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cone 6 test (woes)

updated sat 15 may 99


Sharon Miranda on tue 11 may 99

Dear clayarters:
Please help! I'm trying to develop cone 6 glazes (for Miller 610 white
clay) and the going is slow. (This business of being self-taught....)

I sent in the recipes for the glazes I'm working with - will repeat them
if necessary - they are floating blue, pinnell's weathered bronze green,
metallic black. Am also testing for midrange majolica and any other white
base glaze that will work as a majolica.

Each time I tested (twice now), I have not filled the kiln. It is about
half full. Because pinnell's glazes have been pitting, I have used a
program that soaks at cone 6 and fires down slowly.

For the most part the glazes are running badly, onto the shelves. This
would seem to me to indicate overfiring, even tho the cones are down more
or less correctly (cone5 is flat, cone 6 completely bent). I'm concluding
that a kiln that is not full is quite different, but this raises the
problem of how to know what works or not. I didn't want to commit to a
whole (full) kiln of tests.

The test glazes - midrange majolica and all the other base glazes, did not
show with any accuracy what they are, because of the overfiring, I guess.

Here is the schedule of firing:

cone 6 -2200o

5 segments:
1 - 300o per hr to 1100
2 - 500o per hr to 1950
3 - 108o per hr to 2200 - soak for 30 min.
4 - 5000o per hr to 2000o
5 - 150o per hr to 1200o

I have an envirovent installed. For this schedule I followed the skutt
manufacturer's suggestions.

Floating blue glaze works no matter what, metallic black runs when
overfired, pinnell's glaze runs like hell and when it doesn't it pits like
hell, (I put it on thick and yes, I smoothed it out with fingers and
sponge). Pete Pinnell has said that wax resist may be part of the problem
with the pitting, but I've found it pits no matter what.

Please send me your comments, suggestions!

Thanks, Sharon (pitted out...)

The Buchanans on wed 12 may 99

Are both thick and thin applications running? Also a very slow firing may
need to drop back a cone. Try a firing to cone 5. You can always refire and
not lose to tests. I don't think half a load should make a difference since
you are controling the cooling.

Judi B.
-----Original Message-----
From: Sharon Miranda
Date: Tuesday, May 11, 1999 1:44 PM
Subject: cone 6 test (woes)


Bruce Girrell on wed 12 may 99


I empathize with you. I am self-taught as well and go through stages of
frustration with glazes.

Besides the recipe and firing temperature, other things can affect the melt.
It sounds like you have a good handle on the recipes and temperature, so
I'll ignore them. There are three other things that I can think of right now
that could do what you describe.

1) The materials that you are working with do not have the same chemical
composition as the materials used by those who developed the recipes
(possible cause of runniness as well as pinholing).

2) The particle size of your materials may be smaller, making them melt at a
lower temperature (possible cause of runniness).

3) There are interactions between the clay body and glaze that are peculiar
to the clay/glaze combination (possible source of pinholing).

There may be some wonderful analytical tools available to test all this
stuff, but for your situation, I would suggest Ian Currie's approach. See
Start with "Outline of Systematic Recipe Approach" and follow the text from
there. Yes, this approach will require more tests, but they will be very
organized tests and yes, there is some work involved to make the test tiles
and glaze batches, but you should get your results with one or two
experiments per glaze. Not a bad tradeoff. You're going to have to fire some
more tests anyway, why not make them count?

Good luck.
BTW You are the first person I have seen on Clayart who actually reported
success with Floating Blue. Perhaps you are luckier than you suspect.

Bruce Girrell
who is currently getting fed up with pinholing and bubbles in gerstley
borate based recipes and is about to do the Ian Currie thing, as well.

Mason Batchelder on thu 13 may 99

Dear Sharon,
I was in the class at Arrrowmont where we tested those Pinnell receipes and
there were few students who got the great looking test results and Pete said
also that the source of the strontium was going to be important. My tests
there were fine but at home I ordered the strontium form Highwater Clay and
could not reproduce the glazes.
Good luck I will be watching for your replies.
Margaret Arial

Linda Arbuckle on thu 13 may 99


One of the inexact parts of electric firing with a kiln sitter is
establishing what people really mean when they give a cone number for a

If you say you fire to the cone in the kilnsitter , you'll usually get a
cone lower visually in front of the peep. The sitter cone is horizontal
and has a rod on top of it, so it deforms a bit sooner than the same
cone at the prescribed 8 degree angle as a witness cone.

Another point... I do all electric firing in my studio, and use small
(junior) cones in the sitter and for witness cone packs. At school, some
people use standard cones as witness cones. You can use either, but
gravity and mass kick in, and they melt at slightly different
temperatures due to size differences. So, you want to be consistent in
what you use to calibrate your firing, and ask someone when they say
"cone 6" exactly what they mean: sitter or witness, standard or junior
witness cones?

The tendency, I find, is to cite the sitter cone. So, I'd try firing
those glazes to a visual cone 5 with a 6 in the sitter. Weathered Green
and Dixon Satin are popular glazes at UF, and I think most people are
firing them lower than you are.


Linda Arbuckle
Graduate Coordinator, Assoc. Prof.
Univ of FL
School of Art and Art History
P.O. Box 115801, Gainesville, FL 32611-5801
(352) 392-0201 x 219

Karen Gringhuis on fri 14 may 99

Margaret, Sharon & anyone else - about that strontium (or other
oxides) -- w/ some materials, I think "better" grades exist than
what is usually sold to potters. It may take some research &
phone calls to find them, but it may be worth it. I have on
ocassion asked the supplier if they carry a better grade
(this could be quality or fineness) & if not, do they know whe
who does; then I've talked to the mfgr. who is usually very
helpful. Several grades of iron oxide exist and this may
be true for other products. Talk to a ceramic engineer.
We are WORTH IT! (did anyone ask Pinnell where he gets
his strontium when he's home?)

Don't be afraid to talk to a mfg. or mine company for the
straight story on anything. Within my experience, I think
they are so surprised to talk to a potter (on non-engineer)
that they really help.

Good luck. Karen Gringhuis