The iconic red sandstone used to build the Smithsonian Castle, one of Washington’s most recognizable buildings, was quarried by slaves, including some who were once most likely owned by Martha Washington, according to new historical research to be published Thursday.

The discovery by anthropology professor Mark Auslander adds to what has been a years-long reckoning with slavery’s role in Washington landmarks, including the Capitol and the White House, and adds nuance to the historical portrait of the Smithsonian Castle, which was built between 1847 and 1855.

Auslander, a native Washingtonian, said in an interview that the Smithsonian has been reluctant over the years to address whether slave labor might have played a part in the history of the Castle. “It’s just an area of total silence,” said Auslander, whose findings are to be unveiled in Southern Spaces, an online, peer-reviewed journal published in cooperation with Emory University’s Robert W. Woodruff Library. “The Smithsonian hasn’t gone through the truth-and-reconciliation process that a lot of institutions have gone through. But I think there’s a willingness to do so.”...