In its emergent form pottery ware made by throwing or building-up is not stable but sensitive to, amongst others against water, chemical influences and vibrations. Therefore the ceramic piece is first dried until it has reached its constant weight. This is done by storing at ambient temperature and then at temperatures between 100 °C – 120 °C. Even after this the moulded blank can still easily break and absorb more moisture. Another temperature treatment follows, the so-called firing which depends on the chosen body, shape and thickness of the specific object. The transition from moulded blank to solid piece takes place only after the firing.
The pottery product can just consist of ceramic body which is described as an unglazed product and is fired only once.
In order to achieve coloured or particularly resistant surfaces, the dried object is provided with a glaze. This piece is normally subjected to two firings: the biscuit firing and the glost firing.
Biscuit firing:
In this first firing, residual moisture, crystalline-bound water as well as organic components are dissolved and burned from this dried blank. Through this the moulded blank hardens and changes to a stable condition as a fired body. The final temperature and the kiln curve are thus chosen so as to ensure that no damage occurs to the blank. Normally the first part of the firing is operated with low heating rates, e.g. 60 °C to 100 °C per hour depending on the size and form of the part. When the temperature reaches about 650 °C the reaction processes are complete and the heating-up speed can be increased to e.g. 80 °C – 120 °C/h. When a final temperature of 920 – 950 °C is achieved, a holding time of about 15 – 30 minutes follows in order to allow uniform temperature distribution. After the kiln aggregate has cooled, the ceramics are taken out at ambient temperature.
Glost firing:
The porous articles resulting from the biscuit firing are given a glaze before the second firing. This can be applied e.g. with a paintbrush, with a spraygun or by dipping. During the glost firing (also known as the glaze firing or second firing) process, the glaze melts on the fired body and forms a vitreous surface. Thus, the final temperature complies with the glaze. The fired body and the glaze must be adapted to each other, otherwise cracks or fragmentation can occur. The heating-up speed to the required final temperature is normally a little higher than with the biscuit firing, about 80 °C to 120 °C per hour; the holding time at the final temperature is about 30 minutes. After cooling down the finished object can be taken out of the kiln.