Late last year, Matthew J. Mosca, a specialist in historic paint finishes, gazed up at what looked like a scrap of wallpaper. It was jutting up from an old coat of red paint covering the walls of a pantry inside the yellow-brick farmhouse where the 19th-century artist Thomas Cole displayed paintings that revolutionized American ideas about art and wilderness.

But when Mr. Mosca climbed a ladder, he found it was not wallpaper, but a bold black decorative pattern known as a Greek key that had been hand-painted onto the plaster itself. Excited, he took this discovery to Jean Dunbar, a consultant in historic interiors who was working with him on the refurbishment of the Thomas Cole homestead.