Vince Pitelka on wed 1 dec 10
Jonathan Byler wrote:
We have recently been having some problems with a glaze that we found from
Edouard Bastarache called bottle blue. It worked great for a bit, and
recently we have been having some trouble with it spalling and flaking off
of some pots, mostly at the lips/rims or cups and bowls. We haven't notice=
d
this with any other glazes recently and were wondering what the cause might
be, and what to do as a remedy.
Jonathan -
I didn't run the recipe in Insight, but I cannot see anything obvious in th=
e
glaze recipe that would cause this. What you are referring to is shivering=
,
not spalling. It is the opposite of crazing, and is caused by excessive
glaze compression. Slight glaze compression, where the clay shrinks ever s=
o
slightly more than the claybody, is desirable. But if you have excessive
glaze compression, you get shivering, which is of course dangerous, because
those little slivers and flakes are razor sharp. The solution is usually t=
o
slightly increase glaze shrinkage, and a very small change can do it. I ha=
d
this problem once when I was working full time as a studio potter, and
solved the problem by substituting ball clay for some of the kaolin, on the
theory that kaolin contains more alumina. It doesn't sound like much of a
change, but it worked for me. Another choice would be to decrease the
feldspar, but I'd use a glaze calc program for that.
Has there been any change in your claybody? Sometimes a glaze is very clos=
e
to the limit of glaze compression, in which case the slightest change can
cause shivering. This can happen if the claybody shrinks very slightly mor=
e
than normal during vitrification.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
jonathan byler on wed 1 dec 10
someone mentioned to me yesterday that the rutile might be at
issue.... is this a common problem that people have when using rutile
in glazes? if so, can you remedy the problem by mixing up titanium
dioxide and red iron oxide in the proper proportions to sub for the
rutile?
On Dec 1, 2010, at 7:12 PM, Edouard Bastarache wrote:
> Jon,
>
> it has never happened to me here over the years, using different
> sources of rutile, like the 120-mesh one produced locally by Quebec
> Iron and Titanium.
>
> Gis,
>
> Edouard Bastarache Spertesperantisto
> Sorel-Tracy
> Quebec
>
> http://www.flickr.com/photos/30058682@N00/
> http://edouardbastarache.blogspot.com/
> http://blogsalbertbastarache.blogspot.com/
> http://cerampeintures.blogspot.com/
> http://www.facebook.com/edouard.bastarache
>
>
>
>
>
> ----- Original Message ----- From: "jonathan byler" > >
> To:
> Sent: Wednesday, December 01, 2010 5:01 PM
> Subject: glaze spalling with bottle blue
>
>
>> We have recently been having some problems with a glaze that we found
>> from edouard bastarache called bottle blue.
>> it worked great for a bit, and recently we have been having some
>> trouble with it spalling and flaking off of some pots, mostly at the
>> lips/rims or cups and bowls. We haven't noticed this with any other
>> glazes recently and were wondering what the cause might be, and what
>> to do as a remedy.
>> we normally fire to about ^10 with the ten somewhere between
>> touching
>> and flat.
>> custer spar 40.8
>> whiting 17
>> strontium carb 5.3
>> epk 20.9
>> silica 16
>> _____
>> rutile 4.80
>> cobalt carb 2.55
>> any ideas?
>>
>
>
jonathan byler on wed 1 dec 10
We have recently been having some problems with a glaze that we found
from edouard bastarache called bottle blue.
it worked great for a bit, and recently we have been having some
trouble with it spalling and flaking off of some pots, mostly at the
lips/rims or cups and bowls. We haven't noticed this with any other
glazes recently and were wondering what the cause might be, and what
to do as a remedy.
we normally fire to about ^10 with the ten somewhere between touching
and flat.
custer spar 40.8
whiting 17
strontium carb 5.3
epk 20.9
silica 16
_____
rutile 4.80
cobalt carb 2.55
any ideas?
Edouard Bastarache on wed 1 dec 10
Jon,
it has never happened to me here over the years,
using different sources of rutile, like the 120-mesh
one produced locally by Quebec Iron and Titanium.
Gis,
Edouard Bastarache
Spertesperantisto
Sorel-Tracy
Quebec
http://www.flickr.com/photos/30058682@N00/
http://edouardbastarache.blogspot.com/
http://blogsalbertbastarache.blogspot.com/
http://cerampeintures.blogspot.com/
http://www.facebook.com/edouard.bastarache
----- Original Message -----
From: "jonathan byler"
To:
Sent: Wednesday, December 01, 2010 5:01 PM
Subject: glaze spalling with bottle blue
> We have recently been having some problems with a glaze that we found
> from edouard bastarache called bottle blue.
>
> it worked great for a bit, and recently we have been having some
> trouble with it spalling and flaking off of some pots, mostly at the
> lips/rims or cups and bowls. We haven't noticed this with any other
> glazes recently and were wondering what the cause might be, and what
> to do as a remedy.
>
> we normally fire to about ^10 with the ten somewhere between touching
> and flat.
>
> custer spar 40.8
> whiting 17
> strontium carb 5.3
> epk 20.9
> silica 16
> _____
>
> rutile 4.80
> cobalt carb 2.55
>
>
> any ideas?
>
William & Susan Schran User on thu 2 dec 10
On 12/1/10 5:01 PM, "jonathan byler" wrote:
> We have recently been having some problems with a glaze that we found
> from edouard bastarache called bottle blue.
> it worked great for a bit, and recently we have been having some
> trouble with it spalling and flaking off of some pots, mostly at the
> lips/rims or cups and bowls. We haven't noticed this with any other
> glazes recently and were wondering what the cause might be, and what
> to do as a remedy.
If everyone is using the same clay body, then sometimes over working the ri=
m
while throwing sometimes changes the chemistry of the clay (finer clays
washed away from the surface) and this can lead to shivering, the glaze
peels away from the rim after firing. It's a coefficient of expansion
problem.
If it's happening on one clay but not others, then again, a COE issue,
requiring adjustment of the glaze or not using that clay and glaze together=
.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
L TURNER on thu 2 dec 10
Sometime during the past year I read an article in one of the journals
that addressed the problem of shivering from the rims of glazed forms.
The exact journal reference I can't recall, but the main causal
parameters were the clay body, the design and thickness of the rim,
and the thickness of the applied glaze. It is indeed a COE problem,
but only a localized problem not a general mismatch.
What I remember is that thin or "sharp", rims were common to the
failures analyzed. Thin and sharp rims do not provide a large surface
area for the glaze to adhere without high stresses, thus the glaze
comes off.
If only rims are shivering, I would give to modification to rim design
and glaze application a higher priority than a wholesale revision to
the glaze recipe.
Regards,
L. Turner,
The Woodlands, TX
David Finkelnburg on thu 2 dec 10
Jonathan,
To understand what's happening it would be helpful to know, is this
problem occurring before or after, or during firing? Is it shivering (glaz=
e
pops off after the piece comes out of the kiln), drying cracks (glaze
doesn't get into the kiln intact) or does the glaze fall on the shelves and
melt there during firing? If what you have is shivering then it's clearly a
glaze fit issue.
Dave Finkelnburg
http://www.mattanddavesclays.com
jonathan byler on thu 2 dec 10
thanks all for the replies about the shivering... I will try a slight
modification of the glaze in hyperglaze to see if we can adjust the
coefficient of expansion down just a bit and see if that helps. it is
strictly flaking off on the rims, and happened with some of my work
(strictly amateur at throwing but with some skill) and also to some of
gary's work (strictly professional grade potter with 35+ years of
experience under his belt). we were using the same clay, thrown on
the same day if I am not mistaken. I can't imagine that he was doing
something worse to the rims of his pots than I was. and none of these
edges was razor thin, more like a nice 1/4" diameter rounded lip. and
we have seen plenty of work done with the same clay and different
glazes that had relatively sharper lips that had no problems at all.
I don't think that that particular firing was over reduced (at least
no more than any other firings we have done).
Could be that this batch was mixed up wrong by someone. wouldn't be
the first time that it happened.
anyway, your conclusions are much the same as ours, so we will see
what we can do. it's a nice looking glaze when it works.
best,
jon
On Dec 1, 2010, at 8:36 PM, Vince Pitelka wrote:
> Jonathan Byler wrote:
> We have recently been having some problems with a glaze that we
> found from
> Edouard Bastarache called bottle blue. It worked great for a bit, and
> recently we have been having some trouble with it spalling and
> flaking off
> of some pots, mostly at the lips/rims or cups and bowls. We haven't
> noticed
> this with any other glazes recently and were wondering what the
> cause might
> be, and what to do as a remedy.
>
> Jonathan -
> I didn't run the recipe in Insight, but I cannot see anything
> obvious in the
> glaze recipe that would cause this. What you are referring to is
> shivering,
> not spalling. It is the opposite of crazing, and is caused by
> excessive
> glaze compression. Slight glaze compression, where the clay shrinks
> ever so
> slightly more than the claybody, is desirable. But if you have
> excessive
> glaze compression, you get shivering, which is of course dangerous,
> because
> those little slivers and flakes are razor sharp. The solution is
> usually to
> slightly increase glaze shrinkage, and a very small change can do
> it. I had
> this problem once when I was working full time as a studio potter, and
> solved the problem by substituting ball clay for some of the kaolin,
> on the
> theory that kaolin contains more alumina. It doesn't sound like
> much of a
> change, but it worked for me. Another choice would be to decrease the
> feldspar, but I'd use a glaze calc program for that.
>
> Has there been any change in your claybody? Sometimes a glaze is
> very close
> to the limit of glaze compression, in which case the slightest
> change can
> cause shivering. This can happen if the claybody shrinks very
> slightly more
> than normal during vitrification.
> - Vince
>
> Vince Pitelka
> Appalachian Center for Craft
> Tennessee Tech University
> vpitelka@dtccom.net; wpitelka@tntech.edu
> http://iweb.tntech.edu/wpitelka
Ron Roy on thu 2 dec 10
Ho Jon.
You need to INCREASE the expansion to get rid of shivering.
I have the feeling there was an error in the mixing - was that when
the trouble started?
The ideal way to find the right situation between glaze and clay is to
get to grazing - then drop the expansion till the crazing stops - then
do some testing to see if you can make the combination craze - and if
you can drop it a bit more.
I am interested in what kind of clays you are using - it is possible
that some may be generating cristobalite which can lead to shivering.
RR
Quoting jonathan byler :
> thanks all for the replies about the shivering... I will try a slight
> modification of the glaze in hyperglaze to see if we can adjust the
> coefficient of expansion down just a bit and see if that helps. it is
> strictly flaking off on the rims, and happened with some of my work
> (strictly amateur at throwing but with some skill) and also to some of
> gary's work (strictly professional grade potter with 35+ years of
> experience under his belt). we were using the same clay, thrown on
> the same day if I am not mistaken. I can't imagine that he was doing
> something worse to the rims of his pots than I was. and none of these
> edges was razor thin, more like a nice 1/4" diameter rounded lip. and
> we have seen plenty of work done with the same clay and different
> glazes that had relatively sharper lips that had no problems at all.
> I don't think that that particular firing was over reduced (at least
> no more than any other firings we have done).
>
> Could be that this batch was mixed up wrong by someone. wouldn't be
> the first time that it happened.
>
> anyway, your conclusions are much the same as ours, so we will see
> what we can do. it's a nice looking glaze when it works.
>
> best,
> jon
>
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