On what page?
In the 1860 census there were 488,000 free blacks in the entire United States. Apparently every free black man in the country sneaked into the South so he could fight for the good old CSA.
There were doubtless a good many slaves who went along with their masters to battle, and probably a good many of them were allowed to arm themselves and fight, and were perhaps even accepted by other men in the regiment on an informal basis. However, it cannot seriously be contended that a slave can genuinely volunteer for anything.
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