A cremator is not designed to cremate more than one human body at a time. Crematoria may occasionally make exceptions for cases such as still-born twins, or a baby and mother who died during childbirth. The chamber where the body is placed is called a "cremation chamber" or "retort" and is lined with heat-resistant refractory bricks. Refractory bricks are designed in several layers. The outermost layer is usually simply an insulation material, "e.g.", mineral wool. Inside is typically a layer of insulation brick, mostly calcium silicate in nature. Heavy duty cremators are usually designed with two layers of fire bricks inside the insulation layer. The layer of fire bricks in contact with the combustion process protects the outer layer and must be replaced from time to time.