Cordierite-mullite kiln shelf. 15.5" (394 mm) diameter, 1/2" (16 mm) thick. This is the recommended size for 8-sided top-loading kilns like Euclid's R-85.
Buy 10 of any 1 kiln furniture item & pay 10% less.
The low-expansion property that allows this shelf to withstand repetitive kiln firings also applies to lower temperature applications. Kiln shelves make excellent pizza stones!