clifton wood on fri 10 jun 05
hi, everyone.
here's the question: why would a variant of a glaze with a LOWER COE
craze more than a variant with a higher one?
please read on for the specifics:
i've been working with tony hansen's cone 6 clear, referred to as GA13
in Bailey's book.
i fire at cone 6 ox.
it crazes on one of my clay bodies, but not the other.
below is the recipe:
Soda Feldspar 15
Frit 3124 9
Frit 3110 9
China clay 21
Wollastonite 22
Flint 24
s/t 100
Bentonite 2
i ran a bunch of different tests, and fixed the crazing by adding 6%
frit 3249.
sounds good, right?
yeah, but.
i also am trying to fix crazing on cushing's vc71.
and i don't know much about this.
so... i decided to learn a little more than nothing about coeffients of
expansion.
using glazemaster software, i input the original glaze recipe, which had
a COE of 67.63
then, i put in the "fix" - added 6% frit 3249 - the new COE was 66.03
well, that directionally made sense... lower COE, less crazing, right?
MAYBE?
then, i put in the tests that didn't work.... adding frit 3134, or EPK,
or lithium... and they all had higher COEs than the original. well,
that made sense. because they all crazed.
then came the flummoxing point.
i put in the flint test... the base recipe with 15% flint added... it
had a COE of 63.79
This COE is LOWER than the original recipe... but flint batch crazed
just as badly.
so - can anyone give me a little tutorial on why this might have
happened? besides the hypothesis that we mixed up the test batch
incorrectly?
and... if anyone is feeling like a martyr & would like to discuss the
cushing glaze with me off list, let me know.
yours in glaze confusion....
clifton, norwich & sabra - www.catclay.com
Ron Roy on sat 11 jun 05
Hi Clifton?
First of all - expansion calculation is not infalible - although the
numbers do help a potter to know in which direction the solution is going.
Second - when you added the silica you simply may not have added enough -
an equally important way to lower expansion is to lower KNaO - replace it
with MgO or even Li2O.
The VC 71 has a ratio of 8.5 - this would indicate at least a semi gloss
type glaze - but it is described as a satin matte - it may well be
underfired - there is not much boron there. So when you add more silica you
will certainly make it melt less. I think it is an underfired glaze - which
are more likely to be devirified (recrystalized) during cooling - and that
may be the problem - it is well know that calculated expansion of matte
glazes is unreliable.
I would try upping the 3124 and cutting some KNaO out - then try adding
more silica.
Let me know if you need more on this - I'll be glade to help.
RR
>here's the question: why would a variant of a glaze with a LOWER COE
>craze more than a variant with a higher one?
>
>please read on for the specifics:
>
>i've been working with tony hansen's cone 6 clear, referred to as GA13
>in Bailey's book.
>
>i fire at cone 6 ox.
>
>it crazes on one of my clay bodies, but not the other.
>
>below is the recipe:
>
>Soda Feldspar 15
>Frit 3124 9
>Frit 3110 9
>China clay 21
>Wollastonite 22
>Flint 24
> s/t 100
>Bentonite 2
>
>i ran a bunch of different tests, and fixed the crazing by adding 6%
>frit 3249.
>
>sounds good, right?
>
>yeah, but.
>
>i also am trying to fix crazing on cushing's vc71.
>
>and i don't know much about this.
>
>so... i decided to learn a little more than nothing about coeffients of
>expansion.
>
>using glazemaster software, i input the original glaze recipe, which had
>a COE of 67.63
>
>then, i put in the "fix" - added 6% frit 3249 - the new COE was 66.03
>
>well, that directionally made sense... lower COE, less crazing, right?
>
>MAYBE?
>
>then, i put in the tests that didn't work.... adding frit 3134, or EPK,
>or lithium... and they all had higher COEs than the original. well,
>that made sense. because they all crazed.
>
>then came the flummoxing point.
>
>i put in the flint test... the base recipe with 15% flint added... it
>had a COE of 63.79
>
>This COE is LOWER than the original recipe... but flint batch crazed
>just as badly.
>
>so - can anyone give me a little tutorial on why this might have
>happened? besides the hypothesis that we mixed up the test batch
>incorrectly?
>
>and... if anyone is feeling like a martyr & would like to discuss the
>cushing glaze with me off list, let me know.
>
>yours in glaze confusion....
>
>
>
>clifton, norwich & sabra - www.catclay.com
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
David Hewitt on sun 12 jun 05
Clifton,
This is a very interesting subject and the results can often appear to
be contradictory.
The fact that your glaze crazes on one clay body and not another is, I
am sure, not surprising to you, but the conflicting results from the
changes in glaze recipe is the real question.
My first reaction to the first change that you made was that altering
the COE from 66.73 to 66.03 was that the change was not that
significant, albeit in the right direction. Other factors such as glaze
thickness, the firing cycle and the position of the pieces within a kiln
may be more significant between one firing and another.
You then go onto outline some of the other changes you made, but
specifically detail one with additional silica which gave you a COE of
63.79 and which crazed.
It is almost invariably true that with stoneware glazes that firing
higher improves glaze fit. This is explained by the free silica in the
body converting to cristobalite which contracts on cooling to give
greater shrinkage of the body. With some porcelains and earthenware
bodies, however, this free silica gets involved with alumina and fluxes
to form a glass within the body matrix and is thus not available for the
silica conversion of quartz to cristobalite. This then makes for a worse
glaze fit and is, I think the explanation for your recipe with the
increased silica, even though not fired to a higher temperature.
Mike Bailey and I have recently done a series of tests to investigate
glaze fit on six very different glaze recipes and eight porcelain clay
bodies, each tested at firing to cone 6, 8 and 9. The results we think
will be published in a future issue of Ceramic Review, possibly the next
Issue, no.214, and will elaborate on this question.
You may also like to look at my web site and click on 'Calculating
Crazing', which details the result of earlier published work by us on
this subject including the effect of some different clay bodies..
http://www.dhpot.demon.co .uk
David
In message , clifton wood writes
>hi, everyone.
>
>here's the question: why would a variant of a glaze with a LOWER COE
>craze more than a variant with a higher one?
>
>please read on for the specifics:
>
>i've been working with tony hansen's cone 6 clear, referred to as GA13
>in Bailey's book.
>
>i fire at cone 6 ox.
>
>it crazes on one of my clay bodies, but not the other.
>
>below is the recipe:
>
>Soda Feldspar 15
>Frit 3124 9
>Frit 3110 9
>China clay 21
>Wollastonite 22
>Flint 24
> s/t 100
>Bentonite 2
>
>i ran a bunch of different tests, and fixed the crazing by adding 6%
>frit 3249.
>
>sounds good, right?
>
>yeah, but.
>
>i also am trying to fix crazing on cushing's vc71.
>
>and i don't know much about this.
>
>so... i decided to learn a little more than nothing about coeffients of
>expansion.
>
>using glazemaster software, i input the original glaze recipe, which had
>a COE of 67.63
>
>then, i put in the "fix" - added 6% frit 3249 - the new COE was 66.03
>
>well, that directionally made sense... lower COE, less crazing, right?
>
>MAYBE?
>
>then, i put in the tests that didn't work.... adding frit 3134, or EPK,
>or lithium... and they all had higher COEs than the original. well,
>that made sense. because they all crazed.
>
>then came the flummoxing point.
>
>i put in the flint test... the base recipe with 15% flint added... it
>had a COE of 63.79
>
>This COE is LOWER than the original recipe... but flint batch crazed
>just as badly.
>
>so - can anyone give me a little tutorial on why this might have
>happened? besides the hypothesis that we mixed up the test batch
>incorrectly?
>
>and... if anyone is feeling like a martyr & would like to discuss the
>cushing glaze with me off list, let me know.
>
>yours in glaze confusion....
>
>
>
>clifton, norwich & sabra - www.catclay.com
--
David Hewitt
Web:- http://www.dhpot.demon.co.uk
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