Post #584899 by Babalu on Thu, Apr 14, 2011 12:25 AM

Hey guys,

I've had this happen before too. Glaze will continue to cure long after it has been fired. Dunting could be the cause when you see long vertical cracks like this. I have seen where this will happen when there is a thicker layer of glaze on the bottom inside of the mug than are on the inside walls. Anyway the below is a short explanation that I pulled off of the Internet:

Dunting
This is a special type of crack which occurs from stresses caused during firing and cooling.
This stress primarily occurs during two critical points of firing called silica inversions which occur at 1063¢ª F or 573¢ª C and 439¢ª F or 226¢ª 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 during cooling.
These cracks appear as long, clean, body cracks with sharp edges.
If the pottery or ceramic is glazed, the glaze edges are sharp.
They can be vertical, horizontal or spiral.

There are three main reasons why cooling dunts occur.
The first occurs as you cool through the first silica inversion at 1063¢ª F.
At this inversion the body contracts suddenly.
The more silica, quartz, in the body, the more contraction.
Since different parts of the pottery or ceramic reach this temperature at different times, it doesn't all contract together and that causes stress 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.
The top will cool faster than the bottom, causing a crack around the bottom wall.

The second occurs as you cool through the 439¢ª F inversion.
A similar thing happens as above.
But, pottery and ceramic artists sometimes like to open their kilns at about this temperature to see their pieces and this will make it much worse.

The third type of cooling dunt occurs months or even years after firing.
Sometimes the pottery or ceramic might split right in half after three months.
This is usually the result of thermal shock.
In this case the clay and glaze expand at different rates when exposed to temperature variations and this change causes the object to crack.
To be more specific, the body contracts more than the glaze.
If the glaze is weaker it will shiver.
If the clay is weaker the object will crack.