When Ruth Duckworth titles her work—a rare occurrence—she uses large names: The Guardian (1992), The Spirit of Survival (1996), The Creation (1984). The other 79 pieces in the show, Ruth Duckworth, Modernist Sculptor, at the Smithsonian American Art Museum Renwick Gallery are untitled, suggesting that Duckworth is chasing after something unutterable. When she first went to school as a teenager, she announced her desire “to paint like Rembrandt, draw like Dürer, and sculpt like Michelangelo—pretty…