Scarva Earthstone ES130G White Earthenware Grogged Clay From  £20.33

I have had shivering twice using glazes in their recommended range . Once I bisque fired to 1100, then glaze fired at 1040,:the second time I single fired ton1100. The firing range given is 1060 - 1220, but the comment is the clay matures between those two temperatures. As I am getting shivering I assume the clay has NOT matured, and the firing range given is misleading. I will now retest at a higher temperature.