From about 1640 to 1795, historians say, perhaps 15,000 slaves were buried there in a forgotten wasteland. Many had died before the age of 12. Some had died within two years of arriving in chains. The photographs, “African Burial Ground” and “Three Placards” solemnly concretize a sentiment, a place, a time, and, in effect, become monuments themselves. . . . The overall site, the equivalent of nearly seven acres, is the oldest and largest African burial site in North America, according to the National Park Service. . .