a ceramic base having a center;

a ceramic, generally planar, vessel support member defining a generally planar vessel support surface defining a central aperture therein and a periphery, the vessel support member forming an annulus between the central aperture and the periphery;

a ceramic wall extending from the base to the annulus formed by the vessel support member, the vessel support member and the wall defining a window therein, the window extending about the center of the base by an angle, the angle being between 45 degrees and 150 degrees; and

wherein the window has an upper boundary defined by a portion of the periphery extending across a single camber path to reach an opposite boundary of the annulus.