The coarse, very hard alumina particles serve to erode all the rough surface asperities. The action of the polishing forces also causes a deagglomeration of the Siliceous Earth particles. The kaolinite then covers the surface, and the released fine silica grains break up the alumi-na crystals which likewise disintegrate into lamella-type fragments. Jointly with the kaolinite, they build up a rooftile-like protective layer on the surface, which is already smooth and free from loose particles.