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Re: flaking glazes


From: Marvin Bartel (marvinpb_at_TeacherArtExchange)
Date: Sun Mar 07 2004 - 12:03:59 PST

>... problems with glaze cracking off pottery after they
come out of the kiln? . .

There are at least two reasons glazes can flake off after firing.

1. In some cases another layer of slip or engobe was under the glaze
and did not stick. Maybe the engobe was too thick or not formulated
to melt enough to fuse to the body.

In the case of colored slip made from only clay and colorant oxide,
maybe the clay was already too dry when the slip was added for
decoration or color. Use the slip thinner or sooner when the clay is
still wet.

For engobe, it may need the addition of something to help it melt a
bit more. Try 20 to 40 percent glaze added to the engobe mixture.
The glaze that is added can be the color of the engobe, or it can be
clear or white glaze, but engobe opacity and color would change.

2. In some cases the cooling contraction is more for the clay than
for the glaze and it causes glaze chips to pop off. On high fired
ware it is called shivering and on low fire pottery it is called
shelling . It is most often along the edges and rims. To prevent
this, the clay and/or the clay has to have its coefficient of
expansion changed.

The solutions for shelling and shivering depend on the particulars of
the problems. Feel free to contact me directly if you need help to
remedy this.

Cooling too fast can cause pots to crack (dunt), might cause more
crazing, but fast cooling probably does not cause glazes to flake off.

Marvin Bartel, Ed.D., Professor of Art Emeritus
Goshen College, 1700 South Main, Goshen IN 46526
studio phone: 574-533-0171
"You can't never know how to do it before you never did it before."
... a kindergarten boy working with clay for the first time.