For decades now, dealers in early American folk art have been ransacking barns and attics, dragging back to the cities truckloads of their quarry: samplers and fracturs, whirligigs and tavern signs, painted chests, quilts, scrimshaw, wooden Indians and running-horse weather vanes peppered with goose bumps from 50 years of target practice by farm boys. It is an industry with scholarly spinoffs.
The first exhibition of American folk art—which, roughly speaking, means the work of late 18th and 19th century rural (or at least not cosmopolitan) artists and artisans with little or no formal...