Many thanks for your help.
> I love clay because is it is wonderfully simple to work with.
We live close to Mother Earth [the soil]
New Clay Site!
I hope the information provided in this e-Learning to Japanese ceramic art
will be useful to you.
----- Original Message -----
From: "Marvin Bartel" <firstname.lastname@example.org>
To: "TeacherArtExchange Discussion Group"
Sent: Monday, October 13, 2008 6:43 AM
Subject: Re: [teacherartexchange] Glaze Firing with Cracking
> Angela wrote:
>>I just did a glaze firing and I experienced a tile project that cracked
>>one that broke in half. I have the option on my kiln to do a fast glaze -
>>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
>>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.
> FLAT FORM:
> 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.
> GLAZE DRIPS:
> 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
> 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
> Home Page in Ceramics
> Home Page as an artist
> "We need to remember that we are created creative and can invent new
> scenarios as frequently as they are needed." -- Maya Angelou
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