gordon jones on sun 25 jan 04
hey what did i do wrong this time.
just removed a load of porcelain pots and they had wart /tumor sized bumps
on them,new never seen this before. only one of the glazes did it though.
glaze was a little thck with just one application but still . the bumps are
not the glaze,you can actually see that the clay swelled up like it was
going to burst. maybe air that didnt show up in the bisque firing and the
moisture from the glaze soaked in what do i know,the more i learn the more
questions i come up with.duh
gordon earthbound arts
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Hank Murrow on mon 26 jan 04
Dear Gordon;
Sounds like you have had a few disappointing experiences. This one was
most likely precipitated by glazing ware that didn't have its
combustibles burned out in the bisque. If they burn out in the glaze
fire, they cause blisters in the body. Try raising your bisque
temperature by a couple of cones, or hold the kiln at top temperature
for 40 minutes before allowing it to cool normally.
Cheers, Hank in Eugene
On Jan 25, 2004, at 10:48 AM, gordon jones wrote:
> hey what did i do wrong this time.
> just removed a load of porcelain pots and they had wart /tumor sized
> bumps
> on them,new never seen this before. only one of the glazes did it
> though.
> glaze was a little thck with just one application but still . the
> bumps are
> not the glaze,you can actually see that the clay swelled up like it was
> going to burst. maybe air that didnt show up in the bisque firing and
> the
> moisture from the glaze soaked in what do i know,the more i learn the
> more
> questions i come up with.duh
Ron Roy on mon 26 jan 04
Overfired clay develops regular small bumps all over the ware - inside and
out - which grow progressively larger as the over firing continues. This is
because gases are generated as the clay breaks down and the trapped gases
expand.
If the bumps are different sizes and randomly situated on the ware it is
because there is air in the clay. This would not be uncommon in a vitrified
porcelain - the cure is to wedge the air out before forming. Porcelain
takes more wedging than stoneware by the way.
It may be that the clay is coming with air in it - slice a pug from the box
into many pieces and if you can't see any air bend the slabs and that will
show any small air pockets.
If the clay is deaired properly from you supplier - then you are putting
the air into the clay - perhaps through improper wedging.
Wedge up 5 lb. of clay (start with 50 wedges) cut into slices and look for
air. Wedge as many times as you need to get the air out.
RR
>Unfortunately, that is bloating. I had it happen when i fired down a piece
>cone 5 porcelain to the cone 6 schedule in mastering cone 6. It only happened
>with one piece, but wow, it was ugly.
>
>Laura Davis,
>Cincinnati, OH
>
>
>"just removed a load of porcelain pots and they had wart /tumor sized bumps
>on them,new never seen this before. only one of the glazes did it though.
>glaze was a little thck with just one application but still . the bumps are
>not the glaze,you can actually see that the clay swelled up like it was
>going to burst. maybe air that didnt show up in the bisque firing and the
>moisture from the glaze soaked in what do i know,the more i learn the more
>questions i come up with.duh"
>
>______________________________________________________________________________
>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
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Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
wayneinkeywest on mon 26 jan 04
Gordon:
I work also in porcelain and have had this happen from time to time. The
only explanations I could find are:
I didn't wedge enough, and there was air trapped in the clay (break one of
the pots at the "bump" and look.)
Also discovered that not all porcelains are the same (duh!). Mixing clay
from two different suppliers, or two different formulas can do that as well.
Must have been a reaction between the two bodies.
Sigh...we live, we learn...
Wayne Seidl
> hey what did i do wrong this time.
> just removed a load of porcelain pots and they had wart /tumor sized bumps
> on them,new never seen this before. only one of the glazes did it though.
> glaze was a little thck with just one application but still . the bumps
are
> not the glaze,you can actually see that the clay swelled up like it was
> going to burst. maybe air that didnt show up in the bisque firing and the
> moisture from the glaze soaked in what do i know,the more i learn the more
> questions i come up with.duh
>
> gordon earthbound arts
>
> _________________________________________________________________
> Check out the new MSN 9 Dial-up — fast & reliable Internet access with
prime
> features! http://join.msn.com/?pgmarket=en-us&page=dialup/home&ST=1
>
>
____________________________________________________________________________
__
> 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.
>
Earl Krueger on mon 26 jan 04
Gordon,
For what it's worth, my first-ever mug I fired to about cone 4.
Glaze didn't melt fully but I just had to try it out with coffee.
Then I decided to fire it higher to get a better glaze melt.
Washed the mug and let it dry.
Fired to cone 6.
Mug was full of "warts and tumors".
Same clay and glaze fired directly to cone 6 does not do this.
My conclusion; After firing to cone 4 the clay was still
porous and absorbed water. When I fired to cone 6
this internal water could not easily escape due to glaze
and caused the bloating.
Others may disagree with my conclusion.
Earl...
Bothell, WA, USA
Vanessa Miller on mon 26 jan 04
Unfortunately, that is bloating. I had it happen when i fired down a piece
cone 5 porcelain to the cone 6 schedule in mastering cone 6. It only happened
with one piece, but wow, it was ugly.
Laura Davis,
Cincinnati, OH
"just removed a load of porcelain pots and they had wart /tumor sized bumps
on them,new never seen this before. only one of the glazes did it though.
glaze was a little thck with just one application but still . the bumps are
not the glaze,you can actually see that the clay swelled up like it was
going to burst. maybe air that didnt show up in the bisque firing and the
moisture from the glaze soaked in what do i know,the more i learn the more
questions i come up with.duh"
william schran on mon 26 jan 04
Gordon wrote: >load of porcelain pots and they had wart /tumor sized bumps...
....only one of the glazes did it though.... glaze was a little thick
with just one application but still . the bumps are not the
glaze,you can actually see that the clay swelled up like it was
going to burst. maybe air that didn't show up in the bisque firing<.
Odd - I would first think possible air pockets or material in the
porcelain that burned after the clay body was vitrified causing
bloating or over firing the clay that would also cause bloating - but
only on pots with the one glaze - hmmm...
Does the glaze contain any soluble materials? How long after you
applied the glaze were the pots fired? New glaze or clay body?
Bill
Anne Melvin on tue 27 jan 04
Hi Gordon,
The bumps happened to me also. Being a newbie with porcelain and also
being very impatient, I bisque fired pots to Cone 06 as fast as possible
because I wanted to get on with the glazing. Bad move. After listening
to John Hesselberth's always sage advice and attending a Jeff Zamek
workshop, I found the best bisque fire for Cone 10 porcelain for me is
slooooow and to Cone 04. Something to try anyway. The warts also show up
more often on re-wedged clay.
Hope this helps,
Anne in Avon, CT
Still on a quest for the perfect pot and making all the mistakes along the
way... sometimes more than once.
Chris Schafale on tue 27 jan 04
Earl,
More likely, refiring the mug caused the clay body to be overfired, which
caused the bloating. Doubt very much that it was the water.
Chris
On 26 Jan 2004 at 21:20, Earl Krueger wrote:
> Gordon,
>
> For what it's worth, my first-ever mug I fired to about cone 4.
> Glaze didn't melt fully but I just had to try it out with coffee. Then
> I decided to fire it higher to get a better glaze melt. Washed the mug
> and let it dry. Fired to cone 6. Mug was full of "warts and tumors".
> Same clay and glaze fired directly to cone 6 does not do this.
>
> My conclusion; After firing to cone 4 the clay was still
> porous and absorbed water. When I fired to cone 6
> this internal water could not easily escape due to glaze
> and caused the bloating.
>
> Others may disagree with my conclusion.
>
> Earl...
> Bothell, WA, USA
>
> ______________________________________________________________________
> ________ 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.
--
Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh, NC)
www.lightonecandle.com
NEW email: chris at lightonecandle dot com
(insert the @ and . as appropriate)
william schran on tue 27 jan 04
Earl wrote:>My conclusion; After firing to cone 4 the clay was still
porous and absorbed water. When I fired to cone 6
this internal water could not easily escape due to glaze
and caused the bloating.<
Or the cone 6 firing could be too high for you clay body and that
caused the bloating.
Bill
Jon Pacini on tue 27 jan 04
Greetings All, Hi Gordon---- As Porcelain gets close to total
vitrification it becomes very soft, any gas that has been trapped/sealed in
the body in the body expands from the heat and blows up in the wall of the
pot like a balloon. It's commonly called bloating. This can be from organics
that have not been fully burned out or pockets of air in the Porcelain. Ron
suggested testing for air in the clay. If there's no air in the pug or from
your wedging, then you need to look closer at your firing technique. Too low
a bisque can contribute to this problem, as can too fast a firing. Soaking
is important in a bisque, the heat needs to penetrate into the ware. This is
important when nesting ware inside one another, particularly in electric
kilns. All of these situations can lead to not allowing the organics to burn
out properly.
Over firing will make the problem worse by softening the porcelain to
the point it allows the ballooning effect, but it's not the root cause, the
organics/gas trapped in the body is.
Best regards
Jon Pacini
Clay Manager
Laguna Clay Co
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