It's fun to speculate about how he accomplished this artwork. I agree with you others, that the original sheet of glass was probably bonded to a black substrate. However, that might not hold all of the shards, and the surface would harbor many dangerously sharp edges and points of the broken pieces. Could an intact sheet of glass be pressed against the surface to keep it clean and reduce the hazards?
Maybe our friends at ArtGlass can shed some light on this innovative technique, since they were a sponsor. I'll ask.