Veena Raghavan on sun 12 dec 99
Another question for the experts. I would really appreciate some advice or
explanation on what is going on here.
The studio where I do my work is having a problem with refiring platters at
Cone 6. They do not seem to be able to survive the refiring, they crack. In
one case, the piece actually cracked in half.
Is this a normal situation? Can any precautions be taken to prevent this
from happening?
Thank you in advance for your help.
All the best.
Veena
Veena Raghavan
75124.2520@compuserve.com
Craig Martell on mon 13 dec 99
Veena sez:
>The studio where I do my work is having a problem with refiring platters at
>Cone 6. They do not seem to be able to survive the refiring, they crack. In
>one case, the piece actually cracked in half.
Hello Veena:
It may be from cristobalite. Usually, cristobalite will start forming in
bodies at around cone 6 or so and the rate of development accelerates as
you climb in temperature to cone 10. The formation of cristobalite is an
on-going process so if your clay has sufficient free silica and is a bit
low in feldspar you could very well be developing enough cristobalite
during the second firing to cause dunting. Are the cracks clean and sharp
with no glaze flow over the edges? This would indicate a dunt.
It may be necessary to slow the temp rise and the cooling rate of firing #2
to allow even heating and adjustment for the pieces. Platters are
notorious for uneven heating and expansion.
regards, Craig Martell in Oregon
Stephen Parry-Thomas on mon 13 dec 99
Hello Veena,
There could be a few reasons for your platters cracking. It sounds to me that
your work is dunting.
Cooling the kiln to quickly will cause a large platter to crack. The outer
edge of the platter will be cooler than the centre, this will put the platter
under a thermal stress, or shock resulting in a crack. To get around this
problem I have introduced some grog into the clay making the body more open.
During firing try to raise the platter up , by firing on a saddle or stilts,
- made from high firing clay usually in a long triangle shaped strip. This
will increase the air flow around the platter when cooling, reducing the
thermal stress. Also try keeping the platter away from any spy hole or doors.
If you fire the platter near the door of the kiln place a row of kiln props
between the platter and door, the props will help protect the platter from a
cold spot in the kiln which is present by all kiln doors.
If the crack is a clean break through the glaze then I am sure its caused by
dunting , cooling to quickly. If the glaze seems to matured over and into the
crack, then your platter could have cracked, or dunted in the biscuit firing
and then the fault may not have been spotted before the platter has been
glazed. Hope this helps.
Stephen Parry-Thomas
www.morrigancraftpottery.co.uk
Cindy Strnad on mon 13 dec 99
Veena,
I've refired a lot of ^6 platters to bisque temperature or glaze temperature
and never had any cracking problems. It may be in your clay, or even in your
glaze, however it's worth noting that when refiring a glazed piece, you
should go very slowly. The glazed surface doesn't release gasses easily,
even with an unglazed foot, and too-quick heating can cause pots to crack or
even explode.
Cindy Strnad
Earthen Vessels Pottery
Custer, SD
Ron Roy on mon 13 dec 99
This probably happening on the way up. If so the glaze will have melted
around the edge of the crack - it will not be sharp. This can be too fast
firing during the quartz inversion at 573C and/or improper glaze fit. If
it's happening to all platters - with different glazes on them - it's
probably just too fast firing through the quartz inversion.
If the edges of the crack are sharp then the cracking happened on the way
down after the glazes have set - in that case it's probably improper glaze
fit and/or too fast cooling.
RR
>----------------------------Original message----------------------------
>Another question for the experts. I would really appreciate some advice or
>explanation on what is going on here.
>
>The studio where I do my work is having a problem with refiring platters at
>Cone 6. They do not seem to be able to survive the refiring, they crack. In
>one case, the piece actually cracked in half.
>
>Is this a normal situation? Can any precautions be taken to prevent this
>from happening?
>
>Thank you in advance for your help.
>
>All the best.
>
>Veena
>
>Veena Raghavan
>75124.2520@compuserve.com
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Jeanne Wood on tue 14 dec 99
Hi Veena,
I don't presume to call myself an expert, but I can
tell you how I have refired plates and platters
successfully. I refire quite a few pots, if I'm not
totally satisfied with their looks we try it again,
and again.
Are you firing electric? I have more success re-firing
with electric.
Make sure not to load the re-fired pots on the bottom
of the kiln.
I take the kiln up to temperature verrrry slowly,
especially the first few hours. I've found this to be
the most important part of refiring.
Good luck with it!
-Jeanne
--- Veena Raghavan <75124.2520@compuserve.com> wrote:
> ----------------------------Original
> message----------------------------
> Another question for the experts. I would really
> appreciate some advice or
> explanation on what is going on here.
>
> The studio where I do my work is having a problem
> with refiring platters at
> Cone 6. They do not seem to be able to survive the
> refiring, they crack. In
> one case, the piece actually cracked in half.
>
> Is this a normal situation? Can any precautions be
> taken to prevent this
> from happening?
>
> Thank you in advance for your help.
>
> All the best.
>
> Veena
>
> Veena Raghavan
> 75124.2520@compuserve.com
>
__________________________________________________
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Veena Raghavan on wed 15 dec 99
Ron,
Thank you very much for the explantion of what could be happening
with the cracking of refired cone 6 glazed platters.
None of the cracked platters were mine, but I will check about the
cracks, as to whether the glaze melted around the edge or whether it was
sharp. I do know that the glazes were different on at least two of the
platters.
Thank you for your help.
Veena
Veena Raghavan
75124.2520@compuserve.com
Veena Raghavan on thu 16 dec 99
Dear fellow Clayarters,
I would like to thank all of you for your very helpful and
informative replies to my post on the problem in the studio where I do my
work. I have collected all your advice and am going to pass it on tomorrow.
I hope that this collective information will enable us to refire platters
without their cracking, as has been the case recently.
I would like to take this opportunity to say to everyone on Clayart
that being a part of this group has been a wonderful resource for me. It
has given me a feeling of belonging to a group of very giving, in many case
very knowledgeable people, who willing share their experience and know-how
with those of us who do not know the ins and outs of so many technical
issues. Much of the glaze formulation material is Greek to me. I understand
much much more than I did a year ago, but hope that I will get to a good
glaze formulating workshop in the year 2000 (Ron, hope you see this!) Being
a part of Clayart has given me a new faith in people, something that I was
beginning to lose, because of various things that have happened to me in
the past year. Thank you one and all for your faith in what you do, for
your strong convictions, and for your deep enduring love of clay. I hope I
will get a chance to meet more Clayarters in the years to come.
I look forward to reading and talking to you all in the New Year.
Happy Holidays and Happy New Year.
Veena
Veena Raghavan
75124.2520@compuserve.com
Heidrun Schmid on thu 16 dec 99
------------------
Hi Jeanne,
when you say you refire =22very slowly=22 in the first few hours, how slow =
is
that? 50=B0C/h per hour, 100=B0C/h? And for how many hours do you go slow =
like
that? 3 hours, 4 hours? I have an electronic control on my kiln, so could
program my refirings easily. Most of the time the first refiring works,
second time many pieces crack, but I have had also clay starting to form
bubbles, shortly before =22exploding=22 which made fore some =EDnteresting =
small
bowls...
Maybe there is a limit to how many times clay can be refired to the highest
temperature? And the consequent refirings should be lower?
Heidrun in cool (26=B0C) Bangkok
Jeanne Wood wrote:
=3E ----------------------------Original message----------------------------
=3E Hi Veena,
=3E I don't presume to call myself an expert, but I can
=3E tell you how I have refired plates and platters
=3E successfully. I refire quite a few pots, if I'm not
=3E totally satisfied with their looks we try it again,
=3E and again.
=3E Are you firing electric? I have more success re-firing
=3E with electric.
=3E Make sure not to load the re-fired pots on the bottom
=3E of the kiln.
=3E I take the kiln up to temperature verrrry slowly,
=3E especially the first few hours. I've found this to be
=3E the most important part of refiring.
=3E Good luck with it=21
=3E -Jeanne
=3E
=3E --- Veena Raghavan =3C75124.2520=40compuserve.com=3E wrote:
=3E =3E ----------------------------Original
=3E =3E message----------------------------
=3E =3E Another question for the experts. I would really
=3E =3E appreciate some advice or
=3E =3E explanation on what is going on here.
=3E =3E
=3E =3E The studio where I do my work is having a problem
=3E =3E with refiring platters at
=3E =3E Cone 6. They do not seem to be able to survive the
=3E =3E refiring, they crack. In
=3E =3E one case, the piece actually cracked in half.
=3E =3E
=3E =3E Is this a normal situation? Can any precautions be
=3E =3E taken to prevent this
=3E =3E from happening?
=3E =3E
=3E =3E Thank you in advance for your help.
=3E =3E
=3E =3E All the best.
=3E =3E
=3E =3E Veena
=3E =3E
=3E =3E Veena Raghavan
=3E =3E 75124.2520=40compuserve.com
=3E =3E
=3E
=3E =
=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F==
5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F
=3E Do You Yahoo=21?
=3E Thousands of Stores. Millions of Products. All in one place.
=3E Yahoo=21 Shopping: http://shopping.yahoo.com
Dave Eickholt on thu 16 dec 99
My experience with refire plates is extensive, I fire cone ten crystalline ,
a very slow heat rise for refires is a must.[electric kiln programable
computer] 80 degrees a hr. till it hits 1200 degrees then 100 degrees a hr.
till temp. slow cooling a must [ a fiber blanket over the top of the kiln
helps . Also try mixing alumina hydrate with water till you get a paste and
use a foam brush to apply to the battom of the plate
. Be sure that the alumina hydrate does not get on the glaze surface, it will
leave rough patches in the glaze. the shelves must be well kiln washed and
very flat. Good luck Dave
Veena Raghavan on fri 17 dec 99
Dave,
Thanks so much for your post in reply to my question about refiring
platters. The firing suggestions you have made probably will not work in
this studio, but your suggestion about the alumina hydrate is very
interesting. I have been making sure that all of my platters have alumina
hydtrate in the wax, or that there is a coating of alumina hydrate on the
platter's bottom before the wax, but I have been using a thin coat not a
paste. I will use your suggestion next time and will pass it along to the
studio.
Thanks again and all the best for the holidays and the New Year.
Veena
Veena Raghavan
75124.2520@compuserve.com
Jeanne Wood on fri 17 dec 99
Greetings Heidrun,
I usually refire beginning with the lid cracked open &
on low for 2-4 hours, making sure not to close the lid
until no moisture escapes. Then on low with the lid
closed for another 2-4 hours. Then turn it up slowly
after that(I fire pots slowly & gradually anyway).
Can't tell you how many degrees/hr. as my pyrometer
isn't working and I haven't replaced it.
It is not unusual for me to refire up to 4 times,
unless I see I am really wasteing my time & energy on
a lost pot. I refire at similar temperatures they were
first fired, unless I want to apply a glaze which
requires a different temp.
I'm not sure about the bubbles (bloating?) which
sounds a little overfired temperature-wise.
I haven't found they break after several firings,
however they seem to get heavier (I haven't actually
weighed the difference, so I'm not sure). A little
strange as extra layers of glaze don't seem like they
would add extra weight.(?)
Good luck with it.
-Jeanne
--- Heidrun Schmid wrote:
> ----------------------------Original
> message----------------------------
> ------------------
> Hi Jeanne,
> when you say you refire "very slowly" in the first
> few hours, how slow is
> that? 500C/h per hour, 1000C/h? And for how many
> hours do you go slow like
> that? 3 hours, 4 hours? I have an electronic control
> on my kiln, so could
> program my refirings easily. Most of the time the
> first refiring works,
> second time many pieces crack, but I have had also
> clay starting to form
> bubbles, shortly before "exploding" which made fore
> some mnteresting small
> bowls...
> Maybe there is a limit to how many times clay can be
> refired to the highest
> temperature? And the consequent refirings should be
> lower?
> Heidrun in cool (260C) Bangkok
>
> Jeanne Wood wrote:
>
> > ----------------------------Original
> message----------------------------
> > Hi Veena,
> > I don't presume to call myself an expert, but I
> can
> > tell you how I have refired plates and platters
> > successfully. I refire quite a few pots, if I'm
> not
> > totally satisfied with their looks we try it
> again,
> > and again.
> > Are you firing electric? I have more success
> re-firing
> > with electric.
> > Make sure not to load the re-fired pots on the
> bottom
> > of the kiln.
> > I take the kiln up to temperature verrrry slowly,
> > especially the first few hours. I've found this to
> be
> > the most important part of refiring.
> > Good luck with it!
> > -Jeanne
> >
> > --- Veena Raghavan <75124.2520@compuserve.com>
> wrote:
> > > ----------------------------Original
> > > message----------------------------
> > > Another question for the experts. I would really
> > > appreciate some advice or
> > > explanation on what is going on here.
> > >
> > > The studio where I do my work is having a
> problem
> > > with refiring platters at
> > > Cone 6. They do not seem to be able to survive
> the
> > > refiring, they crack. In
> > > one case, the piece actually cracked in half.
> > >
> > > Is this a normal situation? Can any precautions
> be
> > > taken to prevent this
> > > from happening?
> > >
> > > Thank you in advance for your help.
> > >
> > > All the best.
> > >
> > > Veena
> > >
> > > Veena Raghavan
> > > 75124.2520@compuserve.com
> > >
> >
> >
> _________________________=5F________________________
> > Do You Yahoo!?
> > Thousands of Stores. Millions of Products. All
> in one place.
> > Yahoo! Shopping: http://shopping.yahoo.com
>
__________________________________________________
Do You Yahoo!?
Thousands of Stores. Millions of Products. All in one place.
Yahoo! Shopping: http://shopping.yahoo.com
Heidrun Schmid on sun 19 dec 99
Jeanne and Dave, thanks for the advice about refiring the glazed pots. I
will keep
it and try on my next unsuccesful pieces which will be sometime in
January,
I guess. Our sons are coming for Xmas and I will be "lazy", not working
in
my small studio. Merry Xmas and a happy New year to all on the list. I
have
learned so much from all your discussions, it is unbelievable, and there
is
always something new to learn. The "Neverending Story" in clay!
Heidrun in Bangkok
Jeanne Wood wrote:
> ----------------------------Original message----------------------------
> Greetings Heidrun,
> I usually refire beginning with the lid cracked open &
> on low for 2-4 hours, making sure not to close the lid
> until no moisture escapes. Then on low with the lid
> closed for another 2-4 hours. Then turn it up slowly
> after that(I fire pots slowly & gradually anyway).
> Can't tell you how many degrees/hr. as my pyrometer
> isn't working and I haven't replaced it.
> It is not unusual for me to refire up to 4 times,
> unless I see I am really wasteing my time & energy on
> a lost pot. I refire at similar temperatures they were
> first fired, unless I want to apply a glaze which
> requires a different temp.
> I'm not sure about the bubbles (bloating?) which
> sounds a little overfired temperature-wise.
> I haven't found they break after several firings,
> however they seem to get heavier (I haven't actually
> weighed the difference, so I'm not sure). A little
> strange as extra layers of glaze don't seem like they
> would add extra weight.(?)
> Good luck with it.
> -Jeanne
>
> --- Heidrun Schmid wrote:
> > ----------------------------Original
> > message----------------------------
> > ------------------
> > Hi Jeanne,
> > when you say you refire "very slowly" in the first
> > few hours, how slow is
> > that? 500C/h per hour, 1000C/h? And for how many
> > hours do you go slow like
> > that? 3 hours, 4 hours? I have an electronic control
> > on my kiln, so could
> > program my refirings easily. Most of the time the
> > first refiring works,
> > second time many pieces crack, but I have had also
> > clay starting to form
> > bubbles, shortly before "exploding" which made fore
> > some mnteresting small
> > bowls...
> > Maybe there is a limit to how many times clay can be
> > refired to the highest
> > temperature? And the consequent refirings should be
> > lower?
> > Heidrun in cool (260C) Bangkok
> >
> > Jeanne Wood wrote:
> >
> > > ----------------------------Original
> > message----------------------------
> > > Hi Veena,
> > > I don't presume to call myself an expert, but I
> > can
> > > tell you how I have refired plates and platters
> > > successfully. I refire quite a few pots, if I'm
> > not
> > > totally satisfied with their looks we try it
> > again,
> > > and again.
> > > Are you firing electric? I have more success
> > re-firing
> > > with electric.
> > > Make sure not to load the re-fired pots on the
> > bottom
> > > of the kiln.
> > > I take the kiln up to temperature verrrry slowly,
> > > especially the first few hours. I've found this to
> > be
> > > the most important part of refiring.
> > > Good luck with it!
> > > -Jeanne
> > >
> > > --- Veena Raghavan <75124.2520@compuserve.com>
> > wrote:
> > > > ----------------------------Original
> > > > message----------------------------
> > > > Another question for the experts. I would really
> > > > appreciate some advice or
> > > > explanation on what is going on here.
> > > >
> > > > The studio where I do my work is having a
> > problem
> > > > with refiring platters at
> > > > Cone 6. They do not seem to be able to survive
> > the
> > > > refiring, they crack. In
> > > > one case, the piece actually cracked in half.
> > > >
> > > > Is this a normal situation? Can any precautions
> > be
> > > > taken to prevent this
> > > > from happening?
> > > >
> > > > Thank you in advance for your help.
> > > >
> > > > All the best.
> > > >
> > > > Veena
> > > >
> > > > Veena Raghavan
> > > > 75124.2520@compuserve.com
> > > >
> > >
> > >
> > _________________________=5F________________________
> > > Do You Yahoo!?
> > > Thousands of Stores. Millions of Products. All
> > in one place.
> > > Yahoo! Shopping: http://shopping.yahoo.com
> >
>
> __________________________________________________
> Do You Yahoo!?
> Thousands of Stores. Millions of Products. All in one place.
> Yahoo! Shopping: http://shopping.yahoo.com
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