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A: Cooling dunts-tiny or even unseen cracks that occur on cooling-have several causes, including excessive stress arising out of volumetric changes that occur due to displacive beta to alpha (b?a) changes in crystalline forms, uneven thermal contraction due to uneven temperature, rapidity of annealing, or incomplete mineralogical changes.
Understanding Phase ChangesThe most commonly held reason for cooling dunts is the cracking that may occur from the quartz phase change on cooling. This change, known as quartz inversion, occurs suddenly at a temperature of 573°C (measured at the quartz grain) and can be very destructive due to its suddenness and the magnitude of volume change. This change occurs rapidly, as it is a result of distortion (not breakage) of the Si-O bond.
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.
Thermal ExpansionThe same question of range applies to thermal expansion. If a piece is very thick, there may be a different amount of thermal contraction experienced between the inside and the outside of the piece. Since the outside will cool more quickly than the inside, it will shrink more and set up stresses between the sections that could lead to cooling cracks. The same is true for a long piece that is exposed to different temperatures from one end to the other.
Glassy Bond AnnealingIn reality, some cooling cracks that are blamed on quartz inversion are really a result of stresses developed in the glass bond that forms in the ceramic. Usually if the bond is properly annealed, there is no problem. However, if the constituents are such that the naturally occurring stress between the glass bond and the non-plastic ingredients are too much for the non-plastic material, it will crack on cooling, and will continue to crack over a long period of time. In fact, delayed cracking can be an indicator of this type of problem.
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.
Other CausesA more subtle problem occurs when the expected reactions are not completed properly. Without getting into a detailed discussion, suffice it to say that some reactions may stop at sub oxides and be in intimate contact with completely oxidized and changed minerals. The difference between the two sets up stresses that can cause delayed cracking after cooling. This is sometimes mistaken as cooling dunts, but can only be stopped by firing a little longer during heating to complete the reactions throughout a large ceramic.
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.