Spalling
Spalling is a ceramic defect that occurs when the glaze or engobe detaches from the clay body in small fragments or flakes. This problem is mainly caused by compressive stresses in the glaze layer, caused by an incompatibility between the coefficient of thermal expansion of the glaze and that of the clay body. These stresses, if too high, exceed the strength of the glaze, causing it to separate.
This defect affects both the aesthetics and the functionality of the pieces, especially in utilitarian objects such as tableware or liquid containers, where spalling can compromise the impermeability. For example, a cup with spalled glaze on the inside not only loses its visual appeal, but can also absorb liquids, becoming unusable or unsanitary.
To prevent spalling, it is essential to ensure thermal compatibility between the glaze and the clay body, apply the glaze in uniform layers and avoid firing cycles with abrupt temperature changes. Also, a correct preparation of the surface of the piece, free of dust or grease before glazing, improves adhesion and reduces the risk of this defect.Interesting Fact
In the "naked raku" technique, the glaze cracks and flakes off, leaving char marks and unique patterns on the surface of the piece.Helpful Tip
To avoid spalling, make sure that the coefficient of thermal expansion of the glaze and the clay are well aligned, test firing as necessary.
