Due to the low thermal conductivity of zirconium oxide frameworks, it is generally recommended to use a low heat rate during the firing of the ceramic in the furnace. However, zirconium oxide, in fact, has a better thermal conductivity than the veneering ceramic and the basis for such recommendation is wrong. During the firing of the ceramic, the amount of applied energy, in relation to the needed amount of energy, is a deciding factor for the degree of the ceramic sinterization. This amount of energy is controlled, during the firing procedure, through temperature and time. For the control through the time, there are two parameters available: the heat rate and the holding time at final temperature. The total amount of energy needed is determined by the energy necessary for both the veneering ceramic and the material of the supporting framework. These are determined by the respective mass and the respective specific quantity of heat necessary.
Zirconium oxide has a higher specific heat quantity compared to e.g. preciuos alloys, therefore the same mass of zirconium oxide needs more energy to warm up compared to precious alloys. Since the sintering process takes place from the outer to the inner layers, and thus, only through the layered ceramic veneering, the thermal conductivity plays only a negligible role.
To achieve a homogeneous firing (sintering degree) it is important for the object to have a low temperature gradient, that means a possibly uniform heat distribution. This can also be achieved through a low heat rate; this procedure is very time consuming, though. Another way is to have a sufficient pre-heating of the firing object before beginning to increase the temperature. This procedure has the advantage of a uniform temperature throughout the whole firing object. Throughout the subsequent heating, even with a heat rate of 100°C/min, only a minimal temperature gradient will be formed, thus resulting in a uniform firing.
Consistent pre-heating has the additional advantage that the sintering process starts simultaneously also at the surface of the supporting framework and prevents, for example, the retraction of the ceramic from the margins or its uncontrolled lifting or tearing.