>I just did a glaze firing and I experienced a tile project that cracked and
>one that broke in half. I have the option on my kiln to do a fast glaze - 5
>hours or a slow glaze cycle - about 7 hours. My thought is that the
>fast glaze heated up to fast and caused the cracking. Does anyone have some
>thoughts of why this happened?
There are several things that might cause this. Did the cracking happen during the cooling or prior to the glaze melting? We can tell by looking at the glaze next to the crack to see if it is a sharp break (cracked after the glaze hardened) or a soft looking edge of the glaze.
COEFFICIENT OF EXPANSION (most materials expand when heated and contract when cooled):
Cracking after the glaze has hardened (during the cooling) can be the result of a mismatch between the coefficient of expansion between the fired glaze and the fired clay. I once saw a whole load of a student's stoneware pottery break into pieces as it cooled because he made a mistake in his clay formulation. He omitted the feldspar in his clay recipe, and it contracted more than his glaze as it cooled.
QUARTZ INVERSION (free quartz in the clay is too brittle when converted too fast):
Some clay is particularly vulnerable to thermal shock, even when the clay and glaze has compatible coefficient of expansions. Faster firing and cooling can adversely effect the thermal shock resistance because the quartz inversion process can leave the clay more brittle if the temperature rises or lowers too fast at quartz inversion temperatures.
Since tile is flat and because it is fired directly on the kiln shelf, it is subject to greater stress during cooling. A bowl shape can undergo similar stress and simply bend a small amount as needed, but flat clay cannot release uneven expansion stresses by bending. Therefore, it is quite likely to crack when stress from any cause happens.
KILN SHELF EFFECT:
When we crack the lid, the top tile gets some cool air and kiln shelf holds the residual heat. This cools the top surface of the tile while the kiln shelf holds the heat on the bottom of the tile. This adds stress as the tile contracts during cooling.
Once in a while a tile breaks when the glaze trips over the edge or I fail the clean the bottom and the glaze attaches the tile to the shelf and it cannot contract while cooling. The solutions to this are to use more kiln wash, use less glaze, and/or to clean the tile better prior to firing.
1. The first thing to try might be to slow the cooling rate and wait for 200 F or lower before cracking lid or removing the peeps.
2. A slower firing cycle would cost a bit more, but may produce a less brittle product.
3. Switching to another clay body may help. If you are formulating your own clay, let me know the recipe and firing temp, and I can make a suggestion that might fix it.
4. Sometimes just mixing in a bunch of fine white silica sand makes clay less apt to crack in drying and firing. I buy silica sand at a building supply store at $5 for a 50 pound bag. I routinely include at least 5 percent silica sand, but more could be used. I used to use grog, but this is less expensive. Some clay may cease to be plastic enough with too much sand. Some common sands should be avoided because they have pieces of lime that cause breakage when fired. With common local sand, mix some in a sample and do an experimental firing before risking too much clay or artwork.
I love clay because is it is wonderfully simple to work with. I also appreciate that it is so complicated and challenging that no one can live long enough to learn everything about it.
Marvin Bartel, Ed.D., Professor of Art Emeritus
Adjunct in Art Education
Goshen College, 1700 South Main, Goshen IN 46526
studio phone: 574-533-0171
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"We need to remember that we are created creative and can invent new scenarios as frequently as they are needed." -- Maya Angelou