Holding the temperature during a firing has many uses. Here are several:
1) Firing a few degrees cooler than normal and holding the temperature there for 10 – 20 minutes can heal pinholes in ceramic glazes, heal bubbles in fused glass, and bring out more intense colors in china paints.
2) Holding the temperature forms crystals in crystalline glazes. Some firing formulas include several heating and cooling segments with holds.
3) Temperature hold helps to improve the heat distribution inside the kiln. This may be especially useful when fusing glass in a ceramic kiln.
4) If the witness cone (the one placed on the ceramic shelf inside the kiln) is under-fired in Cone-Fire mode, add hold to the next firing. This is a simple way to correct under-firing. (Glass artists: Cone-Fire mode is used in ceramic firing. You will probably never use this mode.)
5) Hold allows the gases in ceramic greenware to burn out. This prevents firing problems such as pinholes, bloating, carbon coring, etc.
6) Firing to a lower temperature than normal and adding a few minutes of hold sometimes gives greater control of the firing. For instance, it may be easier in some kilns to control the stages of glass fusing by using hold.
Be careful how you use hold. It can easily over-fire the ware if you inadvertently enter 2 hours (02.00) instead of 20 minutes (00.20) of hold time. Experiment with small amounts of hold.
Temperature hold improves many ceramic glazes, but certainly not all of them. This is a matter of experimentation. Too much hold time could even alter the fit between the clay body and glaze.
According to the Orton Ceramic Foundation, one to two hours of hold is equivalent to firing to the next hotter pyrometric cone. This, too, is a matter of experimentation.