The technology of porcelain stoneware revolutionized the ceramic sector bringing innovation, not only in the production process, but also in the final product which become to be employed in for new applications. Today, porcelain stoneware is the best-selling ceramic material in the world. In recent years, with the advent of new technologies and production plants, there has been a huge leap in the range of formats which has led to production of the so called "large slabs". The development of ceramic products with large dimension induced the redefinition of interior architecture. Large porcelain stoneware ceramic slabs are in fact used as coating for doors, wardrobes, kitchen doors, kitchen and bathroom tops. From the large formats it is then possible to obtain a whole series of smaller formats through the use of different cutting techniques. One above all is that of the "split cut" which consists in engraving the slab coming out of the kiln and breaking it along the groove. The large slabs production, in addition to bringing many advantages, has however to face some issues in the manufacturing steps, especially in relation to the possible stress and tensions that could occur during cooling. It has been observed that excessive tension inside large slabs induces breakage and defects at the moment of split cutting. Hence the need to understand the origin of the stress states inside the large slabs that cause them to break when cut. In particular, it was observed that, keeping the same firing cycle and the same ceramic body, ceramic products made with different types of engobes showed different behaviours when cut. This evidence therefore led to the formulation of a hypothesis according to which the origin of the problem could lie in the chemistry and mineralogy of the engobe used. A series of engobes realized with different formulation has been deeply analysed from chemical and mineralogical point of view before and after firing in order to evidence the phase evolution during the production cycle. The results obtained from XRF and from Rietveld refinement on the pristine and fired samples indicates a rather complex picture in which the origin of the tensions inside the large slabs may be linked to the degree of fusibility of the engobe used.