When firing large pieces such as sanitaryware, it is possible to develop enough crystobalite formation in the first fire to cause problems with both cooling and reheating. Cristobalite has phase changes at very low temperatures (ranging from 200-270°C) that can cause the ware to crack when it is first heated in refire. If you check the ware in refire, you may find that cracks have developed before 270°C, and not actually in cooling. If this is the case, you must fire very slowly for those first 270°C.
Examination of the cracked pieces can provide valuable information about the cause of fracture. If the body appearance along the break is shiny, it is probably due to the quartz inversion; if the broken surface is dull, it is a lower temperature occurrence, and is probably attributable to cristobalite.
The more evenly the kiln fires, the narrower the range, since more of the ware in the kiln will reach the critical temperature at the same time. However, the temperature inside the ware will also vary, especially if it is a thick walled piece.
Once the glass is annealed and the residual stresses are removed, the ware is more capable of resisting the normally existing stresses. When not annealed, it can easily crack even with a small extra stress. In practice, microcracks develop and, over time, become major cracks in the ware. Once annealed, the microcracks do not occur. The problem here can be solved by either substituting a stronger non-plastic or by changing the body formula enough to change the thermal expansion of the glass bond and reduce the stress between the bond and non-plastic.
Part of the reason for the confusion between annealing and quartz inversion is that the cure requires slower cooling over the same range of temperatures. Slower cooling between about 1400°F and 900°F is usually the cure for the dunting problem caused by either condition. The more uniform the temperature in the kiln, the narrower that range. However, the annealing must always be over a range as the glass slowly releases its built-in stress.
Sometimes this problem is brought about by not having enough oxygen in the atmosphere. It can be solved by either increasing the air circulated inside the kiln or by allowing more time for oxygen in the kiln to enter the body.
Always remember that when dealing with ceramics there is usually more than one possible cause for an observed event. In the case of cooling dunts, several factors may even be occurring together. So in your hunt for answers, don't just stop at one solution-look for others as well.