Paulette Carr on sat 28 oct 06
Alistair,
I do not consider myself an expert on frits, but I am becoming =20
"somewhat" of an expert on low fire. Almost 1 year ago, I began to =20
work on low-fire glazes for a very large commission. The changes in =20
energy costs prompted me to begin the investigation to find a good =20
sculptural/tile, low-fire white body, and make the glazes that would =20
fit the clay body, be resistant to abrasion, markers, cleaning =20
agents, and have the "look" that I was after. After extensive =20
testing, development, testing, consulting good books, Digitalfire =20
articles, the Clayart archives and several Clayarters, and testing, I =20=
feel like I am beginning to understand the materials and conditions. =20=
With this caveat, I will offer you my advise:
I believe that Lili is on to something when she said that the clay =20
may be the problem. As I read your post, I noted that you were using =20=
a red earthenware body, covered by a white slip. The crazing may be =20
due to the fit of your glaze on this slip, and/or may be due to the =20
absorption of both the red and white clays =96 hence, they are not =20
fired to the points where they are tight enough for the swelling to =20
be minimal. As Lili suggested, the body and/or slip will swell when =20
wet, stretching the glaze until it breaks/crazes.
Here is where I disagree with Lili: The Coefficent of Expansion of =20
your glaze is lower than the glaze that Lili suggested, so your glaze =20=
would be less likely to craze than the one that she suggests. In =20
fact, glazes with COE=92s in your range shiver off many of the white =20
talc-containing bodies that I tested.
I believe that you can solve your problem by bisque firing to ^02 =20
(1102c @ 60c/hr =96 consult an appropriate pyrometric cone chart). It =20=
is even more important to fire slowly with earthenware to avoid =20
temperature gradients and cracking. This is probably more important =20
for talc-containing bodies which are more prone to cracking than red =20
earthenware, but since you are using a commercial body, and do not =20
know what is in it, treat it with every advantage, until you can test =20=
the limits with things that are not precious. The higher bisque =20
firing will strengthen your red body, and reduce the absorption. You =20=
should test it to find out what it is in any case, both at ^04 and =20
^02. With regard to glazing, if you brush, there will be no problem, =20=
and if you dip, you should still be able to dip with some slight =20
modification due to the decreased absorption.
Glaze-fire as before ... at around ^04, unless you believe that the =20
glaze is better when fired higher. The reason that I refer to cones, =20=
as opposed to temperature, is that what you want to measure is heat =20
work (change in temperature with respect to change in time). Your =20
firing schedule is just as important as the glaze, clay body, =20
application, etc. Use cones throughout your kiln during a firing to =20
see what your pieces are really experiencing.
Your original glaze may still not fit your body, but you can =20
determine this after you have eliminated some of the swelling due to =20
large absorption. Test your glaze-fit with thermal shock tests like =20
those described in Mastering Cone 6 Glazes. If you do not have =20
access to the book, contact me, and I will give you the very simple =20
protocol. If the glaze does not fit your clay body (maybe =20
shivers???), try a line blend with your original and the glaze that =20
Lili published. I suspect that her glaze will work well under these =20
new conditions.
Good luck!
My best,
Paulette Carr
Paulette Carr Studio
Member/Potters Council
St. Louis, MO
Date: Mon, 23 Oct 2006 16:24:25 -0400
From: Lili Krakowski
Subject: Alistair's clear
...
I ran Alistair's glaze through GlazeMaster (TM) and got this:
Recipe:
Frit 3195 62
Frit 3134 9.5
Silica 9.5
Kaolin 19
Formula
Na2O .330
K2O .003
MgO .010
Cao .650
Al2O3 .603
B2O 1.019
SiO2 3.665
Coefficient of Expansion 66.47
Alistair wrote he fires at between c05 and 04, and after ten days =20
out of
the kiln this glaze crazed ON THE
BODY HE IS USING, after several washings.
So of course ONE reason might be that the clay is underfired and absorbs
water, expands and pries the glaze apart.
I do not really know how I would try blends on this....To me the =20
boron seems
extraordinarily high.
May I suggest to Alistair a glaze I have used, which works on most =20
bodies.
Karen Stevens's 04 Clear
Frit 3134 55.56
Kaolin 23.81
Flint 20.63
Na2O .315
K2O .003
MgO .002
CaO .679
Al2O3 .299
B2O3 .627
SiO2 3.252
Coefficient of Expansion 71.61
I know it often is suggested that one fire a glaze a tidge higher, a bit
lower....The problem with that, IMO, or why I rarely do it, is that this
often throws other glazes out of kilter. In kilns that either can be =20=
made
to fire unevenly, or which just do, hot and cold spots can be =20
reserved for
particular glazes. But in most kilns--electric ones, anyway, this fine
tuning cannot be done. One then either must fire the whole kiln to this
"adjusted" temperature, or start having problems with other glazes.
I would suggest to Alistair that he just fire his CLAY alone, no =20
glaze, to
the temperature wanted, and then test it for absorption. No point =20
testing
glazes if the clay is at fault.
Date: Sat, 21 Oct 2006 12:44:54 +0100
From: Alistair Gillies
Subject: Ferro frits / crazing
Hi,
Any frit experts want to help me?
For the last month I have been developing a glaze to fit my red =20
earthenware
body [I thought],and being fairly sure that I had a glaze that fitted I
have a kiln full of pots on as I write.
Looking at my clear test today after 10 days out of the kiln, and after
doing hot/cold water tests on the samples it has started to craze - it
certainly had not two days ago. The versions coloured with copper and =20=
iron
have not crazed.........yet.
The recipe is below - do we think that I might save this kiln load by =20=
firing
higher? or is it a write off.
Ferro 3195 - 62
Ferro 3134 - 9.5
Flint - 9.5
China Clay - 19
Firing at 1075c, on Spencroft clay over white slip.
I think my next step is to take out the 3134 altogether.
Thanks for your thoughts in advance,
Alistair
Ironbridge, England
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