Slaves also made up a crucial group of labor for the state. One scholar estimates that 2/5 of chattel slaves mentioned in documents were not born slaves but became slaves due to accumulating debt, being sold by family members, or other reasons. However, one surprising feature of this period is that slaves seem to have been able to accumulate some assets and even property during their lifetimes such that they could buy their freedom. Extant documents give details about specific deals for slaves' freedoms negotiated with slaveowners.