The two street facades are covered in terra-cotta.
Like many buildings in Pittsburgh, the museum's two street facades are coated in cream-colored glazed terra-cotta.
Terra-cotta was a popular surface embellishment at the turn of the century because of its decorative beauty and self-cleaning properties. Basically, terra-cotta has a layer of protective film which keeps its pores from absorbing water and dust. Because of the layer of film, rainwater easily washes away any dust on the surface.
fact via and via / image via