Since the mid-1970s Richard Serra has been exploring the sculptural fundamentals of mass and gravity to create work that produces a simultaneous sense of balance and precariousness. Prop consists of a 60-inch-square sheet of lead that is held flat against the gallery wall and about three feet off the floor by a rolled lead cylinder that leans against it. The apparent instability of this arrangement leads the viewer to focus on the weight of the lead sheet and on the simultaneous assertion and defiance of the force of gravity. Serra's piece in the Minneapolis Sculpture Garden, Five Plates, Two Poles (1971), produces a similar kind of tension with five large steel plates propped up against each other.