At the September opening for Phoenix artists Jay Atherton and Cy Keener's installation Buoyancy, visitors were a little reluctant to duck beneath its gently undulating surface. The huge, floor-echoing rectangular canvas, hung from the ceiling, was fine for kids to scamper beneath. But we adults, balancing white wine and hors d'oeuvres, found the head clearance uncertain at its saggy silver center. Sometimes you had to stoop to pass under it; other times its middle would rise to accommodate the brave. Around the edges of the tarp, further danger: basalt rocks hung from twine, gently swinging if grazed by passersby and a toe-breaker if dropped. (Disappointingly, these fake counterweights are actually attached to the rafters, independent of the Mylar canvas.) As the name implies, Buoyancy heaves and sighs in response to indoor wind and temperature, the latter supplied by body heat or the skylight above the atrium. It's neither lighter than air nor part of the air, but a plane bisecting the air—a dividing line or thermal boundary. In shape if not function, it's a roof beneath the roof, a floor above the floor. Viewed from the mezzanine, it looks like a trampoline (pity it's not strong enough for kids to romp upon). Appropriate for a gallery at an architectural firm, the piece gets you thinking about the somewhat arbitrary aspect of a roofline—where the pencil is first laid on paper (or cursor on screen), where indoors and outdoors are first demarcated.