Average filling factor of clusters in trays 1 (spherical acrylic beads, 2 mm in diameter), 3 (spherical acrylic and glass beads, 2 and 1 mm in diameter, respectively), and 5 (spherical glass and ovoid copper beads, both about 1 mm in diameter). Diamonds show the minimum cluster filling factor immediately after clustering onset, and asterisks show the final filling factor after compaction through wall collisions.