Shape 2 Polymer Ribs are a remake of Mudtools founder Michael Sherrill’s favorite old rib and a workhorse for throwing, smoothing and compressing. Mudtools' polymer pottery ribs are made of a unique flexible material that will not develop burrs or cracks under normal use. With use over time the rib will become even more polished. The color of the rib indicates its softness. Red=very soft. Yellow=soft. Green=medium. The red is the most popular and most recommended.