Few things sting like opening the kiln to find a piece you spent hours on split down the middle. The good news is that kiln cracks aren't random. Almost every one traces back to one of a handful of specific causes, and most are preventable once you know what to look for.
| Crack | Looks like | Usual cause |
|---|---|---|
| S-crack | S or spiral in the bottom of a thrown form | Uneven compression / trapped water when throwing |
| Dunting | Sharp cracks after firing, often through the wall | Cooling too fast / thermal shock |
| Explosion / blowout | Piece blown apart, shards on shelf | Moisture flashing to steam in bisque |
| Rim/edge cracks | Cracks starting at the rim | Drying unevenly before firing |
There are two danger zones in every firing. The first is around 212°F (100°C), where any remaining water turns to steam. Go too fast here and a piece can literally explode. The second is quartz inversion near 1063°F (573°C), where the silica in your clay physically changes volume. Rush the kiln through that point, up or down, and you invite dunting.
Patience at both ends of the firing. A slow preheat clears moisture before it can flash, and a full, unhurried cool carries pieces gently through quartz inversion. A programmable controller lets you build both into every firing automatically, so you're not standing over the kiln guessing. The temptation to open the lid early to "save time" is behind more dunting than almost anything else.
A controller with programmable preheat and controlled cooling prevents most thermal cracking. Not sure what your kiln can do, or ready to upgrade? Call or text me, Spencer, at (801) 839-5882.
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