What exactly is moisture penetration?

Every tile covering with a high water absorption capacity - including vitreous group BIII as per DIN EN 14411 E > 10% - will absorb moisture if the installed tiling is exposed to water. As a result, the tile body acquires a darker appearance in the middle than at the edge. This discoloration is promoted by water-permeable joints, by layers forming moisture barriers in the substrate (water becomes trapped between the moisture barrier and the glaze), and by high humidity in bathrooms with inadequate ventilation or air circulation. In principle, the ceramic body will always dry out again. The greater the extent to which a room is used, the more important it is to ensure thorough and regular airing.