Abstract art is a term that includes a wide variety of media: monochromatic color fields, hard-edged abstraction, and its flat colors, and the sharply defined edges and optical illusions inherent in a geometric form. For its new exhibition “Printing Abstraction,” the Saint Louis Art Museum draws from its holdings of abstract art created by printmakers. The show is something of an expansion of the museum’s ongoing main exhibition, “Graphic Revolution: American Prints 1960 to Now”, in that it offers more examples of the printmakers’ art and the key role it’s played in the promulgation of abstract art.