13th Amendment Loophole

When I watched Lincoln the other week, I noticed something about the 13th Amendment I had not been aware of before. The movie's narrative follows the political struggle to pass the 13th Amendment through Congress. At the end, the text of the 13th Amendment is read out loud:

Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.

I highlighted the above section because it's a big fat loophole in the U.S. Constitution just waiting for some brutal geniuses to think up the prison industrial complex. 150 years later, we have mass incarceration and states of the Old Confederacy have considered using prison labor to do the work that slaves would have done prior to the Emancipation Proclamation.