Gary Harvey on thu 24 jun 04
Here is my problem. Until here lately I have been using commericial =
purchased glazes. I have started with Ron and John's book testing and =
making my own glazes. I think I applied the glazes to thick because =
before firing the glaze cracked inside the two pitchers I was firing. I =
rubbed them and reglazed those area again. When they came out of the =
kiln they had crawled areas inside in the same area I had the problem =
in. I was wondering because the glaze went on weird and I thought I =
might have mixed them wrong. How long should they be mixed? I mixed =
these about 20 minutes and then put them thru my sieve multiple times. =
Commercial glazes seem to have the same coarsness or feel of paint but =
these seem to have a coarser or rougher feel. Is this normal or I am =
paranoid? Gary Harvey, Palestine TX
Snail Scott on tue 29 jun 04
At 02:36 PM 6/24/04 -0500, you wrote:
>...before firing the glaze cracked inside the two pitchers I was
firing...How long should they be mixed?
Your glaze coating was too thick.
The amount of mixing time is less critical than the
thickness of the glaze. Many factors affect this.
The proportions of water to dry materials in the glaze
slurry is of course very important, but the thickness
of the coating on the clay is also affected by the
absorbency of the clay and the method of application.
Some people mix their glazes to a measured specific
gravity using a hydrometer, while others use more
informal 'by eye' assessment, or mix their glaze using
exact amounts of water. Each of these methods has its
pros and cons, but none of them will work until you
can figure out how thick the glaze needs to be on
YOUR work for best results. It's different for every
glaze, and every clay; no one else's measurements will
be quite right. Trial and error will be needed.
You didn't say if you were applying the glaze by
brushing, dipping, spraying, or what. If brushing, it
can be easy to lose track of the thickness of any given
layer, but the absorbency of the bisque is not a big
factor. Dipping is usually the easiest way to get an
even coating, but glaze consistency, absorbency of
bisque, and technique of dipping all play a role in
good results. It's very common to accidentally get the
glaze too thick inside necked forms like pitchers,
since when the glaze is poured in, it tends to sit
longest in the bottom of the form before being swished
around and poured out. It also tends to run back down
if any excess glaze isn't poured out fully. If this
may be your case, you can minimize the problem by
pouring glaze rapidly in and out so that all inside
surfaces get equal contact, and by holding it upside
down until the glaze has fully dried. If this is not
sufficient, try a thinner glaze mixture, too. Also
remember that if you glaze the inside first, the clay
will be fully dry and quite absorbent, but the moisture
from the inside glaze coating will dampen the clay and
make it less absorbent when the outside is glazed next.
So, the perfect glaze thickness for the outside may not
be right for the inside unless you let the work dry
between steps.
>...Commercial glazes seem to have the same coarsness or feel of paint but
these seem to have a coarser or rougher feel.
This is normal. Commercial glazes are more processed
than the raw materials purchased by most studio
ceramists. While finer-mesh materials do tend to melt
more smoothly than coarser ones, the difference is not
a big one, and is definitely not the cause of your
crawling.
If you want glazes as smooth as commercially-made ones,
running the mixture through a ball mill would help it
happen, and adding lots of CMC, glycerin, or other
consistency-modifying additives will give it that nice
brushability. If you aren't brushing the glaze, though,
that's not a major concern. As for particle size, not
everything is best when ultra-refined - it depends on
the results you're looking for.
-Snail Scott
Edwardsville, Illinois, USA, Earth
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