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process of testing [cone 6] glazes

updated fri 15 jan 10


lili krakowski on thu 14 jan 10

Paula et Al.=3D20

(Al must be the best informed person on earth by now!)

=3DE3=3D80=3D80Although we often compare cooking and pottery some processes=
being so similar, glaze has its own and rather fixed "rules."

In many ways it is more like buying clothes. It is a lot about fit and =3D
style and looks.

In glazing, as, alas, in clothes, the body underneath matters a whole =3D
whole lot.

That is why I urge potters to have at least 3 clay bodies (for their =3D
firing temp), to have test-tiles of each, and to test each new glaze on =3D
each body. That allows one to see instantly (after the firing!) whether =3D
it is the glaze or the body that disappoints.

Obviously, if on the clay body you use in your studio the new glaze =3D
looks like the dog's breakfast, while it looks great on another body, =3D
you know, or may know, what to do next.

If you do not want to do that, at least stripe your test tiles with =3D
white and black slip if you use a buff clay or red body. On white clay I =
would use just some RedArt or similar slip, plus some mix of red and =3D
white clay to create a buff.

When testing transparents --"clear" glazes-- you may want to test over =3D
your favorite slips and underglazes.

Unless you once fire, I suggest you wash your test tiles the night =3D
before glazing--or something like that--so that the bisque is damp but =3D
not wet. While clean bisque is essential, both too dry and too wet can =3D
affect the test.

The thickness of application matters. Truth to tell, if you are an =3D
experienced glazer, you "automatically" know a proper thickness of =3D
application. There are a number of ways of checking.

Most glazes "do best" at the thickness of cream. A little thicker than =3D
light cream, a little thinner than heavy cream. About like maple syrup =3D
or sorghum--thinner than molasses.=3D20

DIP your test tile at least twice. Ok. You thin the glaze to what you =3D
think is right. Dip the whole tile (see below) , allow the glaze to dry =3D
a bit, re-dip halfway...That gives you two thicknesses to compare.

My test tiles are L shaped. Many use a hollow tube --in both cases the =3D
base must be totally free of glaze--and a good allowance must be made =3D
towards the base so that, if the glaze runs, it will not hit the shelf!

Many use a small bowl as test tiles--the outside being left glaze free.

One can get a small metal cup sold for testing paint thickness for =3D
spraying. One fills it with glaze and then counts to learn how long it =3D
takes for the cup to empty. This method has never worked for me--but you =
can improvise and try it by using a tiny plastic "glass"--such as =3D
dressing is served in, or meds dispensed--and poking a hole at the =3D
bottom to check drip speed.

Another method is to scratch the applied glaze and see how deep the =3D
scratch is. Between one and two millimeters works for most glazes.

Now comes the bad part. Not all glazes are at their best at the same =3D
thickness--either in liquid--ready to be applied--form or on the pot =3D
proper. You really must test each glaze and remember what thickness is =3D

I know nothing practical about downfiring. Or cools and holds. BUT they =3D
have naught to do with the original thickness of application...Even =3D
there you have to check application to make decisions.

And thickness of application has naught to do with the temp at which you =
fire. Yes, I can think of exceptions but I am not going to....As a rule =3D
for each temp, each firing method, each glaze, there is an "ideal".

And it is not as bad as it sounds. Here it IS like cookery. You have to =3D
test, check, adjust, readjust--and then PERFECTION.

Lili Krakowski
Be of good courage

Lis Allison on thu 14 jan 10

On Thursday 14 January 2010, lili krakowski wrote:
> And it is not as bad as it sounds. Here it IS like cookery. You have to
> test, check, adjust, readjust--and then PERFECTION.

One of things I found I had to learn was what various glaze problems
actually look like. What does underfired look like? What does too thin
look like? What does poor colour response look like? What does a blister
look like, as compared to a crater, pinhole, bubble or goodness knows what
else (if you think Al is smart, Goodness is even more so, Lili!). Those
are just some examples.

Experience matters, but it takes time and effort to get it! Paula, maybe
you can get some help from another local potter who can look at your tests
and discuss them with you?


Elisabeth Allison
Pine Ridge Studio