the household. Circumcision and ritual bath (t’vilah) were indicators of being part of the Jewish family, while (ironically to our eyes) still being held as property. I do not think this was thought of as “conversion” in the sense that we mean it today. But it was indication of responsibility: on the part of the slave to Jewish law, and on the part of the slave-owner to treat the slave as one also bound by Jewish law. Philosophically, Master and Slave both had a higher Master in G-d.