All of our Mudtools polymer pottery ribs are made of a unique flexible material that will not develop burrs or cracks under normal use. We've found that over time the rib becomes even more polished. Each polymer shape is available in a variety of hardness. The Shape 5 Polymer Ribs have much in common with our Shape 1 Polymer Ribs, only on a bigger scale. They offer more surface coverage, making them great for smoothing a slab of clay.