The ultimate example of the distinction between the two trades is that in the greatest Islamic empire, the Ottoman Empire, after the sons of the first two sultans, no sultan mounted the throne who had not been born of a concubine. The Ottoman ruling family did not marry because they regarded the royal family as above any alliance. Occasionally, marriage would be used to ensure the loyalty of a Turkish tribe, but overwhelmingly the fertility of the Ottomans was through concubines.
Why could Islamic slaves assimilate into the surrounding society so more easily than American blacks could?
Here we get to a further dimension of the difference between the two trades. Slavery in the West, because it was so cruel and had become so disreputable, required some kind of excuse or extenuation -- the idea of biological discrimination. Essentially, the concept of race developed and was popularized. The sort of pseudo-scientific view, in distinction from the pseudo-religious view, came about during the Victorian age, the 19th century, when you had Darwin's theory of evolution. You could irresponsibly and intellectually dishonestly subscribe to the idea that certain races were inferior.
But the Koran, on the other hand, prohibits racism?
The Koran very explicitly attacks it. According to the Prophet, Islam comes to do away with these distinctions of tribe and nation and color. There is a strong argument made by Patricia Crone that, initially, Mohammed was most influential in a political rather than a religious sense. He supplanted this intertribal rivalry by uniting a large part of the Arabian people into a political unit, and, of course, it then became an imperial power.
Was there no stigma attached to being black in Islam?
Nothing is ever quite so simple. There did develop an attitude toward color. There were distinctions in market value and general consumer appreciation between one sort of black slave and another. Some of this was aesthetic. One tends to think that anyone who looks like one's own people is more beautiful. For instance, the Ethiopians and the Nubians were highly favored because they had sharpish noses rather than flat noses and they were lighter colored. Clichés developed so that you had so-called Negro slaves for hard work and you had Ethiopians and Nubians for concubinage.
But this was never institutionalized. This is another key to the difference between the two empires. Of course, there were Islamic pseudo scientists in the Middle Ages who said differences of character and temperament were the consequences of climate -- those who lived too far from the sun in the North had frigid temperaments, and those who were immediately beneath the sun were given to too much merriment and too little thought.
But in the context of the development of Islam it would have been a real break with tradition had it been institutionalized in law. This is important for the assimilation aspect too, because once you were freed, there was no discrimination in law against you.
They weren't confined to an underclass after they were freed?
Many of them might have been, although the client/patron relationship was a sort of protection if you were in need -- that is, if your previous owner was a true practicing Muslim. And there isn't this history of separation. The nature of the Atlantic trade and therefore the survival of racism in the West has been one of segregation. In America, separation was the social clarion call and as bad in the Northern states as in the Southern. Generally, the geographical separation -- the kind of separation in individual churches where blacks were seated in one part of the congregation and whites in another -- produced this enormously creative black diaspora in America, as well as infinite suffering.
There wasn't this separation in Islam. Whites didn't push blacks off the pavement. They didn't refuse to allow a black singer to sing in Constitution Hall. They didn't forbid restaurants to serve them. I don't think that there's any disputing that slavery was a more benevolent institution in Islam than it was in the West.
Also, it is irrational to make the exclusive connection between slavery and color that existed in the West because there were white slaves in Islam in significant numbers.
In comparable numbers to black slaves?
With the enormous expansion of Islam and the conquests of huge territories, there were certainly large numbers of white slaves in the early periods. But, to be cautious, white slaves became increasingly more difficult and expensive to obtain. Black slaves became far more numerous than white ones. Certainly, when you get to the 19th century, which was the cruelest century, there were many more black slaves than white ones in Islam.