Wednesday, January 13, 2010

Dunting

Dunting sounds like some sort of Lacrosse pass. It's not.
I have a large serving dish that days after coming out of the kiln.....it's going PING...and after very careful inspection I see a tiny hairline crack, barely noticeable. A few hours later, another sound and another. For three days not it has been pinging and stressing out. I had to look this one up. I had this once before I wondered what it might be.


Dunting is a special type of crack which occurs from stresses caused during firing and cooling. These stresses primarily occur during two critical points of firing called silica inversions which occur at 1063 degrees F (573 degrees C), and 439 degrees F (226 degrees C). At these inversion points, the structure of the silica molecules rearranges. It is important to fire slowly through these two temperatures, and electronic kiln profiles often do this for you automatically while they are heating.

Most dunting however is caused in cooling. These cracks appear as long, clean, body cracks with sharp edges. If the ware is glazed, the glaze edges are sharp. They may be vertical, horizontal, or spiral.


There are 3 main reasons why cooling dunts occur.


The first occurs as you cool through the first silica inversion at 1063 degrees F. At this inversion the body contracts suddenly. The more silica (quartz) in the body, the more contraction. Since different parts of the pot reach this temperature at different times, it doesn't all contract together, and that causes stresses which can crack. Take for example a tall pot. The top will cool much faster than the bottom, because the bottom has the whole temperature of the kiln shelf keeping it warm. So the top will cool faster than the bottom, causing a crack around the bottom wall.


The second occurs as you cool through the 439 degree F inversion. A similar thing happens as above. However, potters sometimes like to open their kilns at about this temperature to see their pots, and they can make this much worse.


The third type of cooling dunt occurs months or even years after firing. For example, a pot might split right in half after 3 months. This is likely the result of thermal shock. In this case the clay and glaze expand at different rates when exposed to temperature variation, and this change causes the object to crack. To be more specific, the body has contracts more than the glaze. If the glaze is weaker it will shiver (see above). If the clay is weaker the object will crack.- taken from Bigceramicstore.com


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