Hardening is a preliminary firing at low temperature, generally between 650°C and 700°C, which is carried out to eliminate the organic matter present in the decoration applied to the ceramic and to fix the colors before the application of the glaze. This process ensures that the decorations remain stable during subsequent stages, such as glazing or a final firing at higher temperatures. Hardening does not completely vitrify the ceramic, as the low temperature does not reach the maturation point of the clay.
This firing is especially important in techniques such as majolica or metal oxide decoration, where the designs painted on the surface must remain sharp and adherent to the clay body during handling and subsequent firings. For example, in decorated ceramics, hardening prevents colors from fading or altering when the liquid glaze is applied to the decorated piece.
Interesting Fact
At around 573 °C, quartz inversion occurs (from alpha to beta phase), which implies a change in the crystalline structure of the material. Although "hardening" does not vitrify the clay, this point is critical during low-temperature firing, because if heating or cooling is not carefully controlled, the internal stresses generated by this change can cause cracks in the piece.
Helpful Tip
Make sure the parts are completely dry before subjecting them to hardening to avoid explosions or cracks in the kiln.
