Pfeiffer, Dan R (Dan) on thu 9 feb 06
I would like more info, if anyone has a clue as to why re-firing a pot
sometimes makes it worse. We have been having a reoccurring problem in
that some of the pots in a load will have quite a number of pin holes
and on added a bit more glaze to the holes and re-firing we get more
holes that we started with. On one the whole top half of the pot was a
solid mass of pin holes.
We do a slow bisque to ^05 or 04 and did not see any change so I think
if there is a problem with the bisque it is more than the temp unless we
need to go lower? We have been working to slow the bisque up even more
in the hope this will help from having to re-fire.
Our glaze need to have a very low COE to fit the clay and we made it so
it also has a low LOE tiring to fix the pinholes. I do not have the
formula at hand but it is a very simple one with Custer,EPK, fine silica
and wollastonite. This makes a nice glass and with a long hold( 2 hours
or so) on the way down at about 1950F this does work most of the time.
It does seem to be more of a problem in green and blue. All too often
there is just one or two pinholes on a piece too big to ignore that
needs to be reworked. :(=20
=20
So the question is after firing to ^10 how could there be any carbon or
other stuff left in the body after a glaze firing? I would think this
would rule out any problem in the bisque causing a problem in a second
firing.
=20
This would seem to leave only the oxides to break down and outgas
leaving holes. Does this sound right? Would firing lower on a re-fire,
maybe to ^ 9 help or not? What should be a minimum temp to get a ^9-10
glaze to re-melt and still not make things worse?
=20
So many question and so little time! I hope to build a test kiln in a
few weeks and maybe this will help.
Dan & Laurel in Elkmont Al=20
Pfeiffer Fire Arts=20
www.pfeifferfirearts.com
Potters Council Members=20
Ivor and Olive Lewis on fri 10 feb 06
Dear Dan Pfeiffer,=20
You say <<...We have been having a reoccurring problem in that some of =
the pots in a load will have quite a number of pin holes and on added a =
bit more glaze to the holes and re-firing we get more holes that we =
started with. ...>> and wonder how it could be possible for gas to come =
out in even greater quantities on refiring to Cone 10. and continue =
<<...This would seem to leave only the oxides to break down and outgas =
leaving holes. Does this sound right?...>>
Yes, it does. My hypothesis, based on experience and study, is that the =
oxides we use are not as stable as the pundits would like us to believe. =
Nor are they "Pure" in the sense of 99.999% what it says on the packet. =
Many are no better that 90% and some which look white as powders are no =
better than 98%. This is why John and Ron will tell us that we must have =
the full analysis to do good calculations and glaze formulations.
That hypothesis. These bubbles are due to the breakdown of Iron Fe2O3 to =
FeO with the evolution of Oxygen. It seems to happen as Cone 10 is =
approached. You cannot prevent this without changing your firing =
program. It is question of basic chemical principles. To give guidance I =
would need to know about your firing facility and current firing =
program. Do you wish to continue our discussion ?.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ron Roy on sat 11 feb 06
Hi Dan,
This is usually the case when the clay is overfired - it starts melting too
much - produces gas and the gas causes the pin holes as it escapes through
the glaze.
If there is iron in the clay - and it does not get enough oxygen during the
burning out of organic matter in the bisque fire - and the iron gets
reduced - it becomes a flux - and helps overfire the clay. It is sometimes
worse where the clay is thicker and across the bottoms of pots. Happen more
when pots are stacked in the bisque.
It could also be that the clay is not formulated properly for the cone you
are firing to. Have you ever done an absorption test?
RR
>I would like more info, if anyone has a clue as to why re-firing a pot
>sometimes makes it worse. We have been having a reoccurring problem in
>that some of the pots in a load will have quite a number of pin holes
>and on added a bit more glaze to the holes and re-firing we get more
>holes that we started with. On one the whole top half of the pot was a
>solid mass of pin holes.
> We do a slow bisque to ^05 or 04 and did not see any change so I think
>if there is a problem with the bisque it is more than the temp unless we
>need to go lower? We have been working to slow the bisque up even more
>in the hope this will help from having to re-fire.
>Our glaze need to have a very low COE to fit the clay and we made it so
>it also has a low LOE tiring to fix the pinholes. I do not have the
>formula at hand but it is a very simple one with Custer,EPK, fine silica
>and wollastonite. This makes a nice glass and with a long hold( 2 hours
>or so) on the way down at about 1950F this does work most of the time.
>It does seem to be more of a problem in green and blue. All too often
>there is just one or two pinholes on a piece too big to ignore that
>needs to be reworked. :(
>
>So the question is after firing to ^10 how could there be any carbon or
>other stuff left in the body after a glaze firing? I would think this
>would rule out any problem in the bisque causing a problem in a second
>firing.
>
>This would seem to leave only the oxides to break down and outgas
>leaving holes. Does this sound right? Would firing lower on a re-fire,
>maybe to ^ 9 help or not? What should be a minimum temp to get a ^9-10
>glaze to re-melt and still not make things worse?
>
>So many question and so little time! I hope to build a test kiln in a
>few weeks and maybe this will help.
>Dan & Laurel in Elkmont Al
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
clennell on sat 11 feb 06
Sour Cherry Pottery
> Hi Dan,
>
> This is usually the case when the clay is overfired - it starts melting too
> much - produces gas and the gas causes the pin holes as it escapes through
> the glaze.
>
> If there is iron in the clay - and it does not get enough oxygen during the
> burning out of organic matter in the bisque fire - and the iron gets
> reduced - it becomes a flux - and helps overfire the clay. It is sometimes
> worse where the clay is thicker and across the bottoms of pots. Happen more
> when pots are stacked in the bisque.
>
> It could also be that the clay is not formulated properly for the cone you
> are firing to. Have you ever done an absorption test?
>
> RR
>
>> I would like more info, if anyone has a clue as to why re-firing a pot
>> sometimes makes it worse. We have been having a reoccurring problem in
>> that some of the pots in a load will have quite a number of pin holes
>> and on added a bit more glaze to the holes and re-firing we get more
>> holes that we started with. On one the whole top half of the pot was a
>> solid mass of pin holes.
Dan and RR: Ron's answer is interesting in that i was just considering
refiring some woodfired work that got hot on one side and dry on the other.
It had been fired to Cone 10. Cones down but no heat work. If i fire it
again to cone 10 considering "the heat work" am i really firing the pots to
more like 11 or 12.
So I'm planning due to RR"s post of putting the work in a cooler spot in my
gas kiln. Now will oxidized iron which gave me a muddy brown get to be sweet
green refired in reduction. I'll find out.
I fired the woodkiln too fast (my excuse is I was tired from Georgia) 700F
to Cone 10 in 6 hours. Work is pretty pale and lifeless. Hoping refiring
will turn the gravel into diamonds.
Cheers,
Tony
Tony and Sheila Clennell
Sour Cherry Pottery
4545 King Street
Beamsville, Ontario
CANADA L0R 1B1
http://www.sourcherrypottery.com
Lee Love on sun 12 feb 06
On Sun, 12 Feb 2006 17:35:27 -0500, Liz Willoughby wrote:
>on the pots became rougher. I think this is what happened. Refiring
>the glaze melted IT more, but the unmelted wood ash seemed much more
>obvious, and it wasn't hot enough to melt the ash.
Liz,
Sometimes it helps to sand the ash surface before refiring. I
sometimes put rice straw grass on my pots in an area where I flatten cone
13. The straw is mostly silica, and always comes out ruff. It is sharp and
will cut flesh easily. I smooth it with the grinding stone I use to
smooth pot feet with. Sometimes this fixes it, but other times I refire
after smoothing the ash.
Also, when wood firing in heavy reduction, sanding and polishing
can take to surface oxidation off, to expose a richer, brighter surface
underneath. Pots out of my teacher's yohen chamber are sanded, polished
with 3M pads and then rubbed with with a cotton cloth. These pots are very
dull before they are polished. After polishing, they are very rich and
colorful.
--
李 Lee Love 大
愛 鱗
in Mashiko, Japan http://mashiko.org
http://seisokuro.blogspot.com/ My Photo Logs
http://ikiru.blogspot.com/ Zen and Craft
"We are such stuff / As dreams are made on, and our little life / Is rounded
with a sleep."
--PROSPERO Tempest Shakespeare
Liz Willoughby on sun 12 feb 06
Tony C. says:
>
>Dan and RR: Ron's answer is interesting in that i was just considering
>refiring some woodfired work that got hot on one side and dry on the other.
>It had been fired to Cone 10. Cones down but no heat work. If i fire it
>again to cone 10 considering "the heat work" am i really firing the pots to
>more like 11 or 12.
>So I'm planning due to RR"s post of putting the work in a cooler spot in my
>gas kiln. Now will oxidized iron which gave me a muddy brown get to be sweet
>green refired in reduction. I'll find out.
>I fired the woodkiln too fast (my excuse is I was tired from Georgia) 700F
>to Cone 10 in 6 hours. Work is pretty pale and lifeless. Hoping refiring
>will turn the gravel into diamonds.
>Cheers,
>Tony
>
Hi Tony,
Tony remember the wood firing that our group had last Oct? When we
fired the kiln too fast??? Ron refired a few of my pots, they did
come out with better color (mostly shino), however the ash that was
on the pots became rougher. I think this is what happened. Refiring
the glaze melted IT more, but the unmelted wood ash seemed much more
obvious, and it wasn't hot enough to melt the ash. Refiring should
be in the cooler part of the kiln, but if you have a piece (that is
not precious, or is that possible?) that has a lot of ash on it, I
would put it in the hottest part of the kiln to see if you can melt
the ash. Maybe it would really require a few hours of cone 11 to
melt it, preferably in a wood kiln, or maybe not.
Sounds like your travels and workshops went well.
Liz
p.s. been away a while seeing the ancient Mayan cities in Mx. Came
home sick, nice to be able to read clayart again. Supposed to snow
tonight, how nice to be back to winter.....just kidding......how can
winter be 37C? imagine what summer is like in the Yucatan.
I am knitting a black sparkly classy boa for Portland.
--
Liz from Grafton, Ontario, Canada
"Three grand essentials to happiness in this life are . . . something
to do, something to love, and something to hope for."
Joseph Addison
Hank Murrow on sun 12 feb 06
On Feb 12, 2006, at 2:35 PM, Liz Willoughby wrote:
> Tony remember the wood firing that our group had last Oct? When we
> fired the kiln too fast??? Ron refired a few of my pots, they did
> come out with better color (mostly shino), however the ash that was
> on the pots became rougher. I think this is what happened. Refiring
> the glaze melted IT more, but the unmelted wood ash seemed much more
> obvious, and it wasn't hot enough to melt the ash. Refiring should
> be in the cooler part of the kiln, but if you have a piece (that is
> not precious, or is that possible?) that has a lot of ash on it, I
> would put it in the hottest part of the kiln to see if you can melt
> the ash. Maybe it would really require a few hours of cone 11 to
> melt it, preferably in a wood kiln, or maybe not.
Dear Liz;
When you think about it, it must take some real heat or some real time
'at heat' to melt ashy with the clay. One is trying after all to 'mix'
glaze ingredients in the kiln.....and they are not 300 mesh! So lots of
heat work is necessary to effect a melt. I routinely re-fire both salt,
soda, and woodfire in my fiber kiln and they just keep getting better.
Some of those beautiful pots from the kilns at Iga in Japan have been
fired a dozen times to get all the beauty into/out of them.
Cheers, Hank
www.murrow.biz/hank
ps: The roof trusses went up on the new Anagama Kiln Temple at the U of
Oregon here in Eugene today. 2800 #s each truss, six in all........and
anyone coming down to Eugene before or during NCECA must see this
Temple of Fire. It is simply gorgeous. I will install the fiber-lined
20" stainless steel flue after NCECA. Too busy with three shows to fire
for right now!
Dannon Rhudy on mon 13 feb 06
Tony, Liz et al - If you will brush the "dry" side of
your re-fires with just a little soda ash, you're gonna
love what you get.
regards
Dannon Rhudy
---
---
Pfeiffer, Dan R (Dan) on mon 13 feb 06
Ivor,
I would be glad to provide more info in the hope of coming to a better
understanding of what is happing. I will go a take a picture of our firing
chart and problem pots and glaze info and put all this on our web site. It
is about time to start a clayarts page. This will take a day of two to do
but will get back to you when complete.
Thanks,
Dan & Laurel in Elkmont Al
Pfeiffer Fire Arts
www.pfeifferfirearts.com
Potters Council Members
>>>To give guidance I would need to know about your firing facility and
current firing program. Do you wish to continue our discussion <<
Pfeiffer, Dan R (Dan) on mon 13 feb 06
I have yet to do any absorption testing but really should. We use a
highwater's clay and have not given any real thought to what cone is should
be fired at. Box has 6 - 10. It does not melt or bloat at ^12 and does seems
hard at ^10 from the pieces we break up. I have not noticed any more
problems with the clay being thicker, most often the higher outside parts of
the pot will have the most pin holes. It is a white clay so my guess in no
or little iron. I will make some test pieces for the next glaze firing.
Thanks,
Dan & Laurel in Elkmont Al
Pfeiffer Fire Arts
www.pfeifferfirearts.com
Potters Council Members
RR >>>>This is usually the case when the clay is overfired - it starts
melting too
much - produces gas and the gas causes the pin holes as it escapes through
the glaze.
If there is iron in the clay - and it does not get enough oxygen during the
burning out of organic matter in the bisque fire - and the iron gets
reduced - it becomes a flux - and helps overfire the clay. It is sometimes
worse where the clay is thicker and across the bottoms of pots. Happen more
when pots are stacked in the bisque.
It could also be that the clay is not formulated properly for the cone you
are firing to. Have you ever done an absorption test?>>>>
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