It was 230 years ago Sunday that Robert Carter III, the patriarch of one of the wealthiest families in Virginia, quietly walked into a Northumberland County courthouse and delivered an airtight legal document announcing his intention to free, or manumit, more than 500 slaves. He titled it the “deed of gift.” It was, by far, experts say, the largest liberation of Black people before President Abraham Lincoln signed the District of Columbia Emancipation Act and Emancipation Proclamation more than seven decades later. On September 5, 1791, when Carter delivered his deed, slavery was an institution, a key engine of the...