Earl Brunner on mon 26 may 03
Glaze can be "fine" and the clay can be "fine" and the co-efficient of
expansions still not match. If you are taking the glaze to maturity it
shouldn't matter on the length of firing. Basically. One of the things
they "might" be suggesting by suggesting a need for a longer firing
schedule could have to do with the silica particle size in their glaze.
If they used coarser silica, it might need more time to dissolve into
the glaze mix
. How much is or gets dissolved into the mix affects the co-efficient
of expansion. How long it takes you to get to temperature shouldn't
make much difference as long as the "heat work" takes place. As long as
the glaze has matured, the short answer is that time is not necessarily
a limiting factor.
One of the drawbacks to using premixed glazes is that when you have a
problem like this, analyzing and solving the problem is a bit like
working in the dark. If altering the firing doesn't solve the problem
then, you either have to change the glaze or change the clay. You can
do this by altering the clay or glaze, or by replacing them.
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of
Ray.Warren@ANIXTER.COM
Sent: Monday, May 26, 2003 2:10 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Glazed and Confused - CRACKING
OK, terminal confusion has set in. My wife is struggeling with the
following;
Clear and transpartent glazed cracking days after firing.
The Clay is "buff" purhased from Great Lakes Clay.
The clear glaze is GL100 from Great Lakes Clay.
Everything seems fine but then cracking 3 days or later after being
removed from Kiln.
Many posts on here suggest incompatibility between clay body and glaze.
Great Lakes assures us glaze is fine. Should be since its there clear
and
there Buff Stoneware.
Great Lakes said " it should take 12 hours to fire " - To cone 6 ??
We are using a used Duncan kiln which seems to be in excellent condition
but reaches cone 6 in 5 hours ??
Adjusted the Kiln sitter arm since cone didn't seem bent very far when
kiln was tripped off.
Test firing again now [ HAPPY MEMORIAL DAY].
Thanks in advance for your experience and help.
Ray Warren
Vida's number one fan
Paul Lewing on mon 26 may 03
on 5/26/03 1:10 PM, Ray.Warren@ANIXTER.COM at Ray.Warren@ANIXTER.COM wrote:
> Clear and transparent glazed cracking days after firing.
> Everything seems fine but then cracking 3 days or later after being
> removed from Kiln.
> Many posts on here suggest incompatibility between clay body and glaze.
> Great Lakes assures us glaze is fine. Should be since its their clear and
> their Buff Stoneware.
Ray, the many posts are posts are correct. There obviously is some
incompatibility between the clay and the glaze. The fact that it took three
days to show up indicates that it's probably not too far off, but the
coefficients of expansion do not match, despite the fact that it is their
glaze and their clay. Assuming that they have done any testing to determine
if this glaze usually fits this body in their kilns, this doesn't
necessarily mean that it's right for you. Incidentally, I know nothing
about the clay, the glaze, or the company, so I have no idea if they've done
the world's most rigorous testing regimen, or if they're just blowing smoke
at you.
Glaze fit is affected not only by the chemistry of the clay and the glaze,
but also by how big and how thick your work is, how thick you put your glaze
on, and by the heating and cooling cycle of your kiln.
> Great Lakes said " it should take 12 hours to fire " - To cone 6 ??
> We are using a used Duncan kiln which seems to be in excellent condition
> but reaches cone 6 in 5 hours ??
That does seem a tad fast, and that may indeed be the problem. Or it may at
least explain the difference between your results and theirs.
> Adjusted the Kiln sitter arm since cone didn't seem bent very far when
> kiln was tripped off.
This is good regular practice, but if the cone was bent at all, chances are
the firing temperature wasn't too far off.
It's a little hard to tell you what would fix your problem, since I assume
you don't know the recipe for the glaze. If you'd tell us a bit more about
the problem and what facilities you have to remedy it, we might be able to
help more. When you say the glaze is cracking, do you mean a network of
fine cracks (crazing), or that pieces of the glaze are cracking off
(shivering)? And do you have scales and raw materials and all that stuff,
or do you rely solely on commercial glazes?
Paul Lewing, Seattle
John Rodgers on mon 26 may 03
You need to put some free standing witness cones at various points in
your kiln to find out what is going on. Do not depend solely on the kiln
sitter performance. Put a free standing witness cone on a post near a
peephole where you can see it when the plug is pulled. Then as the
firing reaches maturity, pull the plug every few minutes and look at the
witness cone on the post. Compare it's position to the shut off point
when the kiln goes off.
You will learn a ot about your kiln and what it is really doing.
Good luck.
Regards,
John Rodgers
Birmingham, AL
Ray.Warren@ANIXTER.COM wrote:
>OK, terminal confusion has set in. My wife is struggeling with the
>following;
>
>Clear and transpartent glazed cracking days after firing.
>The Clay is "buff" purhased from Great Lakes Clay.
>The clear glaze is GL100 from Great Lakes Clay.
>Everything seems fine but then cracking 3 days or later after being
>removed from Kiln.
>Many posts on here suggest incompatibility between clay body and glaze.
>Great Lakes assures us glaze is fine. Should be since its there clear and
>there Buff Stoneware.
>Great Lakes said " it should take 12 hours to fire " - To cone 6 ??
>We are using a used Duncan kiln which seems to be in excellent condition
>but reaches cone 6 in 5 hours ??
>Adjusted the Kiln sitter arm since cone didn't seem bent very far when
>kiln was tripped off.
>Test firing again now [ HAPPY MEMORIAL DAY].
>
>Thanks in advance for your experience and help.
>Ray Warren
>Vida's number one fan
>
>______________________________________________________________________________
>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.
>
>
>
Ray.Warren@ANIXTER.COM on mon 26 may 03
OK, terminal confusion has set in. My wife is struggeling with the
following;
Clear and transpartent glazed cracking days after firing.
The Clay is "buff" purhased from Great Lakes Clay.
The clear glaze is GL100 from Great Lakes Clay.
Everything seems fine but then cracking 3 days or later after being
removed from Kiln.
Many posts on here suggest incompatibility between clay body and glaze.
Great Lakes assures us glaze is fine. Should be since its there clear and
there Buff Stoneware.
Great Lakes said " it should take 12 hours to fire " - To cone 6 ??
We are using a used Duncan kiln which seems to be in excellent condition
but reaches cone 6 in 5 hours ??
Adjusted the Kiln sitter arm since cone didn't seem bent very far when
kiln was tripped off.
Test firing again now [ HAPPY MEMORIAL DAY].
Thanks in advance for your experience and help.
Ray Warren
Vida's number one fan
Tony Hansen on tue 27 may 03
No way around it, the glaze does not fit.
It is best to make your own transparent glaze using a recipe that
has adjustability for thermal expansion, then you have control.
Here is an example:
http://digitalfire.ab.ca/cermat/education/226.php
Check this web page for more info on thermal expansion:
http://digitalfire.ab.ca/cermat/education/198.php
Check here for information on the approach used in industry
to reduce crazing in glazes:
http://digitalfire.ab.ca/cermat/education/4.php
-------8<--------
OK, terminal confusion has set in. My wife is struggeling with the
following;
Clear and transpartent glazed cracking days after firing.
The Clay is \"buff\" purhased from Great Lakes Clay.
The clear glaze is GL100 from Great Lakes Clay.
Everything seems fine but then cracking 3 days or later after being
removed from Kiln.
Many posts on here suggest incompatibility between clay body and glaze.
Great Lakes assures us glaze is fine. Should be since its there clear and
there Buff Stoneware.
Great Lakes said \" it should take 12 hours to fire \" - To cone 6 ??
We are using a used Duncan kiln which seems to be in excellent condition
but reaches cone 6 in 5 hours ??
Adjusted the Kiln sitter arm since cone didn\'t seem bent very far when
kiln was tripped off.
Test firing again now [ HAPPY MEMORIAL DAY].
Thanks in advance for your experience and help.
Ray Warren
Vida\'s number one fan
______________________________________________________________________________
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.
--------
Tony Hansen, Digitalfire Corp.
Snail Scott on tue 27 may 03
At 04:10 PM 5/26/03 -0500, you wrote:
>Everything seems fine but then cracking 3 days or later after being
>removed from Kiln.
>Many posts on here suggest incompatibility between clay body and glaze.
Another cause of delayed crazing is moisture
absorption into a non-vitrified clay body.
I'm not familiar with the Great Lakes clays,
but is it truly matured at ^6, or is it one
of those so-called '^6-10' clays which is
really just a ^10 clay being flogged to the
mid-fire market?
-Snail
MarjB on tue 27 may 03
Afternoon Ray - before some possible solutions can be suggested I think we
require more information ? Are you using guide cones ? Are you certain
that you are reaching cone 6 ? Have you tested the absorption percentage of
the clay body? How large is your kiln? Do you have infinite heat switches?
How quickly does it cool ? Answers to these questions might offer the
directions to the desired results. MarjB
----- Original Message -----
From:
To:
Sent: Monday, May 26, 2003 5:10 PM
Subject: Glazed and Confused - CRACKING
> OK, terminal confusion has set in. My wife is struggeling with the
> following;
>
> Clear and transpartent glazed cracking days after firing.
> The Clay is "buff" purhased from Great Lakes Clay.
> The clear glaze is GL100 from Great Lakes Clay.
> Everything seems fine but then cracking 3 days or later after being
> removed from Kiln.
> Many posts on here suggest incompatibility between clay body and glaze.
> Great Lakes assures us glaze is fine. Should be since its there clear and
> there Buff Stoneware.
> Great Lakes said " it should take 12 hours to fire " - To cone 6 ??
> We are using a used Duncan kiln which seems to be in excellent condition
> but reaches cone 6 in 5 hours ??
> Adjusted the Kiln sitter arm since cone didn't seem bent very far when
> kiln was tripped off.
> Test firing again now [ HAPPY MEMORIAL DAY].
>
> Thanks in advance for your experience and help.
> Ray Warren
> Vida's number one fan
>
>
____________________________________________________________________________
__
> 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.
>
Stephani Stephenson on wed 28 may 03
Ray
If I am reading your post right, it is the glaze which is 'cracking',
not the clay?
If so
Sounds to me like you have a case of delayed crazing.
Crazing happens when the glaze shrinks more than the clay body during
cooling.
So it is fitting like stretch plastic film, stretched too tight over
something.
Even though it looks smooth and ok when it comes out of kiln it is under
tension.
Delayed crazing can show up a few hours, a few days or a few years
later. As the piece is exposed to moisture from the air or from water
, the fired clay expands . The glaze does not and begins to crack or
craze to relieve the increasing tension, from the expanding clay. Even
minute moisture from the atmosphere can cause this
On earthenware, dinnerware and lowfire tile this can be a problem as
these cracks are inlets for further moisture, bacteria, etc. . When
crazing is extreme the glaze surface can further deteriorate, and the
edges of the crazed 'platelets' can become raised .
I am not familiar with Great Lakes Clay. What do you know about your
clay body? If you are using a high expansion clear glaze and a porous
clay body, which can absorb moisture, you will likely see delayed
crazing .
For example I once used a sculpture body rated to cone 6, yet it was
still very porous at cone 6 and also had a very low shrinkage rate.
. Usually I didn't glaze it but used stains and engobes on it. however
once I did try glazing it for a kid's class project and every glaze I
had crazed on it, showing up the day after firing.
If the clay body is very vitrified at the temp you are firing to, it
will not be absorbing moisture, and the crazing may be solely be a
question of fit
Crazing is sometimes encouraged and enjoyed as a decorative effect, as
in 'crackle' glazes and some raku glazes.
penetrating sealants applied to lowfire bodies can protect against the
absorption of moisture and can stave off some cases of crazing, but
finding or modifying the glaze to lower its expansion (and subsequent
contraction on cooling), or finding a claybody with less porosity and
absorption or a bit higher shrinkage rate may also help.
Sincerely
Stephani Stephenson
steph@alchemiestudio.com
Carlsbad CA
> Ray Warren wrote:
> Clear and transparent glazed cracking days after firing.
> The Clay is "buff" purchased from Great Lakes Clay.
> The clear glaze is GL100 from Great Lakes Clay.
> Everything seems fine but then cracking 3 days or later after being
> removed from Kiln.
>
>
Ron Roy on fri 30 may 03
Hi Ray,
I think 6 hours to cone 6 is too fast - I think 100C per hour is fast
enough for a cone 6 firing and I suggest you try that before you proceed
and see if the glaze still crazes. If it still crazes I would be interested
to hear what Great lakes says.
Make sure cone 6 bends at least half way down.
If they need to have the glaze adjusted - so it does not craze on that body
- refer them to me.
RR
>OK, terminal confusion has set in. My wife is struggeling with the
>following;
>
>Clear and transpartent glazed cracking days after firing.
>The Clay is "buff" purhased from Great Lakes Clay.
>The clear glaze is GL100 from Great Lakes Clay.
>Everything seems fine but then cracking 3 days or later after being
>removed from Kiln.
>Many posts on here suggest incompatibility between clay body and glaze.
>Great Lakes assures us glaze is fine. Should be since its there clear and
>there Buff Stoneware.
>Great Lakes said " it should take 12 hours to fire " - To cone 6 ??
>We are using a used Duncan kiln which seems to be in excellent condition
>but reaches cone 6 in 5 hours ??
>Adjusted the Kiln sitter arm since cone didn't seem bent very far when
>kiln was tripped off.
>Test firing again now [ HAPPY MEMORIAL DAY].
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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