American Artists don’t have to be licensed a good thing, that but they do tend to be credentialed. The art world is bristling with degrees from Yale and Cal Arts and hundreds of other academies. In that world, Thornton Dial stands out. He has no formal training and very little schooling of any kind. To be blunt, he can’t read or write. But sometime during his long years as a metalworker in Alabama, he turned to making what he at first simply called “things,” because it would be a long time before he, or anybody else, realized that those things are better described as art. And not just that, but some of the most assured, delightful and powerful art around. Dial’s work has sometimes been described as outsider art, a term that attempts to cover the product of everyone from naive painters like Grandma Moses to institutionalized lost souls like Martn Ramrez and full-bore obsessives like Henry Darger, the Chicago janitor who spent a lifetime secretly producing a private fantasia of little girls in peril. But if there’s one lesson to take away from “Hard Truths: The Art of Thornton Dial,” a triumphant new retrospective at the Indianapolis Museum of Art, it’s that Dial, 82, doesn’t belong within even the broad confines of that category. The show which is on view in Indianapolis through Sept. 18, then travels to New Orleans; Charlotte, N.C.; and Atlanta is a sign that after more than two decades in which his work has settled gradually into the collections of a number of major museums, he may at last be achieving a kind of cultural escape velocity. What he does can be discussed as art, just art, no surplus notions of outsiderness required.