Abstract: An impact decorticator is disclosed having a rotor, preferably a squirrel-cage rotor, mounted for rotation about a substantially horizontal axis in a generally concentric and cylindrical casing. The casing is provided at the upper portion with an inlet opening and in the lower portion with an outlet opening for the material being processed and graded, substantially horizontally extending ripples are formed on the internal surface of the casing between the inlet and outlet openings (in the direction of rotor rotation), the ripples being graded in that they are spaced progressively closer together, become progressively sharper, become progressively deeper and/or approach more closely from the inlet toward a central zone between the inlet and outlet openings.