Ceramic materials, as a rule, do not present high mechanical resistance when subjected to tensile stress. It is generally considered that the compressive strength of ceramic products is about ten times higher than the tensile strength. This particularity becomes very relevant in the manufacture of glazed ceramic products, as tensile stresses can develop in the glaze layer as a result of a combination of several factors. When the tensile stresses to which enamels are subjected are greater than their mechanical resistance, a manufacturing defect called cracking occurs, consisting of a set of capillary-sized cracks that appear in the enamel layer.
Among the main factors responsible for the cracking of glazes are the deficiency in dilatometric agreement entre a massa e o esmalte; e a moisture expansion of the ceramic body. In the case of products that receive an engobes layer, thermal expansion and engobes burning retraction are also important variables that can affect the cracking resistance of the product. The same goes for surface applications such as grit and protective coatings, which influence the tensions developed and supported by the assembly.
These concepts are applied to all glazed ceramic products, such as coverings, porcelain tiles, tableware, sanitaryware, electrical insulators, etc. However, what differentiates them significantly is the expansion due to humidity (EPU), which is generally higher in products with greater porosity, such as BIIb ceramic tiles and tableware produced from masses known as earthenware. In traditional porcelain tiles, porcelain tile, insulators and sanitaryware, the masses are more vitrified and the products obtained usually present reduced humidity expansion. However, EPU should never be neglected, as it occurs in all products, even if in reduced magnitude.
As a result of differences in expansion due to humidity, the precautions that must be taken in the manufacture of each product with regard to the dilatometric agreement between the constituent layers must be different. The higher the EPU of a ceramic product, the dilatometric agreement must predict greater compression in the enamel layer, as this condition tends to change over time, after manufacturing.
Coupling voltages can be predicted from analysis of thermal expansion e firing retraction of the mass, engobes and glazes used in the manufacture of ceramic products. More complete analyses can also be carried out in optical platforms developed especially for this purpose. EPU measurements, in turn, must be carried out using high-precision instruments, since the magnitudes of the dimensional variations found in these analyses are very small.
Although this is a phenomenon that has been known for many years, unfortunately, this pathology still occurs with certain frequency in ceramic products. In this context, seek knowledge about the topic to act assertively in the manufacture of ceramic products and control quality of inputs and the manufactured product continue to be the most effective ways to avoid pathologies of this nature.