Abstract This work reports an experimental study on a long standing, optical, grinding technique. In particular it concerns scratching due to oversize particles in the abrasive and how they are controlled within the grinding process itself. The edge definition of the upper working piece (specimen or lap) controls admission of oversize particles to the space between pieces; a sharp perimeter rejects particles larger than the lap-specimen working distance and completely eliminates scratching. The mechanism discovered appears to be basic to the grinding process and its recognition offers insight into the method's working.