The manufacturing of porous wall tiles is quite ancient, but even with the newest technology it still suffers from 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 can show a slow but inexorable tendency to react with the water present as humidity in the air, which permanently changes its properties. This dimensional expansion of the ceramic pieces occurs over a period of time, which can vary from several days to several years. In the past, this phenomenon was not comprehended and was the biggest problem of porous ceramic bodies. During the past 50 years, however, it has been heavily investigated and is now completely understood. The solution looks easy at first sight: minimise the moisture expansion and increase the compression of the glaze on the body. Indeed, this approach produced excellent results for many years, but today the problem is showing up again under different circumstances.
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