Article 1, Section 9: "No bill of attainder, ex post facto law, or law denying or impairing the right of property in negro slaves shall be passed." In other words--slavery is the law of the land, forever. A closer reading reveals that states cannot challenge this "federal" law.
"She was received as a commonwealth holding, maintaining and protecting the institution known as negro slavery-- the servitude of the African to the white race within her limits-- a relation that had existed from the first settlement of her wilderness by the white race, and which her people intended should exist in all future time."
Given at the time of secession, an excerpt: "Our new government is founded upon exactly the opposite idea; its foundations are laid, its corner- stone rests, upon the great truth that the negro is not equal to the white man; that slavery subordination to the superior race is his natural and normal condition."