Lily Krakowski on tue 5 aug 03
Can you do an experiment for "us"?
Throw and trim and decorate your vases as usual. When they are ready for
the bisque select one and make holes--about the thickness of a
pencil--around the shoulder (top of belly). Bisque and proceed as usual.
If there are no pinholes it would lead me to believe that some gasses
trapped inside a vase (bottle) which escape from a bowl are doing the
pinholing. That would back RR's suggestion of slower firing at crucial
point.
Then:
I have come up with yet another theory: that the glaze on the INSIDE of the
belly is thicker than elswhere--not uncommon with closed shapes. that the
gasses it generates somehow are causing your troubles. Therefore fire one
vase unglazed on the inside.
Could you? Would you?
Lili Krakowski
P.O. Box #1
Constableville, N.Y.
(315) 942-5916/ 397-2389
Be of good courage....
Kathi LeSueur on wed 6 aug 03
mlkrakowski@CITLINK.NET wrote:
> Can you do an experiment for "us"?
>
> Throw and trim and decorate your vases as usual. When they are ready for
> the bisque select one and make holes--about the thickness of a
> pencil--around the shoulder (top of belly). Bisque and proceed as usual.
> If there are no pinholes it would lead me to believe that some gasses
> trapped inside a vase (bottle) which escape from a bowl are doing the
> pinholing. That would back RR's suggestion of slower firing at crucial
> point.
>
> Then:
> I have come up with yet another theory: that the glaze on the INSIDE
> of the
> belly is thicker than elswhere--not uncommon with closed shapes. that the
> gasses it generates somehow are causing your troubles. Therefore fire one
> vase unglazed on the inside.
>
> Could you? Would you?>>
Will do.
Kathi
>
>
>
>
>
>
> Lili Krakowski
> P.O. Box #1
> Constableville, N.Y.
> (315) 942-5916/ 397-2389
>
> Be of good courage....
>
> ______________________________________________________________________________
> 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.
John Baymore on wed 6 aug 03
Finally happened to read this thread....... have a possible theory:
I think I have most of the pertinent facts........ haven't read all the
posts yet..... and if I have missed some important fact ... then ....who
knows .
-Only happens on one particular form.
-Only happens in the belly of the piece...rest is fine.
-Happens no matter where pieces are placed in the kiln.
-Does not really "fix" on a refire.
-You throw them using a rib on the inside of the form.
Glazes don't exist in a vacume. The exist on top of a clay body. The cl=
ay
and glaze interact with each other. ( Also see page
http://www.JohnBaymore.com/moreglaz.html for more on this general concept=
.
)
You throw with a rib on the inside. This usually implies that the bellyi=
ng
of a form is done by a lot of pure stretching and that there is no ongoin=
g
feedback from the tips of the fingers about wall thickness. If you don't=
throw the basic cylinder thicker in the center before stretching..... thi=
s
will cause the wall section in the bellied area to be much thinner than
that at the foot area and the neck area. Same amount of clay....larger
circumference....has to happen.
This thinner section will provide less water absorbtion into the bisque
during the glazing process. This will result in a thinner application of=
the powdered glaze coating in this area. So therefore:
-This slightly thinner glaze coating may be being affected by stuff in th=
e
clay that doesn't do the same thing when there is more glaze in proportio=
n
to the clay. =
-The glaze may have some soluble ingredient (K, NA, Li, ?) that is being
absorbed into the bisque. This chemistry may be causing clay body proble=
ms
like overfiring......... and therby affecting the overlying glaze. Or it=
may be that in the thinner section there is LESS of the material in the
bisque to migrate back out to the drying surface..... and thereby modifyi=
ng
the glaze composition.
My real guess is that it is the first one. (Don't remember seeing the
glaze recipe or formula.)
What I'd try first off is to take one of the forms you have the problem
with and cut it in half at the leatherhard state with a wire or large
knife. Look at the XC. Is there a dramatic difference in clay thickness=
between the area that typically has the pinholes and the areas that don't=
? =
If not.......... stop here . =
Now break open one of the fired pinholled pieces. Look at the cross
section and the pinholes. Does there seem to be ANY correlation between
the two?
If so...... glaze a piece as normal. Then I'd use a brush when the piece=
is quite dry....and add glaze around only the belly of the piece.... and
only on one side. Then fire it. If the part with the thickened glaze is=
not pinholed or less pinholed...... then you are on the right track.
Then repeat the test at least once to conform the observation.
If this is the culprit........... when you pull the cylinder up before yo=
u
start stretching the form into the wide belly......... leave the center
section of the cylinder a bit thicker than the top lip area and the lower=
foot area (unless you typically trim the lower sidewalls and foot area at=
leathehard state to get the right thickness ........ I don't usually, so
this is different than how I work).
YOu'll have to experiment and cut pieces in half until you get the "fee=
l"
of how much extra you need to leave.
The other alternative if this is the culprit is to layer on more glaze in=
the middle section.
Another possible cause of this problem is that the thinner section area o=
f
the piece will heat up faster and cool off faster than the rest of the
piece. A property called thermal lag. If....and that is a really big
IF.... the glaze you are using is really, Really, REALLY close to a
critical point in some aspect of the firing on the way up or at the peak =
of
the firing ... then this lag may be causing the issue. This is REALLY an=
unlikely issue. If it were this finnicky a glaze... I am sure that you
would be seeing other issues in other places.
There is also that the thinner section of the wall can more easily be ful=
ly
permeated fully thru by reducing gases (from both sides) like CO and H th=
an
the thicker sections, resulting in the claybody composition there being
slightly different (due to reduction effects on Fe, etc.)
than in the thicker areas. Ditto for O2 getting into all areas and mayb=
e
oxidizing something differently than in the thick parts. Again....not ve=
ry
likely.... but possible.
Hope one of these thoughts helps. Let me know if you try it.
best,
...................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086-5812 USA
JBaymore@compuserve.com
http://www.JohnBaymore.com
603-654-2752 (studio)
800-900-1110 (studio)
"Earth, Water, and Fire Noborigama Woodfiring Workshop: August 15-24,
2003"
Ron Roy on thu 7 aug 03
I like Johns thinking on this - makes a lot of sense to me.
Johns answers to this list are consistently of excellent quality - if there
was a list of nominees for Clay Arter of the year I would surely nominate
him.
He is a working potter and that makes his contributions even more laudatory
- it's hard enough making a living with pots never mind taking the hours
off to help us with our problems.
RR
>-Only happens on one particular form.
>-Only happens in the belly of the piece...rest is fine.
>-Happens no matter where pieces are placed in the kiln.
>-Does not really "fix" on a refire.
>-You throw them using a rib on the inside of the form.
>
>
>Glazes don't exist in a vacume. The exist on top of a clay body. The clay
>and glaze interact with each other. ( Also see page
>http://www.JohnBaymore.com/moreglaz.html for more on this general concept.
>)
>
>You throw with a rib on the inside. This usually implies that the bellying
>of a form is done by a lot of pure stretching and that there is no ongoing
>feedback from the tips of the fingers about wall thickness. If you don't
>throw the basic cylinder thicker in the center before stretching..... this
>will cause the wall section in the bellied area to be much thinner than
>that at the foot area and the neck area. Same amount of clay....larger
>circumference....has to happen.
>
>This thinner section will provide less water absorbtion into the bisque
>during the glazing process. This will result in a thinner application of
>the powdered glaze coating in this area. So therefore:
>
>-This slightly thinner glaze coating may be being affected by stuff in the
>clay that doesn't do the same thing when there is more glaze in proportion
>to the clay.
>
>-The glaze may have some soluble ingredient (K, NA, Li, ?) that is being
>absorbed into the bisque. This chemistry may be causing clay body problems
>like overfiring......... and therby affecting the overlying glaze. Or it
>may be that in the thinner section there is LESS of the material in the
>bisque to migrate back out to the drying surface..... and thereby modifying
>the glaze composition.
>
>
>My real guess is that it is the first one. (Don't remember seeing the
>glaze recipe or formula.)
>
>What I'd try first off is to take one of the forms you have the problem
>with and cut it in half at the leatherhard state with a wire or large
>knife. Look at the XC. Is there a dramatic difference in clay thickness
>between the area that typically has the pinholes and the areas that don't?
>If not.......... stop here .
>
>Now break open one of the fired pinholled pieces. Look at the cross
>section and the pinholes. Does there seem to be ANY correlation between
>the two?
>
>If so...... glaze a piece as normal. Then I'd use a brush when the piece
>is quite dry....and add glaze around only the belly of the piece.... and
>only on one side. Then fire it. If the part with the thickened glaze is
>not pinholed or less pinholed...... then you are on the right track.
> Then repeat the test at least once to conform the observation.
>
>If this is the culprit........... when you pull the cylinder up before you
>start stretching the form into the wide belly......... leave the center
>section of the cylinder a bit thicker than the top lip area and the lower
>foot area (unless you typically trim the lower sidewalls and foot area at
>leathehard state to get the right thickness ........ I don't usually, so
>this is different than how I work).
> YOu'll have to experiment and cut pieces in half until you get the "feel"
>of how much extra you need to leave.
>
>The other alternative if this is the culprit is to layer on more glaze in
>the middle section.
>
>
>Another possible cause of this problem is that the thinner section area of
>the piece will heat up faster and cool off faster than the rest of the
>piece. A property called thermal lag. If....and that is a really big
>IF.... the glaze you are using is really, Really, REALLY close to a
>critical point in some aspect of the firing on the way up or at the peak of
>the firing ... then this lag may be causing the issue. This is REALLY an
>unlikely issue. If it were this finnicky a glaze... I am sure that you
>would be seeing other issues in other places.
>
>There is also that the thinner section of the wall can more easily be fully
>permeated fully thru by reducing gases (from both sides) like CO and H than
>the thicker sections, resulting in the claybody composition there being
>slightly different (due to reduction effects on Fe, etc.)
> than in the thicker areas. Ditto for O2 getting into all areas and maybe
>oxidizing something differently than in the thick parts. Again....not very
>likely.... but possible.
>
>
>Hope one of these thoughts helps. Let me know if you try it.
>
>
>best,
>
>...................john
>
>
>John Baymore
>River Bend Pottery
>22 Riverbend Way
>Wilton, NH 03086-5812 USA
>
>JBaymore@compuserve.com
>http://www.JohnBaymore.com
>
>603-654-2752 (studio)
>800-900-1110 (studio)
>
>
>"Earth, Water, and Fire Noborigama Woodfiring Workshop: August 15-24,
>2003"
>
>______________________________________________________________________________
>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
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Kathi LeSueur on thu 7 aug 03
ronroy@TOTAL.NET wrote:
>I like Johns thinking on this - makes a lot of sense to me.
>RR
>
><>
>This thinner section will provide less water absorbtion into the bisque
>during the glazing process. This will result in a thinner application of
>the powdered glaze coating in this area. >>
>
Not quite. It also happens on utensil holders which are straight sided
and very uniform in thickness. But, I recently got a message from
another potter who has been battling the problem on the same forms for
several years. A rutile glaze, mine is titanium. Yes, I understand that
rutile is basically titanium with iron. Another thing we have in common
is that we use the same clay supplier even though I'm in Michigan and
she's in the southwest. Additionally, several years ago another potter
in my area decide to try clay from my supplier. He used a white clay
rather than the iron one I use. When I saw him at a show he said he
loved the clay but was getting pitting on vase forms. Same glaze, same
forms, different clay. This makes me wonder if its an ingredient common
to the clay bodies from this supplier. So, when I go pick up chemicals
next week I'll also pick up some clay and see if it makes a difference.
Kathi
--at the rate this is going I'm going to have to rent a hall to fix
dinner for all of you trying to help me with this problem. Ain't Clayart
great!
>
>
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