Of all the materials and methods artists deploy these days, Cai Guo-Qiang’s media are the most, ahem, combustible. The Chinese artist, who was director of visual and special effects for the Beijing Olympics, has created fireworks installations all over the world. He also draws with gunpowder. This video documents the making of the monumental scroll-like pieces exhibited in “Sky Ladder” at the Geffen Contemporary at MOCA, Los Angeles, through July 30. The exhibition, which I saw recently, also includes videos of his in-the-sky projects.