Sherrill Mudtools Polymer Rib for Pottery and Clay Artists, Shape 4, Blue Color, Firm Potters Ribs are essential tools for throwing clay on the wheel, hand building or sculpting. Shaped like the traditional kidney rib to fit comfortably in your hand. Mudtools polymer pottery ribs are made of a unique flexible material that will not develop burrs or cracks under normal use. Available in different sizes and colors (color designates level of flexibility) Shape 4 Ribs are 4.125" x 2" and Made in USA!