WASHINGTON – African-American slaves sweated in the summer heat and shivered in the winter’s cold while helping to build the U.S. Capitol.

Congress took note of their service and sacrifice Wednesday by erecting commemorative plaques inside the Capitol in their honor. Lawmakers said the memorials will ensure that the contributions of slaves in building one of the world’s most recognizable buildings are never again forgotten.

“In remembering the slaves who labored here, we give them in death some measure of the dignity they were so cruelly denied in life,” Senate Republican leader Mitch McConnell of Kentucky said at the unveiling.

The plaques read: “This original exterior wall was constructed between 1793 and 1800 of sandstone quarried by laborers, including enslaved African Americans who were an important part of the workforce that built the United States Capitol.”

Rep. John Lewis, D-Ga., a former civil rights leader who chaired a congressional task force that studied the contributions of slaves to the Capitol, told onlookers that the plaques help reveal a part of the Capitol’s history that has been overlooked by many.

“Imagine, in Washington’s oppressive summer heat and humidity, to chisel and pull massive stones out of a snake- and mosquito-infested quarry,” Lewis said. “Imagine, having to fight through the bone-chilling winter in rags and sometimes without shoes. Just imagine, the United States government paying your owner $5 a month for your labor. This Capitol, the most recognizable symbol of our democracy, was not built by machines. It was built through the backbreaking work of laborers and slave laborers.”

The plaques were unveiled in the Capitol’s historic Mansfield Room. They will next be placed in the Congressional Visitor Center.