NEW ORLEANS — A marathon project is under way in New Orleans to digitize thousands of time-worn 18th-century French and Spanish legal papers that historians say give the first historical accounts of slaves and free blacks in North America.

Yellowed page by yellowed page, archivists are scanning the 220,000 manuscript pages from the French Superior Council and Spanish Judiciary between 1714 and 1803 in an effort to digitize, preserve, translate and index Louisiana’s colonial past and in the process help re-write American history.

“No single historian could ever live long enough to write all the books that are to be written from all these documents,” said Emily Clark, a Tulane University historian who has worked in the papers....