PORTLAND, Maine — Pop artist Jessica Lauren Lipton stood on the dance floor at Flask Lounge on Tuesday, clad only in her underwear. Lipton stayed silent but her voice looped on a sound system. It invited strangers to dip their fingers in powdered charcoal and caress her body. She asked them to leave soot-stained fingerprints on her pale skin, blackened evidence of where their hands had roamed. Half a dozen people did just that.