It appears that slavery is in fact illegal. What is the punishment for practicing it?
Slavery and involuntary servitude is unconstitutional in the U.S. under the 13th Amendment, and Congress was given the power to enact laws enforcing that prohibition.
There is an exception in the 13th Amendment for slavery or involuntary servitude as punishment for a crime. In practice, this means that involuntary chain gangs for people in prison, and military sentences to hard labor are permitted.
The 13th Amendment has also been interpreted to not forbid compulsory military service pursuant to a draft, so long as a non-soldier option is available for conscientious objectors, and does not forbid compelling soldiers to complete their term of enlistment.
There are also isolated circumstances where involuntary servitude has been allowed for civilians, including jury duty, attorneys who are ordered by a court to represent an indigent defendant, doctors who cannot abandon a patient in a critical condition, and babysitters who can't abandon children who would be in peril if the babysitter left.
Federal Penalties Set Forth In Statutes
The answer from user6726 correctly links to the federal laws setting forth the criminal penalties under federal law for slavery related offenses, and a prison sentence of up to twenty years is the penalty for having a slave (i.e. holding someone in peonage).
Those laws also mandate restitution awards and authorize civil lawsuits if a prosecutor fails to bring criminal charges.
Impact Of The U.S. Sentencing Guidelines
A judge sentencing someone for a slavery offense upon a conviction could award any lesser amount of imprisonment instead, however, and would be influenced in exercising his discretion by the United States Sentencing Guidelines which provide an advisory guide on the correct sentence. The relevant U.S. Sentencing Guideline states:
4. PEONAGE, INVOLUNTARY SERVITUDE, SLAVE TRADE, AND CHILD SOLDIERS
Historical Note Effective November 1, 1987. Amended effective November 1, 2009 (amendment 733).
§2H4.1. Peonage, Involuntary Servitude, Slave Trade, and Child Soldiers
(a) Base Offense Level:
(1) 22; or
(2) 18, if (A) the defendant was convicted of an offense under 18 U.S.C. § 1592, or (B) the defendant was convicted of an offense under 18 U.S.C. § 1593A based on an act in violation of 18 U.S.C. § 1592.
(b) Specific Offense Characteristics
(1) (A) If any victim sustained permanent or life-threatening bodily injury, increase by 4 levels; or (B) if any victim sustained serious bodily injury, increase by 2 levels.
(2) If (A) a dangerous weapon was used, increase by 4 levels; or
(B) a dangerous weapon was brandished, or the use of a dangerous weapon was threatened, increase by 2 levels.
(3) If any victim was held in a condition of peonage or involuntary servitude for (A) more than one year, increase by 3 levels;
(B) between 180 days and one year, increase by 2 levels; or (C) more than 30 days but less than 180 days, increase by 1 level.
(4) If any other felony offense was committed during the commission of, or in connection with, the peonage or involuntary servitude offense, increase to the greater of:
(A) 2 plus the offense level as determined above, or
(B) 2 plus the offense level from the offense guideline applicable to that other offense, but in no event greater than level 43.
The base offense level and modification in the language above and additional modifications from other parts of the guidelines are used to produce a final offense level. A pretty typical offense level for this crime would be 25 if no other aggravating circumstances are present.
Then another set of guidelines is used to determine how much of a prior criminal record the convicted defendant had prior to this offense. The sentencing guidelines then have a grid that combined offense level and prior criminal record score to produce a proposed advisory range of guideline sentences for the offense.
That would be a sentence of 57-71 months (about 5-6 years) for someone with no prior criminal records and up to 110 to 137 months (a little under 10 to almost 12 years) for someone with a lengthy and serious criminal record.
Of course, lots of cases of slavery will often also involve other offenses (such as kidnapping, rape or pimping), and sentences for these more aggravated cases would be longer.
Colorado voters narrowly rejected a constitutional amendment proposed by two-thirds majorities in both houses of the state legislature to eliminate this exception in its state constitution in the November 2016 election.
A small number of other states have removed this exception from their state constitutional prohibitions of slavery. Where the exception is eliminated, only voluntary prison labor would be allowed and sentences to hard labor would be prohibited.
State Criminal Penalties
In addition to federal laws imposing criminal penalties for slavery and involuntary servitude, the practice is also outlawed under states laws. For example, the relevant statutes in Colorado state:
§ 18-3-503. Human trafficking for involuntary servitude - human trafficking of a minor for involuntary servitude
(1) A person who knowingly sells, recruits, harbors, transports, transfers, isolates, entices, provides, receives, or obtains by any means another person for the purpose of coercing the other person to perform labor or services commits human trafficking for involuntary servitude.
(2) Human trafficking for involuntary servitude is a class 3 felony; except that human trafficking of a minor for involuntary servitude is a class 2 felony.
§ 18-3-504. Human trafficking for sexual servitude - human trafficking of a minor for sexual servitude
(1)(a) A person who knowingly sells, recruits, harbors, transports, transfers, isolates, entices, provides, receives, or obtains by any means another person for the purpose of coercing the person to engage in commercial sexual activity commits human trafficking for sexual servitude.
(b) Human trafficking for sexual servitude is a class 3 felony.
(2)(a) A person who knowingly sells, recruits, harbors, transports, transfers, isolates, entices, provides, receives, obtains by any means, maintains, or makes available a minor for the purpose of commercial sexual activity commits human trafficking of a minor for sexual servitude.
(b) Human trafficking of a minor for sexual servitude is a class 2 felony.
(c) In any prosecution under this subsection (2), it is not a defense that:
(I) The minor consented to being sold, recruited, harbored, transported, transferred, isolated, enticed, provided, received, obtained, or maintained by the defendant for the purpose of engaging in commercial sexual activity;
(II) The minor consented to participating in commercial sexual activity;
(III) The defendant did not know the minor's age or reasonably believed the minor to be eighteen years of age or older; or
(IV) The minor or another person represented the minor to be eighteen years of age or older.
(3) A person does not need to receive any of the proceeds of any commercial sexual activity to commit an offense described in this section.
(4) Conviction for an offense described in this section does not preclude conviction for an offense described in article 6 or 7 of this title based in whole or in part on the same or related conduct, and the court shall not require the prosecution to elect at trial between such offenses.
In Colorado class 1 felonies are felonies punishable by death or life imprisonment (i.e. first degree murder or kidnapping under circumstances where it is presumed that the victim died).
Other class 2 felonies include second degree murder, aggravated kidnapping where the victim is released alive, gang rape, armed rape and rape involving serious bodily injury. Class 2 human trafficking is punishable by 16 to 48 years in prison (which can be reduced by 25% for good behavior) followed by 5 years of post-release supervision.
Other class 3 felonies including armed robbery, aggravated assault causing serious bodily injury, aggravated rape (other than those that are class 3 felonies), and heat of passion murder. Class 3 human trafficking is punishable by 8 to 24 years in prison (which can be reduced by 25% of good behavior) followed by five years of post-release supervision.
In general, penalties under Colorado's state laws would be more serious than those under federal law pursuant to the U.S. Sentencing Guidelines for the same offense, but in other states, the penalty might be more lenient than under federal law or there might be no specific state law offense for holding someone as a slave.
An Example Of A State Criminal Prosecution For Holding A Slave
These statutes are not just a dead letter.
For example, there is a man who is a Saudi Arabian citizen who is currently in a Colorado State prison for keeping a young woman as a slave doing housework in his home in Colorado.
Homaidan Ali Al-Turki (born 1969) is a Saudi national convicted in a Colorado court for sexually assaulting his Indonesian housekeeper and keeping her as a virtual slave for four years. On August 31, 2006, Al-Turki was sentenced to 28 years in prison on twelve felony counts of false imprisonment, unlawful sexual contact, theft and criminal extortion. In February 25, 2011, He was re-sentenced from 28 to eight years for his good behavior in prison.
A reduction from 28 years to 8 years in a re-sentencing for good behavior in prison is almost completely unheard of, and realistically, reflects a diplomatic negotiation between the U.S. government and the Saudi Arabian government to ease Saudi Arabian government dissatisfaction with the conviction for conduct that would not have resulted in any criminal charged in Saudi Arabia in all likelihood.