According to usps.com:
With the 2013 Emancipation Proclamation (Forever®) stamp, the U.S. Postal Service commemorates the 150th anniversary of the Emancipation Proclamation, which President Abraham Lincoln signed on January 1, 1863.
Lincoln's proclamation, issued nearly two years into the Civil War, declared that all slaves in the rebel states of the Confederacy “are, and henceforward shall be free.” In addition, the document authorized the recruitment of black soldiers into the Union army. Their courage in battle and contributions to the Union's ultimate victory greatly influenced the nation to adopt the 13th Amendment to the Constitution in 1865, outlawing slavery forever.
“In giving freedom to the slave, we assure freedom to the free,” Lincoln wrote in a message to Congress one month before signing the Emancipation Proclamation. A nation “dedicated to the proposition that all men are created equal” at last began the journey toward true liberty and justice for all.
Art director Antonio Alcalá worked with graphic designer Gail Anderson of New York City to produce the stamp. To evoke the look of broadsides from the Civil War era, they employed Hatch Show Print of Nashville, Tennessee, one of the oldest working letterpress print shops in America.
The Emancipation Proclamation stamp is one of a civil rights set being issued in 2013. The Postal Service is issuing two other stamps in 2013 to commemorate significant anniversaries in the struggle for African-American civil rights. One of them celebrates the 100th anniversary of the birth of Rosa Parks; the other has not been unveiled at the time of this writing.
The Emancipation Proclamation stamps are being issued as Forever stamps in self-adhesive sheets of 20. Forever stamps are always equal in value to the current First-Class Mail one-ounce rate.