Pottery and Ceramics Glaze
Cracking, Shivering, Crazing and Dunting
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Have you ever opened the kiln and found a big crack on your favorite piece?
CAUSESCracks usually result from stress in the clay.
There is always some stress in clay because it shrinks as it dries, when it is fired and it also expands and contracts during firing.
Sometimes if the stress is too much for the clay to handle it cracks.
How the piece of pottery or ceramic is made can either minimize cracking or contribute to cracking.
If you sit unfired pottery upside down on its rim, you will put stress on the rim.
A crack may not appear right away, but it could show up later as the pottery dries more or when it is fired.
It is a good idea when turning pottery upside down, to place the rim on a piece of soft foam.
Different clays can handle different amounts of stress without cracking.
The shape of the pottery or ceramic may cause the cracking.
Sharp corners concentrate stress and are more prone to cracking.
Fast drying will tend to cause more stress than slow drying.
Over firing pottery or ceramics or firing it multiple times will tend to make it more susceptible to cracking.
Differences in thickness of the pottery or ceramic will also set up stresses, since the thin areas will dry faster than the thick and the stress won't be evenly distributed.
This is very common when the base of pottery is thicker or thinner than the walls.
Sometimes this will show up as a circular crack around the bottom edge of the pottery.
A thick layer of glaze on the inside of a pottery or ceramic and a thin or no layer on the outside will cause stress which will result in a spiral crack up the sides.
If glaze pools on the inside of a pot, tension is created and the pottery or ceramic may crack or split across the base.
Specific Types of Cracks
Sometimes a piece of glaze will crack off, normally near a rim or at edges.
Some clay may be attached to the glaze piece that cracks off.
This occurs because stress has built up between the clay and glaze that can't be absorbed.
It is often caused by over sponging which takes away the fine clay particles and leaves behind the groggier clay particles which are not elastic enough to absorb the stress.
This is a network of very fine cracks in the glaze.
It is caused by a mis-match between the clay and glaze.
It often will not show up until the pottery or ceramic is cooled or sometimes even until it has been heated and cooled a few times.
Some people believe slow cooling will prevent crazing, but the stress still exists and eventually the crazing will occur.
DuntingThis 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˚ F or 573˚ C and 439˚ F or 226˚ 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˚ 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˚ 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 usuallly 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.
S CracksOne of the most common cracks found in pottery, is the "s" crack, which occurs at the bottom of a pot, in the shape of an s, usually on thrown pieces, but can also happen to a poured ceramic.
The most important thing to remember, is that you should keep the bottom of the pottery or ceramic as dry as possible while throwing or pouring and compress the bottom during throwing and trimming.
Troubleshooting A CrackIf you have a crack, find the point where it is widest.
This will be the point where the crack started and will help you understand what happened.
Cracks in the rim usually were caused by stresses in the raw stage.
Cracks in the base usually occur in the firing.
Another way to determine the cause of a crack is to look at the surrounding glaze.
If the glaze at the end of the crack is sharp, it cracked in the later stages of firing, probably during cooling.
If the glaze is round at the edge of the crack, the crack probably occurred early in firing and the glaze had time to heal over.
This is just few of the reasons pottery or ceramics can crack.
Most clay is pretty tolerant or you would get a lot more cracks.
Don't be afraid to make a piece of pottery or ceramic just because it might get a crack.
If you take heed of the causes above, usually you won't get any cracks.
Even if you do get a crack there are many ways to fix or hide it.
Ok, get to crackin, OOOOP's, slip of the tongue, I mean get busy and create a perfect and beautiful piece of pottery or ceramic!!
"Copyright BigCeramicStore.com, reprinted with permission."
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