The High Museum of Art in Atlanta has the largest collection of Thornton Dial works in the world. It’s now about to get bigger, thanks to a major gift of artworks to the museum from the Souls Grown Deep Foundation. The acquisition totals 54 works by contemporary African-American artists from the South. Thirteen of those are by Mr. Dial, a self-taught artist who used scavenged materials to depict black struggle in the South. The acquisition includes “Crossing Waters” (2006-2011), which refers to the trans-Atlantic slave trade and was the largest painting ever made by Mr. Dial, who died last year.