Sagger clay (also saggar clay) (not to be confused with the fine grained sagger ball clay which fires to a light cream) is a course grained fire clay which gets its name from the saggers or saggars which it is used to make. Saggars are containers used in the kiln to protect and support pieces, including stacking saggars to allow the production of more pieces in one firing.
References
Sagger clay Wikipedia(Text) CC BY-SA