Made from cordierite, a low thermal expansion calcine type of refractory clay. Has heat tolerance of over 2150 degrees F. Smooth and flat, will accept shelf primer to keep glass from sticking to the shelf. Some of my customers dip these in glass separator and cure before use. I think that's a great idea. This shelf is great for smaller kilns including the Skutt Bead kiln. Just make sure to measure your kiln before ordering. These are cut from large slabs before they get to me and sometimes have some edge chipping or minor variations in size which will not diminish performance.