The manufacturing of porous wall tiles is quite ancient but even with the newest technology still suffers of old problems. The study of the state of tension between glaze and body is fundamental in order to avoid some common defects, such as delayed crazing. This defect is due to the volume increase of the body, because of absorption of humidity. The body, in fact, can show a slow but inexorable tendency to react with the water present as humidity in the air, which changes permanently its properties. This dimensional expansion of the ceramic pieces occurs in a period of time which can vary from some days to some years.