The Current Thirteenth Amendment
The current Thirteenth Amendment to the U.S. Constitution reads:
Section 1. 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.
Section 2. Congress shall have power to enforce this article by appropriate legislation.
After the civil war, even the Southern states approved and adopted what is now the Thirteenth Amendment, in contradistinction to the (so-called) Fourteenth Amendment.
The amendment says "Neither slavery, nor involuntary servitude...". According to the founding fathers, as found in the Declaration of Causes and Necessity of Taking Up Arms (1775) and the Declaration of Independence (1776) even voluntary slavery is unacceptable.
While I will be the first to say that the institution of slavery is an abomination, I will also say that calling it involuntary servitude and then justifying it as punishment for some so-called crime is equally an abomination. The ONLY reason it exists is because a gang of perjurers in the US Congress made it so, and it still doesn't apply to "We the People", because they don't have the authority to violate our rights in that way. It ONLY applies to US citizens and proof of that is that a "party" is a "person".
"The Emancipation Proclamation" is what ended slavery, but let's analyse that a bit.