Slavery in the ancient world was not based solely on race like in the Confederate South. Slavery in the ancient world happened to you when your city was conquered or when you were kidnapped by pirates. When a city was defeated the women and children were often enslaved, the men were often slaughtered, though sometimes they were enslaved to work in the mines. Or if you could not pay your bills you could be sold into slavery.
In both ancient and modern times, the slaves who labored under the worst conditions were the slaves who worked in the mines and quarries. The mines were often a death sentence, after the Athenian defeat in the Sicilian expedition in the Peloponnesian War the men were enslaved to work in the mines, none made their way back to Athens, most probably only lived for a few years. We have a passing reference that Nicias rented a thousand or so slaves to Athens to work in the mines. In Soviet Russia the worst gulag camp was the Kolyma gold mine in western Siberia, few survived the gold mines of Kolyma. In the ancient world the slaves working quarries were slightly better off, at least they did not have to work in dark shafts with oil lamps, but accidents were frequent in both mines and quarries.
Our other blog on ancient slavery: http://www.seekingvirtueandwisdom.com/slaves-in-the-ancient-world-blog-1-were-slaves-the-employees-of-the-ancient-world/
And on the Bible and Church Fathers on slavery: http://www.seekingvirtueandwisdom.com/teachings-about-slavery-in-the-bible-and-by-the-early-church-fathers/
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The slaves who worked in the plantation farms worked in slightly less brutal conditions. These mostly male slaves lived in barracks, the Roman slaves usually slept in leg irons, the ancient sources do not mention how the Greek plantation slaves were restrained. These slaves were not allowed to marry or have any sort of family life, they were worked like draft animals. Like the slaves who worked in the mines and quarries, they likely worked very long hours in often back breaking work.
In both ancient and modern times domestic slaves were considered the most fortunate of slaves, rarely were they worked to death. In ancient Greece these slaves were considered to be a part of the oikos, or household. Slaves helped raise the children, they were often wet nurses, sometimes teachers, or they accompanied the boys to the city square or agora in Greece where teachers often held class. Quite close personal bonds would form between the child and their wet-nurse, when Odysseus returned home disguised as a beggar it was his wet-nurse slave who first recognized him.
Slaves also did many other chores, since women in ancient Greece rarely left the house the slaves did the daily shopping for food at the market, they helped with the cooking and gardening, and the female slaves helped with the spinning.
Since slaves were considered property they had no rights. Often household slaves were permitted to have a family, but the masters could sell their slaves at any time, slave couples could be separated from each other and from their children. The masters could beat or abuse their slaves, or even kill them, and there was nothing the slaves could do to prevent this. Professor Garland tells the sad story of a pretty girl slave who was regularly beaten by her mistress, because she suspected that her husband was forcing his way upon her. The Athenian courts only admitted testimony from slaves when it was compelled through torture.
Probably the most fortunate class of slaves were those who lived apart, often with their wives and children. These slaves were the most valuable slaves, they performed all the work that the Athenian citizens thought was beneath them, these slaves were blacksmiths, craftsmen, artisan, tanners, managers of shops and factories, and even captains of trading ships. They paid their owner a commission on what they earned.
There were also slaves who were owned by city-states, including Athens. These slaves were the ancient equivalent of the civil service, some were somewhat independent, especially those who were court or other government officials or clerks. Perhaps some of them were work crews who worked on the roads and other infrastructures, maybe they also lived in barracks, we are just not sure.
How many slaves were there? Some scholars estimate that in ancient Greece as many as one in three people were slaves, many were born into slavery. We do know that slavery was the basis of Athenian democracy, giving owners time to participate in the many government assemblies and juries, and the Spartan Helot slaves enabled the Spartans to be full time warriors. But in emergencies slaves would fight side by side with their masters, Herodotus informs us that a large portion of the armies fighting the Persians were slaves, who were likely granted their freedom if they survived the battle.
SLAVERY IN ANCIENT ROME
A higher percentage of Romans were slaves because so many were enslaved in the wars of conquest by the Roman emperors. Julius Caesar may have enslaved as many as half a million people during his wars in Gaul, which is France today. We know that in Rome criminals could be enslaved to work in the mines and the quarries. Greek slaves were the most highly prized slaves, many were skilled artisans and teachers.
Unlike the Greeks, Romans domestic slaves earned a small wage and could eventually save enough to purchase their freedom. They were not totally free, as former slaves their former masters were now their patrons, and they owed a few days of labor each year to their former masters, and if they ran afoul of the law their freedom could be revoked. This taint of once being enslaved was not inherited, the children of free slaves were truly free. There were many freed slaves in ancient Rome, some became quite wealthy.
We know there was a transition from plantation slaves to serfs after communication and trade lessened after the collapse of the Roman Empire in the west. This probably happened at different times and different places, and was probably a gradual process, but history is silent on the how and when of this transition. Although serfs were often bound to the land, they were allowed to live in simple houses with their families, and their masters could not break up their families, since they were no longer considered to be property.
 Robert Garland, The Other Side of History, Lecture 24, Being a Roman Slave