The firing curve is the time axis (X) in relation to the thermal firing (Y), i.e. a temperature-time-atmosphere curve.
In addition to the composition of the raw materials and the admixture of additives, the length and structure of a firing curve always depends on the individual conditions of the respective production site and can vary greatly depending on the firing method and kiln; often even as a well-guarded trade secret.
Each individual article of a porcelain or tableware series has its own optimum firing curve, which depends not only on the material properties and the geometry of the moulded body itself, but also on the heat radiation of the firing units in the kilns themselves. Deviations from the optimum firing curve can lead to unwanted changes in the physical properties, e.g. Deformationen or Nadelstiche.
The optimum achievement of an ideal firing curve is determined by many years of experience and investigations of thermal analyses. The analyses show the temperature and time points at which the respective reactions take place in the porcelain. The graph above shows a firing curve for hard porcelain in mono firing (single firing) in a 78-metre gas tunnel kiln with active heat recovery. This kiln reaches the maximum temperature for hard firing after 13 hours at 1,370 °C and then “cools down” the product at 900 °C for another 5 hours, slowly decreasing. The process between hour 9 (1,250 °) and 13 (1,370 °) is called glow firing or sintering.