“In Slavery’s Wake,” a new exhibition at the Smithsonian’s National Museum of African American History and Culture in Washington, deals in huge themes and vast numbers. Over four centuries, an estimated 12 million Africans were transported across the ocean on more than 36,000 voyages, an epochal forced migration that reshaped societies on both sides of the Atlantic.

But of the artifacts on view in the show, it’s sometimes the smallest that speak most powerfully.

In a case in one corner, there’s a scattering of cowrie shells excavated at Valongo Wharf in Rio de Janeiro, the point of entry for nearly 900,000 enslaved people, which was lost to history until a construction crew stumbled on the ruins a decade ago.

The shells, which may have been carried by people who endured the Middle Passage, are a rare and remarkable survival. But they are also a metaphor for the difficulties of truly grasping the subject of slavery, Johanna Obenda, one of the show’s curators, explained recently during a tour of the exhibition.

“With this history, oftentimes we get lost in the enormity,” she said. “It’s challenging to even process. But when you see artifacts like this, you start to see faces, people.”