Changes to United States Constitution are rare. In our history only 27 times have amendments to the Constitution been ratified. The Emancipation Proclamation, however, was unlike any amendment. A unique piece of American legal history, and quite different from the 13th Amendment which freed slaves, the two documents do have many similarities.
The Emancipation Proclamation
Legally, the Emancipation Proclamation was a strategically clever move. The president is not endowed by the Constitution to proclaim laws or even bestow civil liberties on specific groups of people. However, the president is empowered with broad wartime powers to protect the general welfare of the United States. Abraham Lincoln made the Emancipation Proclamation under his power as commander in chief in a time of war. It was not approved by Congress or even voted upon. Lincoln issued the proclamation as a king issues an edict. The key similarity between the proclamation and the 13th amendment is the intention of each was to free the slaves.
The 13th Amendment
Unlike the Emancipation Proclamation asserted as a power of the commander in chief, the 13th amendment followed legislative procedures outlined in the Constitution to create a permanent change to the highest law of the United States. It was voted on in Congress and ratified by a majority of states. The relationship between the two documents is the amendment is the official means by which the emancipation was carried out.
Arguably, the purpose of the Emancipation Proclamation was much broader than the purpose of the 13th amendment. The initial pursuit and primary goal of the Civil War was to preserve the union. In pursuit of union preservation President Lincoln came to the conclusion that slaves must be freed. It is unclear to what extent Union citizens and the president were motivated by the immorality of slavery. But, at some point in the Civil War, the moral purpose of freeing the slaves aligned itself with the patriotic purpose of freeing the slaves.
It should be noted the Emancipation Proclamation did not free a relatively small number of northern slaves. It concerned itself with Confederate slaves, omitting Union slaves.The signing of the 13th amendment supports the idea that Lincoln's purpose was to free all slaves.