Caldera-BC Bead Annealing Collar The Caldera-BC collar is a kiln section that stacks onto the Caldera or Caldera-S kiln. The Caldera-BC collar has a 7?? x 2? door on the side for annealing glass beads. The Caldera-BC collar adds 4 ?? of height to the kiln. The collar has no heating elements. The Caldera and Caldera-S, even with a collar, still fire to 2000?F/1093?C. The Collar is made of 2 ?? insulating refractory firebricks.