We've talked about Ivanhoe's focus on anti-Jewish prejudice, but there is also a fair amount of ethnic racism at work in the book. Brian de Bois-Guilbert has two men in his service, whom he calls his "Saracen slaves" (6.67) or his "Moslem slaves" (5.58). ("Saracen" is an old-fashioned term for Arab people.) These are men he has taken captive in the Middle East and brought back to serve him in England. Reginald Front-de-Boeuf also has foreign slave attendants, whom he uses to threaten Isaac with torture at Torquilstone. Their unfamiliar appearance terrifies and impresses Isaac. And Lucas Beaumanoir makes the spectacle of Rebecca's trial even more dramatic by including four African slaves among his attendants.
None of these enslaved characters speak. This marks them as very different from Gurth, the Saxon "thrall," who talks all the time. What's the difference between these slaves and Gurth? Gurth has an established position in the Saxon social order, even if it's right at the very bottom. These foreign slaves, on the other hand, mainly symbolize terror of the unknown.
Neither the Saxon nor the Norman characters seem to regard these men as human beings. For example, as Scott sets the stage for Rebecca's trial, he explains the racism at work in the Normans' use of the African slaves:
Beside this deadly apparatus [of the stake] stood four black slaves, whose colour and African features, then so little known in England, appalled the multitude who gazed on them as on demons employed about their own diabolical exercises. (43.4)
Beaumanoir is counting on the fact that the audience at the trial will look at these African men and be frightened, which tells us a lot about race relations in the 1190s. The fear with which the white characters regard these men of color is totally dehumanizing. The audience looks at these slaves "as on demons." Scott uses this fear and prejudice to show how superstitious and backward a lot of people were in the 1190s.
Hamet and Abdalla are Brian de Bois-Guilbert's two Muslim slaves. Bois-Guilbert captured them in the Middle East and has brought them back to England to serve as his guards. They never speak, and we know nothing of their inner thoughts or feelings. They appear in the book mainly to show (a) how tough Bois-Guilbert is, having brought back two of his enemies to England to serve him; and (b) to demonstrate that Ivanhoe is happening during the Crusades, which were fought between European Christian and Middle Eastern Muslim armies during the Middle Ages. These two slaves show Bois-Guilbert's status as a Crusader.
There is also an ugly moment at Rotherwood early in the book, when Bois-Guilbert is reluctantly dining with Cedric. When Isaac comes to Cedric's hall, Bois-Guilbert protests the idea of eating with a Jew. Cedric is surprised by Bois-Guilbert's racist feelings when he freely eats with Hamet and Abdalla, who are also of a different religion. Bois-Guilbert replies, "my Saracen [Arab] slaves are true Moslems, and scorn as much as any Christian to hold intercourse [which just means "to talk" here] with a Jew" (5.7). In other words, anti-Semitism unites Bois-Guilbert and his two Muslim slaves despite their own religious differences. It's horrifying that this hatred should be cross-cultural.