Wanda Holmes on thu 21 nov 02
I have recently completed a set of glaze tests (cone 6 oxidation) as
part of a series of tests to find the appropriate COE for my clay body
(Clayworld Stoneware Special II). I found some interesting results and
would like Clayarters' perspective on my results and my conclusions.
15 glazes were tested. COEs ranged from 61.4x10e-7 to 65.8x10e-7.
COE's for the glazes that crazed ranged from 63.2 to 65.8. COE's for
glazes that did not craze ranged from 61.4 to 65.5. In other words,
there was a range of COE values in which there were both crazed glazes
and non-crazed glazes, so COE alone did not appear to be a reliable
predictor of crazing.
In further analyzing the results, I noticed that there was a pretty
distinct break based on KNa. The 10 glazes with .15 or less of KNa
(.123 to .149) were generally uncrazed. The COEs ranged from 61.5 to
65.5. The two glazes in this group that did graze were strontium-based
glazes with very low (less than .3) amounts of CaO. Their COEs were
64.2 and 65.8 respectively.
The 5 glazes with KNa of .17 or higher (.179 to .232) were, with one
exception, crazed. The COE's ranged from 63.2 to 65.5. I can't see
anything particularly distinctive about the one that did not craze, so
it remains an unexplained anomaly.
My questions:
1) I intended for these to be clear transparent glazes, but they turned
out to semi-mattes. I had added talc to bring down the COE and I now
realize that the glazes probably had too much MgO to be well melted (as
much as .263). Could the fact that they were not well melted throw off
the relationship between COE and crazing? Ie. Should I ignore these
tests and do another batch with the MgO adjusted?
2) Is it possible that a relatively high amount of KNa in a glaze can
lead to crazing independently of the COE?
3) Is it possible that a relatively high amount of SrO in a glaze can
lead to crazing independently of the COE?
3) Is it possible that a low amount of CaO in a glaze could make it more
susceptible to crazing regardless of the COE?
Thanks,
Wanda
John Hesselberth on fri 22 nov 02
Hi Wanda,
A couple thoughts. First the range of COEs that you tested was quite
narrow. When Ron designed the series of test glazes for the book, he
covered a range of 64 up to 88. To me one of 2 things is happening: 1)
you are right on the edge of crazing at 64 and the calculations are
simply not accurate enough to allow you to slice it that finely or, more
likely in my opinion, 2) you are not using a set of compositions that
gives a fully melted transparent, glossy glaze at cone 6. The latter is
indicated by the fact they turned out to be semimattes. COEs for
semimattes are not useful because of the crystals present.
I would highly recommend you use the series of glazes in the book that
Ron designed and see where you are with them. He designed them to be
fully melted at cone 6 and they have been tested vs. dilatometer results
so they are a fairly good reference set.
My opinion on 2,3 and 4 below is 'no' to each, but as I said above, when
you have crystals forming (or not thoroughly melted) all bets are off on
using COE as a predictor.
Regards,
John
On Thursday, November 21, 2002, at 12:13 PM, Wanda Holmes wrote:
> 1) I intended for these to be clear transparent glazes, but they turned
> out to semi-mattes. I had added talc to bring down the COE and I now
> realize that the glazes probably had too much MgO to be well melted (as
> much as .263). Could the fact that they were not well melted throw off
> the relationship between COE and crazing? Ie. Should I ignore these
> tests and do another batch with the MgO adjusted?
>
> 2) Is it possible that a relatively high amount of KNa in a glaze can
> lead to crazing independently of the COE?
>
> 3) Is it possible that a relatively high amount of SrO in a glaze can
> lead to crazing independently of the COE?
>
> 3) Is it possible that a low amount of CaO in a glaze could make it more
> susceptible to crazing regardless of the COE?
http://www.frogpondpottery.com
http://www.masteringglazes.com
Wanda Holmes on sun 24 nov 02
Thanks, John. The range of COEs was narrow because I have already
determined (by methods similar to what Ron describes)that I have a low
expansion clay body and I am trying to fit a set of glazes to it. Since
the set was not fully melted, I think it best to simply set these
results aside, however your point about the calculations simply not
being accurate enough to finely define the boundary point it a very good
one.
Wanda
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG] On
Behalf Of John Hesselberth
Sent: Friday, November 22, 2002 12:58 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Crazing & COE
Hi Wanda,
A couple thoughts. First the range of COEs that you tested was quite
narrow. When Ron designed the series of test glazes for the book, he
covered a range of 64 up to 88. To me one of 2 things is happening: 1)
you are right on the edge of crazing at 64 and the calculations are
simply not accurate enough to allow you to slice it that finely or, more
likely in my opinion, 2) you are not using a set of compositions that
gives a fully melted transparent, glossy glaze at cone 6. The latter is
indicated by the fact they turned out to be semimattes. COEs for
semimattes are not useful because of the crystals present.
I would highly recommend you use the series of glazes in the book that
Ron designed and see where you are with them. He designed them to be
fully melted at cone 6 and they have been tested vs. dilatometer results
so they are a fairly good reference set.
My opinion on 2,3 and 4 below is 'no' to each, but as I said above, when
you have crystals forming (or not thoroughly melted) all bets are off on
using COE as a predictor.
Regards,
John
On Thursday, November 21, 2002, at 12:13 PM, Wanda Holmes wrote:
> 1) I intended for these to be clear transparent glazes, but they
turned
> out to semi-mattes. I had added talc to bring down the COE and I now
> realize that the glazes probably had too much MgO to be well melted
(as
> much as .263). Could the fact that they were not well melted throw
off
> the relationship between COE and crazing? Ie. Should I ignore these
> tests and do another batch with the MgO adjusted?
>
> 2) Is it possible that a relatively high amount of KNa in a glaze can
> lead to crazing independently of the COE?
>
> 3) Is it possible that a relatively high amount of SrO in a glaze can
> lead to crazing independently of the COE?
>
> 3) Is it possible that a low amount of CaO in a glaze could make it
more
> susceptible to crazing regardless of the COE?
http://www.frogpondpottery.com
http://www.masteringglazes.com
________________________________________________________________________
______
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
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John Hesselberth on sun 24 nov 02
Hi Wanda,
COE calculations work well when you have a single recipe and are trying
to modify it to meet your needs (assuming a fully melted, glossy glaze).
So if you increase K/Na they will give a good prediction of how much
expansion will increase. Or if you increase Mg how much it will
decrease. The calculations begin to lose some accuracy when you are
trying to compare recipes that are significantly different from each
other--particularly if one of those recipes contain oxides whose
expansion coefficients are not well defined, e.g. lithium or strontium.
They lose a lot of accuracy when crystals are involved.
Regards,
John
On Sunday, November 24, 2002, at 08:45 AM, Wanda Holmes wrote:
> however your point about the calculations simply not
> being accurate enough to finely define the boundary point it a very good
> one.
>
> Wanda
>
>
Ron Roy on sun 24 nov 02
Dear Wanda,
I am going to over simplify this because it could go on for ever.
KNaO have high COE's - Take a look at the list in the back of the Hamer
book to get a clear idea on this. One way of stopping crazing is to
replace KNaO with any of the other fluxes - MgO is especially low for
instance.
I solve a lot of fit problems for potters - tweaking the fluxes is part of
the process - adding silica but maintaining the alumina/silica ratio is
productive as well. Boron is good at helping because it adds elasticity to
glazes - up to about 12%.
There are some glazes that will simply not be fixable while maintaining the
same look - those low in silica for instance.
This all sounds very complicated but it is rather a simple process - you
try some thing that lowers the calculated expansion of a glossy glaze - and
soon find out by trial and error - how to do it.
It does involve thinking however - and I can see you are well on your way
in this respect.
RR
>I have recently completed a set of glaze tests (cone 6 oxidation) as
>part of a series of tests to find the appropriate COE for my clay body
>(Clayworld Stoneware Special II). I found some interesting results and
>would like Clayarters' perspective on my results and my conclusions.
>
>15 glazes were tested. COEs ranged from 61.4x10e-7 to 65.8x10e-7.
>COE's for the glazes that crazed ranged from 63.2 to 65.8. COE's for
>glazes that did not craze ranged from 61.4 to 65.5. In other words,
>there was a range of COE values in which there were both crazed glazes
>and non-crazed glazes, so COE alone did not appear to be a reliable
>predictor of crazing.
>
>In further analyzing the results, I noticed that there was a pretty
>distinct break based on KNa. The 10 glazes with .15 or less of KNa
>(.123 to .149) were generally uncrazed. The COEs ranged from 61.5 to
>65.5. The two glazes in this group that did graze were strontium-based
>glazes with very low (less than .3) amounts of CaO. Their COEs were
>64.2 and 65.8 respectively.
>
>The 5 glazes with KNa of .17 or higher (.179 to .232) were, with one
>exception, crazed. The COE's ranged from 63.2 to 65.5. I can't see
>anything particularly distinctive about the one that did not craze, so
>it remains an unexplained anomaly.
>
>My questions:
>
>1) I intended for these to be clear transparent glazes, but they turned
>out to semi-mattes. I had added talc to bring down the COE and I now
>realize that the glazes probably had too much MgO to be well melted (as
>much as .263). Could the fact that they were not well melted throw off
>the relationship between COE and crazing? Ie. Should I ignore these
>tests and do another batch with the MgO adjusted?
>
>2) Is it possible that a relatively high amount of KNa in a glaze can
>lead to crazing independently of the COE?
>
>3) Is it possible that a relatively high amount of SrO in a glaze can
>lead to crazing independently of the COE?
>
>3) Is it possible that a low amount of CaO in a glaze could make it more
>susceptible to crazing regardless of the COE?
>
>Thanks,
>Wanda
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Wanda Holmes on mon 25 nov 02
Thanks, Ron. This is all related to my quest for a copper blue glaze
that will fit my low expansion body. I'm not ready to give up - and,
shoot, the worst thing that will happen is that I'll learn a few things
in the trying.
wanda
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG] On
Behalf Of Ron Roy
Sent: Sunday, November 24, 2002 12:15 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Crazing & COE
Dear Wanda,
I am going to over simplify this because it could go on for ever.
KNaO have high COE's - Take a look at the list in the back of the Hamer
book to get a clear idea on this. One way of stopping crazing is to
replace KNaO with any of the other fluxes - MgO is especially low for
instance.
I solve a lot of fit problems for potters - tweaking the fluxes is part
of
the process - adding silica but maintaining the alumina/silica ratio is
productive as well. Boron is good at helping because it adds elasticity
to
glazes - up to about 12%.
There are some glazes that will simply not be fixable while maintaining
the
same look - those low in silica for instance.
This all sounds very complicated but it is rather a simple process - you
try some thing that lowers the calculated expansion of a glossy glaze -
and
soon find out by trial and error - how to do it.
It does involve thinking however - and I can see you are well on your
way
in this respect.
RR
>I have recently completed a set of glaze tests (cone 6 oxidation) as
>part of a series of tests to find the appropriate COE for my clay body
>(Clayworld Stoneware Special II). I found some interesting results and
>would like Clayarters' perspective on my results and my conclusions.
>
>15 glazes were tested. COEs ranged from 61.4x10e-7 to 65.8x10e-7.
>COE's for the glazes that crazed ranged from 63.2 to 65.8. COE's for
>glazes that did not craze ranged from 61.4 to 65.5. In other words,
>there was a range of COE values in which there were both crazed glazes
>and non-crazed glazes, so COE alone did not appear to be a reliable
>predictor of crazing.
>
>In further analyzing the results, I noticed that there was a pretty
>distinct break based on KNa. The 10 glazes with .15 or less of KNa
>(.123 to .149) were generally uncrazed. The COEs ranged from 61.5 to
>65.5. The two glazes in this group that did graze were strontium-based
>glazes with very low (less than .3) amounts of CaO. Their COEs were
>64.2 and 65.8 respectively.
>
>The 5 glazes with KNa of .17 or higher (.179 to .232) were, with one
>exception, crazed. The COE's ranged from 63.2 to 65.5. I can't see
>anything particularly distinctive about the one that did not craze, so
>it remains an unexplained anomaly.
>
>My questions:
>
>1) I intended for these to be clear transparent glazes, but they turned
>out to semi-mattes. I had added talc to bring down the COE and I now
>realize that the glazes probably had too much MgO to be well melted (as
>much as .263). Could the fact that they were not well melted throw off
>the relationship between COE and crazing? Ie. Should I ignore these
>tests and do another batch with the MgO adjusted?
>
>2) Is it possible that a relatively high amount of KNa in a glaze can
>lead to crazing independently of the COE?
>
>3) Is it possible that a relatively high amount of SrO in a glaze can
>lead to crazing independently of the COE?
>
>3) Is it possible that a low amount of CaO in a glaze could make it
more
>susceptible to crazing regardless of the COE?
>
>Thanks,
>Wanda
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
________________________________________________________________________
______
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
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