Historically, most slaves ancestors were initially captured in wars or kidnapped in isolated raids but some were sold into slavery by their parents as a means of surviving extreme conditions. Most slaves were born into that status. Ancient warfare often resulted in slavery for prisoners and their families who were either killed, ransomed or sold as slaves. Captives were often considered the property of those who captured them and were looked upon as a prize of war. Normally they were sold, bartered or ransomed. It originally may have been more humane than simply executing those who would return to fight if they were freed, but the effect led to widespread enslavement of particular groups of people. Those captured sometimes differed in ethnicity, nationality, religion, or race from the captors but often were the same as the captors. The dominant group in an area might take captives and turn them into slaves with little fear of suffering the like fate, but the possibility might be present from reversals of fortune, as when Seneca warns, at the height of the Roman Empire,
And as often as you reflect how much power you have over a slave, remember that your master has just as much power over you. "But I have no master," you say. You are still young; perhaps you will have one. Do you not know at what age Hecuba entered captivity, or Croesus, or the mother of Darius, or Plato, or Diogenes?
and when various powerful nations fought among themselves anyone might find himself enslaved. The actual amount of force needed to kidnap individual people for slaves could lead to enslavement of those secure from warfare, as brief sporadic raids or kidnapping often sufficed. St. Patrick recounts in his Confession having been kidnapped by pirates, and the Biblical figure Joseph was sold into slavery by his brothers.
Ancient societies characterized by poverty, rampant warfare or lawlessness, famines, population pressures, and cultural and technological lag are frequently exporters of slaves to more developed nations. Today the illegal slave trade (mostly in Africa) deals with slaves who are rural people forced to move to cities, or those purchased in rural areas and sold into slavery in cities. These moves take place due to loss of subsistence agriculture, thefts of land, and population increases.
In many ancient cultures, persons (often including their family) convicted of serious crimes could be sold into slavery. The proceeds from this sale were often used to compensate the victims. The Code of Hammurabi (~1800 BCE) prescribes this for failure to maintain a water dam, to compensate victims of a flood. The convicted criminal might be sold into slavery if he lacked the property to make compensation to the victims. Other laws and other crimes might enslave the criminal regardless of his property; some laws called for the criminal and all his property to be handed over to his victim.
Also, persons have been sold into slavery so that the money could be used to pay off their debts. This could range from a judge, king or Emperor ordering a debtor sold with all his family, to the poor selling off their own children to prevent starvation. In times of dire need such as famine, people have offered themselves into slavery not for a purchase price, but merely so that their new master would feed and take care of them.
In most institutions of slavery throughout the world, the children of slaves became the property of the master. Local laws varied as to whether the status of the mother or of the father determined the fate of the child; but were usually determined by the status of the mother. In many cultures, slaves could earn their freedom through hard work and buying their own freedom; this was not possible in all cultures.
The type of work slaves did depended on the time period and location of their slavery. In general, they did the same work as everyone else in the lower echelons of the society they lived in but were not paid for it beyond room and board, clothing etc. The most common types of slave work are domestic service, agriculture, mineral extraction, army make-up, industry, and commerce. Prior to about the 18th century, domestic services were acquired in some wealthier households and may include up to four female slaves and their children on its staff. The chattels (as they are called in some countries) are expected to cook, clean, sometimes carry water from an outdoor pump into the house, and grind cereal. Most hired servants to do the same tasks.
Many slaves were used in agriculture and cultivation from ancient times to about 1860. The strong, young men and women were sometimes forced to work long days in the fields, with little or no breaks for water or food. Since slaves were usually considered valuable property, they were usually well taken care of in the sense that minimally adequate food and shelter were provided to maintain good health, and that the workload was not excessive to the point of endangering health. However, this was not always the case in many countries where they worked on land that was owned by absentee owners. The overseers in many of these areas literally worked the slaves to death.
In mineral extraction, the majority of the work, when done by slaves, was done nearly always by men. In some places, they mined the salt that was used during extensive trade in the 19th century. Some of the men in ancient civilizations that were bought into chattel slavery were trained to fight in their nation's army and other military services. Chattel slaves were occasionally trained in artisan workshops for industry and commerce. The men worked in metalworking, while the females normally worked in either textile trades or domestic household tasks. The majority of the time, the slave owners did not pay the chattels for their services beyond room and board, clothing etc.
Female slaves, who for the most were from Africa, were long traded to the Middle Eastern countries and kingdoms by Arab traders, and sold into sexual slavery to work as concubines or prostitutes. Typically, females were sold at a lower price than their male counterparts, with one exception being when (predominantly) Irish women captured in Viking raids were sold to the Middle East in the 800-1200 period.