resistance to trampling wear is the resistance of a surface to wear and tear (for unglazed tiles) or to change its appearance (for glazed tiles). According to UNI EN norms, abrasion resistance is classified with PEI grades, where PEI 1 corresponds to the lowest resistance and PEI 5 corresponds to the most resistant tiles. For unglazed tiles, abrasion resistance increases with the decrease of water absorption therefore it is high in the case of porcelain stoneware. The abrasion resistance value can also change depending on the treatments carried out on the tile surface: in the case of terracotta tiles, for instance, the impregnation using natural or synthetic substances and for porcelain stoneware any polishing treatment. For glazed tiles, abrasion resistance exclusively depends on the glaze: light colour glazes tend to show the effects of use more, also due to their more complex cleaning.