There's something special about these slab trays. The clay is rolled out, and placed on a furniture cushion that's covered by a piece of fabric. Then a GR Pottery form mold is used to impress the shape. The fabric leaves it's mark on the back.
 
This tray is 15 in x 7.5 in x .75 in
 
There are multiple layers of decoration using red iron oxide wash, stencils, porcelain slip and underglaze, and finally a clear glaze is applied.