Electric Mid&Hi Fired Ceramics
Thermal Expansion of two glazes and problems
Has anyone done two glazes of different thermal expansion, one on the inside and one on the outside of a bisqued piece? Is it a known piece of information I don't know being new at this? Like thermal expansion has to match inside and outside?
Below is the problem I ran into:
I have two glazes which I tested their thermal expansion and both worked great on our bisqued clay pieces. I decided I'd like to use the one on the inside of a piece, its thermal expansion is 7.2, which we find to be what we need. However the second glaze has a thermal expansion of 6.41 and normally this would be too tight. But we have used this glaze multitudes of time to cover a whole piece/inside and out and there has never been any problems with it. It is a very nice mat, the other glaze is a very nice clear liner glaze.
What I did was glaze the inside with the clear liner glaze, which had 7.2 thermal expansion and glazed the outside with the mat colored glaze 6.41. The inside of the vase did circles of crazing lines around the inside of the bottom and has gradually added more circles of grazing up the inside, as if it were following throwing lines. I am used to crazing being lots of weird unpatterned lines but this is circles going up the inside of the vase. The outside bottom of the vase has a crack. I wouldn't call the crack an S crack because it is a circle crack.
Sharon Canfield
Thanks for your help Kabe.
What we found was I had glazed a miscellaneous pot that was porcelain! At least now I know it isn't the glazes:)
Nov 26, 2012
Dave Hodapp
Sharon:
I have a bucket in my studio with a sign that says "Sometimes the magic works, and sometimes it doesn't". It is currently filled with broken mugs that resulted from using different glazes on the inside and outside of the mugs. Someone gave me some porcelain to try out and I was having trouble finding a clear glaze that did not craze on the clay body. Finally I found an article about low expansion glazes on Digitalfire that had a recipe for a low expansion glaze for cone 6. I tried it out and it was great, no crazing. According to the Digitalfire website, Cone 6 porcelains are the most prone to crazing because they have lower quartz contents than stoneware clays.
My wife had bought a pint of a commercial glaze that she loved the looks of and wanted me to use on some pieces for her. It was a beautiful turquoise glaze and one of its features was that it crazed. Beautiful color and great cracking! So I glazed mugs with the clear on the inside for a food safe liner, and the turquoise crackle on the outside. I also did some vases with the turquoise on the inside and on the outside. So, I fired the pieces and opened the kiln. The vases looked great. But every mug was in pieces on the kiln shelf or had big cracks and fell apart when I picked them up.
I just put the clear glaze recipe into a glaze calculation program and the expansion coefficient is 5.7. I don’t know what it is for the commercial glaze, but I’d guess over 7. So it looks like there was a very large difference in expansion between the two glazes and upon cooling the mugs were torn apart.
So if you are coating a piece with different glazes on the inside and outside, it may pay to look at the thermal expansion coefficients for the two glazes if you can figure them out.
Nov 29, 2012
Sharon Canfield
Thanks, Dave, I did have the thermal expansion for both since I have Insight Glaze Calculation software. What I discovered was I'd grabbed a pottery piece that was porcelain without knowing it:) Fogot that at one time Jim tried working a bit with porcelain and I had grabbed a piece for testing from the top shelf which was all porcelain pieces. We only do stoneware now. When I went back and tested the glazes on our stoneware, it worked just fine.
Dec 10, 2012