Techno Tips

Crazing Driving You Crazy?

What is crazing? Technically, it’s when the hot molten
glaze, which is a form of glass, cools and shrinks on a
pot more than the clay body underneath, resulting in
tiny cracks. In general, things expand when they are
heated and contract or shrink when cooled. Much of
the pottery made since ancient times has crazed glazes.
Much of it cannot be seen by the naked eye without
some help or magnification. Try pouring a whitish
glaze in a dark glazed pot (or a light colored glaze in a
white or clear glazed pot.) The fine granules in the
wet glaze will get caught in the craze lines and make
them more visible.

So what’s wrong with crazed glazes? Well, crazing
weakens the glaze, allows wate and moisture to seep
through, potentially traps bacteria and contributes a
crackled look on the pot’s surface. These are consider-
ations for the functional pot used for edible liquids
and wet foods. It’s arguable how serious it is, as crazed
pots have been in use over many centuries. The likely
consequence is that the pot may break or annoy the
owner, usually resulting in its disposal and replace-
ment. It’s a personal thing, but also a measure of quali-
ty that can differentiate and elevate your pottery.

Usually, with functional pottery we want to eliminate
crazing but for aesthetic and artistic reasons (you sim-
ply like the look) some may actually want to enhance

the effect.

In order to control crazing in glazes, we have a couple
of approaches. We can change the glaze to shrink less,
or we can change the clay body to shrink more, or do
a combination of both.

‘To lower the expansion and contraction/shrinkage of a
glaze, you want to increase the ingredients that have a
low expansion rate, and decrease the ingredients that
have a high expansion rate. A general rule that may
work with some glazes is to increase the silica by 5%
at a time until the crazing is eliminated. This may
result in undesirable changes in the glaze (too runny,
too stiff, changes color, bubbling . . .) making this
technique ineffective.

A mote effective way to change a glaze is to substitute
the high expansion flux ingredients with lower expan-
sion flux ingredients. This is more easily done with

glaze chemistry programs and usually involves replac-

ing fluxes like sodium and potassium with some com-
bination of magnesium, lithium, boron, and calcium.

Alternatively, you can adjust the clay body to shrink
more and compress the glaze more. this can be done
by adding more fine silica to the clay body. It sounds a
bit weird that adding silica can make the clay shrink
more while adding silica to a glaze makes it shrink less.
The key difference is that glaze is molten and com-
bines with the silica in the glass where in the clay
body, it is not melted and allowed to turn into some-
thing called cristabolite. Cristabolite has an interesting
habit of shrinking a lot (about three percent), causing
the body to shrink more than before. The more silica
and the longer you fire, the more cristabolite is created
and the more the body shrinks when it cools. You may
have observed that a glazed pot in a normal firing will
craze, but an identical glazed pot in a very long wood-
firing (which will create more cristabolite) will result
in a glaze with less or no crazing!

In general, altering the clay body is the most effective
approach to fixing crazing as it doesn’t affect any visual
characteristics of the glaze. The crazing will stop and
the glaze will look like it should. The only drawback is
that many potters don’t mix their own clay bodies. If
you have a pugmill, it could be an easy addition. You
can successively add five percent more silica to a clay-
body and fire a series of glaze test tiles. You should see
the crazing get progressively less until it disappears and
then at the other end, the glaze should shiver/pop off
(usually off the lip of a pot). Pick the one with no
crazing, closer to the crazing side as crazing is always
better than shivering.

Hopefully all your glazes are of a similar
expansion/contraction, so that the one clay body will
fit them all. If you can find a commercial clay body
that fits the glazes you are using, that’s ideal. I'd love
to hear which clay bodies fit which glazes. A little
charting can save a lot of testing.

Happy potting. If you have input, questions or even
corrections to what I’ve said, please send them my
way.

don.jung@shaw.ca

Don Jung

November/December 2005

Potters Guild of British Columbia Newsletter