SierraEarthWorks on mon 1 dec 03
I have tried everything to prevent crazing on pieces that I apply a =
clear gloss to. I don't have a problem with my other glazes, but it is =
almost a guarantee that I will see crazing when I use clear. I also =
notice delayed crazing sometimes. I have tried just about everything I =
know to remedy this, but sometimes it still occurs. I also am using =
Duncan IN1001 clear glaze. Is it the glaze? Is it the glazes underneath? =
I heard that crazing may occur if the glazes underneath are underfired. =
Do pieces react to cold temperatures even after a few days? I let the =
kiln cool completely before removing my pieces and when they are =
finished they look fine. It isn't until later, even an hour or so, that =
I will notice crazing. Thanks for the help!!=20
Rachel
Snail Scott on tue 2 dec 03
At 09:41 PM 12/1/03 -0800, you wrote:
>I have tried everything to prevent crazing on pieces that I apply a clear
gloss to...I also am using Duncan IN1001 clear glaze. Is it the glaze? Is
it the glazes underneath?
I don't know of any way to truly prevent
crazing on earthenware. You may forstall
it by finding a clay that fits the glaze
better (or finding a glaze that fits the clay
better), but eventually, they all seem to
craze, becaus the earthenware is still
slightly absorbent. It's not only clear
glossy glazes that craze, either, it's just
that opaque, matte, or colored glazes don't
show the effect as clearly (so to speak).
-Snail
Ron Roy on wed 3 dec 03
Changing the bisque temperature will not affect the fit of clay and glaze -
unless the higher bisque simply results in a thinner application of the
glaze.
If a glaze crazes where thick and not where thin it still means the glaze
will eventually craze - the expansion/contraction of the glazes still needs
to be lowered - or the body expansion/contraction needs to be raised.
Keep in mind - the clay and glaze fit perfectly at top temperature. Even
though the clay and glaze contract at different rates during cooling - the
glaze - while still soft can adjust to the body. It is only when the glaze
freezes (becomes rigid) that fit problems appear.
Best to do the glaze - lowering body expansion is not the best answer -
unless all your glazes are crazing.
RR
>You can attempt to resolve crazing by
>1-bisquing to a higher cone
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
piedpotterhamelin@COMCAST.NET on wed 3 dec 03
You can attempt to resolve crazing by
1-bisquing to a higher cone
2-incorporating more silica in your glaze
3-Combining a high exp and low expansion frit or adding a low expansion frit
4-Bring up your glaze firing a half or full cone
5-Trying tests of this glaze with other clay samples
6-Accept the reality of the beast
--
"Many a wiser men than I hath
gone to pot." 1649
E Wood on wed 3 dec 03
try bisque firing the clay body higher than the glaze firing temperature.
you get a little more shrinkage that way. also, if the look of crazing or
scumming is bothersome, add a little spanish red to the clear you use on
your red clay body - kind of like a concealer!
----- Original Message -----
From: "Snail Scott"
To:
Sent: Tuesday, December 02, 2003 11:48 AM
Subject: Re: Help with crazing!
> At 09:41 PM 12/1/03 -0800, you wrote:
> >I have tried everything to prevent crazing on pieces that I apply a clear
> gloss to...I also am using Duncan IN1001 clear glaze. Is it the glaze? Is
> it the glazes underneath?
>
>
> I don't know of any way to truly prevent
> crazing on earthenware. You may forstall
> it by finding a clay that fits the glaze
> better (or finding a glaze that fits the clay
> better), but eventually, they all seem to
> craze, becaus the earthenware is still
> slightly absorbent. It's not only clear
> glossy glazes that craze, either, it's just
> that opaque, matte, or colored glazes don't
> show the effect as clearly (so to speak).
>
> -Snail
>
>
____________________________________________________________________________
__
> 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 on thu 4 dec 03
How about mixing your own clear glaze. Then you
can adjust it to fit any body you want.
Duncan is mixing 80 or 90% frit with kaolin,
you can do that also.
Start with 40:40:5:15 Frit 3134:3124:Silica:kaolin.
Increase 3124 if it crazes, 3134 if it shivers.
Search for \"base\" here:
http://digitalfire.ab.ca/cermat/education/index.php
-------8<--------
> At 09:41 PM 12/1/03 -0800, you wrote:
> >I have tried everything to prevent crazing on pieces that I apply a clear
> gloss to...I also am using Duncan IN1001 clear glaze. Is it the glaze? Is
> it the glazes underneath?
>
>
> I don\'t know of any way to truly prevent
> crazing on earthenware. You may forstall
> it by finding a clay that fits the glaze
> better (or finding a glaze that fits the clay
> better), but eventually, they all seem to
> craze, becaus the earthenware is still
> slightly absorbent. It\'s not only clear
> glossy glazes that craze, either, it\'s just
> that opaque, matte, or colored glazes don\'t
> show the effect as clearly (so to speak).
>
-------
Tony Hansen, http://digitalfire.com
Personal Contact Page: http://digitalfire.com/services/contact.php
Calculation/Database Software for Ceramic Industry
Ron Roy on fri 5 dec 03
Crazing (and the opposite - shivering) are always the fault of the clay and
the glaze together - either change the glaze or the clay.
Ask your clay supplier to sell you a clay that fits that glaze - some have
special bodies that have a high enough expansion to solve the problem.
Low fire bodies will absorb water over time and eventually craze anyway - a
good reason to move up to cone 6 so you can have a vitrified clay.
RR
>I have tried everything to prevent crazing on pieces that I apply a clear
>gloss to. I don't have a problem with my other glazes, but it is almost a
>guarantee that I will see crazing when I use clear. I also notice delayed
>crazing sometimes. I have tried just about everything I know to remedy
>this, but sometimes it still occurs. I also am using Duncan IN1001 clear
>glaze. Is it the glaze? Is it the glazes underneath? I heard that crazing
>may occur if the glazes underneath are underfired. Do pieces react to cold
>temperatures even after a few days? I let the kiln cool completely before
>removing my pieces and when they are finished they look fine. It isn't
>until later, even an hour or so, that I will notice crazing. Thanks for
>the help!!
>Rachel
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
piedpotterhamelin@COMCAST.NET on fri 5 dec 03
Dear Ron,
Although perhaps, you observed and determined that a higher bisque firing than that of the glaze firing would have no effect other than applied glaze thickness, I beg to differ.
1-Duncan and all the other manufactures of the little jars of glaze instruct on the label to bisque at ^04 and glaze fire at ^05-06. They also instruct to apply at least three coats of glaze so thickness is constant.
2-Jeff Zamek in his book "What every potter should know" on page 137 stated to bisque fire one to cones higher than the glaze firing which might bring the clay/glaze to a better fit.
3-Hamer on page 88 of The Potters Dictionary also stated " If the earthenware body is biscuited at a higher temperature than the glaze will be fired, then this temperature greatly affects craze resistance. Underfiring bisque will cause causing. (BTW) Overfiring of non-cristobalite bodies may also cause crazing by fusing the previously crystalline free silica."
4-My personal experience with my ^05 glazes is that a ^05 bique pot has no crazing compared to a ^06 pot. I must apply my glaze to the same thickness due to the demands of color and gloss response, so I cannot apply a thinner coat, hence, preventing the crazing.
Mr Roy, if the pot is fired higher, then would it not have more of a complete fusion and more shrinkage towards fitting the glaze? Increasing the shrinkage being the same as your suggestion of raising the body contraction.
I only want to learn, never to debate. These are my sources and observations for my cone range using cone 04 red earthenware.
BTW to the original questioner, a former glaze that fitted beautifully on my red clay would craze heavily on a white earthenware body from the same supplier.
--
"Many a wiser men than I hath
gone to pot." 1649
> Changing the bisque temperature will not affect the fit of clay and glaze -
> unless the higher bisque simply results in a thinner application of the
> glaze.
>
> If a glaze crazes where thick and not where thin it still means the glaze
> will eventually craze - the expansion/contraction of the glazes still needs
> to be lowered - or the body expansion/contraction needs to be raised.
>
> Keep in mind - the clay and glaze fit perfectly at top temperature. Even
> though the clay and glaze contract at different rates during cooling - the
> glaze - while still soft can adjust to the body. It is only when the glaze
> freezes (becomes rigid) that fit problems appear.
>
> Best to do the glaze - lowering body expansion is not the best answer -
> unless all your glazes are crazing.
>
> RR
>
>
> >You can attempt to resolve crazing by
> >1-bisquing to a higher cone
>
> 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.
Cindi Anderson on fri 5 dec 03
The reason it is typically recommended to bisque hotter for low fire glaze
is to burn out carbon at the bisque so it doesn't come out during the glaze.
If you bisqued to Cone 06 and glazed to Cone 04, then some additional carbon
could burn out and come through the glaze causing glaze defects. However I
have personally done this (bisque at 06, glaze at 04) and didn't have glaze
defects. I did however have crazing sometimes.
As far as more fusion at a hotter temperature, it has always been my
understanding that there is no fusion happening at these low temperatures.
So their should be no difference between cone 04 and 05.
However your Hamer reference was interesting, and I wonder if anyone has
experience to support that statement.
Cindi
Fremont, CA
piedpotterhamelin@COMCAST.NET on fri 5 dec 03
Hi Cindi
I appreciate your thoughts on the crazing question.
Wouldn't the purpose of your thoughts be for preventing the problem of outgassing causing pinholing and not that of crazing?
The body/glaze fusion at earthenware tempertures is certainly not the same as at higher temperatures which is what I do acknowledge. My question was if the bisquing at a cone or two cones higher actually in a sense, fused a smaller pot that fits the glaze better. Something like making the foot smaller to fit the shoe. The ceramic body gets tighter as temperture increases and when co of effiecients only need to change by .5 while not obvious as a volume change to the eye, it does affect the glaze.
I agree with you that Hamers and Zameks notes on this are interesting. I didn't have the time to look into Parmalee or my other books but I will try. I get frustrated with open ended statements such as this that don't teach us more. Which is why I am here opening my can of worms.
--
"Many a wiser men than I hath
gone to pot." 1649
> The reason it is typically recommended to bisque hotter for low fire glaze
> is to burn out carbon at the bisque so it doesn't come out during the glaze.
> If you bisqued to Cone 06 and glazed to Cone 04, then some additional carbon
> could burn out and come through the glaze causing glaze defects. However I
> have personally done this (bisque at 06, glaze at 04) and didn't have glaze
> defects. I did however have crazing sometimes.
>
> As far as more fusion at a hotter temperature, it has always been my
> understanding that there is no fusion happening at these low temperatures.
> So their should be no difference between cone 04 and 05.
>
> However your Hamer reference was interesting, and I wonder if anyone has
> experience to support that statement.
>
> Cindi
> Fremont, CA
>
> ______________________________________________________________________________
> 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.
Craig Martell on fri 5 dec 03
>Mr Roy, if the pot is fired higher, then would it not have more of a
>complete fusion and more shrinkage towards fitting the glaze? Increasing
>the shrinkage being the same as your suggestion of raising the body
>contraction.
Hi:
"Shrinkage", has nothing to do with glaze fit. Glaze fit, or lack of glaze
fit, is determined by the expansion rates of both the claybody and
glaze. It's important to remember that shrinkage and expansion are not the
same thing.
regards, Craig Martell Hopewell, Oregon
piedpotterhamelin@COMCAST.NET on sat 6 dec 03
I don't think that I used the term shrinkage wisely. My intentions were a visual aid by using the term "shrinkage". More complete fusion is more appropriate.
Parmelee and Harman refer to Rieke's finding that the coefficient of exp of porcelain glazes vary from 27 to 42 and stoneware (vitreous all clay body) from 57-96 (x10-7).
It suggests to me that as temperature increases (and so to the fusion of the clay) this promotes reducing the expansion of the clay body in the second firing and hence bringing the fired clay towards fitting the glaze.
My books don't discuss this relationship of the co-eff of expansion of the fired bisqueware to a glaze or as a factor in making the glaze fit, other than the Zamek and Hamer comments already stated.
Now Parmelee and Harman were not concerned with the earthenware range in this article and I acknowledge that the glaze body interface (which is more completely formed in this heat range than that of earthenware) forms more completely and is more forgiving in these upper temperatures. I am merely suggesting (maybe questioning) that the potentially higher expansion rate of a pot bisqued at 05 and glazed at 04 may have more tendency to craze than a pot that was bisqued at 04 and glazed at 06. By Zamek, Hamer and Harman's notes, I sense that the expansion rate of a body is not constant across the cones but reduces as temperature increases. And that can benefit glaze fit.
Well, there it is .
"Many a wiser men than I hath
gone to pot." 1649
> >Mr Roy, if the pot is fired higher, then would it not have more of a
> >complete fusion and more shrinkage towards fitting the glaze? Increasing
> >the shrinkage being the same as your suggestion of raising the body
> >contraction.
>
> Hi:
>
> "Shrinkage", has nothing to do with glaze fit. Glaze fit, or lack of glaze
> fit, is determined by the expansion rates of both the claybody and
> glaze. It's important to remember that shrinkage and expansion are not the
> same thing.
>
> regards, Craig Martell Hopewell, Oregon
>
> ______________________________________________________________________________
> 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.
Ron Roy on sun 7 dec 03
Dear .......what is your name?
Yes - I see your references but I was responding to the statement you made
with no reference to low fire - just to make sure others would not expect
to influence fit by firing bisque higher for stoneware temperature clay and
glazes.
It also seems to me that firing bisque higher at earthenware temperatures
to stop crazing is contrary to what I understand about the problem. Firing
bisque higher would result in more crystalline silica being turned into
amorphous silica - which does not go through the quartz inversion at 573C.
This inversion helps eliminate crazing because the body gets smaller
suddenly - at that temperature during cooling.
I agree with Hamer that adding silica to a body will help to cure crazing
because of the quartz inversion. I also agree that firing bisque higher
will make the earthenware body tighter and delay the eventual moisture
expansion that will craze all earthenware bodies.
We also need to clarify what kind of earthenware body we are talking about
- if it contains Talc or Dolomite we can expect some enstatite formation -
which will help crazing when fired higher.
I am willing to do the dilatometer tests to compare the two situations -
all I need are samples of the clay fired to cone 06, 04 and 02. If you are
willing to do that part I will send instructions on how to prepare the rods
and you can mail them to me. Just the clay - no glaze needed.
Are there any potters out there who have found that firing earthenware
bisque higher has any effect on crazing? If so I need some samples to test.
Could it be - if the bisque is fired higher - that the load cools slower
and that has the effect of seeming to cure crazing?
Hoping we can find some relative truth in this - RR
>Although perhaps, you observed and determined that a higher bisque firing
>than that of the glaze firing would have no effect other than applied
>glaze thickness, I beg to differ.
>1-Duncan and all the other manufactures of the little jars of glaze
>instruct on the label to bisque at ^04 and glaze fire at ^05-06. They also
>instruct to apply at least three coats of glaze so thickness is constant.
>2-Jeff Zamek in his book "What every potter should know" on page 137
>stated to bisque fire one to cones higher than the glaze firing which
>might bring the clay/glaze to a better fit.
>3-Hamer on page 88 of The Potters Dictionary also stated " If the
>earthenware body is biscuited at a higher temperature than the glaze will
>be fired, then this temperature greatly affects craze resistance.
>Underfiring bisque will cause causing. (BTW) Overfiring of
>non-cristobalite bodies may also cause crazing by fusing the previously
>crystalline free silica."
>4-My personal experience with my ^05 glazes is that a ^05 bique pot has no
>crazing compared to a ^06 pot. I must apply my glaze to the same thickness
>due to the demands of color and gloss response, so I cannot apply a
>thinner coat, hence, preventing the crazing.
>Mr Roy, if the pot is fired higher, then would it not have more of a
>complete fusion and more shrinkage towards fitting the glaze? Increasing
>the shrinkage being the same as your suggestion of raising the body
>contraction.
>I only want to learn, never to debate. These are my sources and
>observations for my cone range using cone 04 red earthenware.
>BTW to the original questioner, a former glaze that fitted beautifully on
>my red clay would craze heavily on a white earthenware body from the same
>supplier.
>
>--
>"Many a wiser men than I hath
>gone to pot." 1649
>> Changing the bisque temperature will not affect the fit of clay and glaze -
>> unless the higher bisque simply results in a thinner application of the
>> glaze.
>>
>> If a glaze crazes where thick and not where thin it still means the glaze
>> will eventually craze - the expansion/contraction of the glazes still needs
>> to be lowered - or the body expansion/contraction needs to be raised.
>>
>> Keep in mind - the clay and glaze fit perfectly at top temperature. Even
>> though the clay and glaze contract at different rates during cooling - the
>> glaze - while still soft can adjust to the body. It is only when the glaze
>> freezes (becomes rigid) that fit problems appear.
>>
>> Best to do the glaze - lowering body expansion is not the best answer -
>> unless all your glazes are crazing.
>>
>> RR
>>
>>
>> >You can attempt to resolve crazing by
>> >1-bisquing to a higher cone
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on sun 7 dec 03
This is a very complex subject - there will be many conflicting examples to
any generalizations.
The role of silica is huge for instance and any generalization such as all
earthenware clays simply will not work.
By the way the cure for crazing is to increase expansion/contration of the
clay body - it must get or stay smaller than the glaze if crazing is to be
avoided.
We use the word expansion but what we mean is expansion during heating -
contraction during cooling - high expansion means high contraction -
expansion and contraction are exactly reversable - no matter how many time
the clay or glaze is reheated and cooled.
It is quite clear when you look at dilatometer charts - cristobalite and
quartz inversions in clay are obvious as are set points, transition curves
and softening points for glazes.
As I said in my previous post - if anyone is interested in finding out if
bisquing clay higher changes expansion rates - I have the dilatometer to
find out.
RR
>I sense that the expansion rate of a body is not constant across the cones
>but >reduces as temperature increases. And that can benefit glaze fit.
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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