joe shaw on sat 11 sep 04
>The first thing I would ask is what is the clay body you are using. (What
>is its range?)I have used an earthenred clay body that was a the peak of
>its range at cone 5 and if you exceeded that it would start to bloat.
>
>John Britt
>www.johnbrittpottery.com
We are still using the ^5 B mix from Laguna, same as always. Many
people fire it to ^6 as we do. Regardless of what glaze temp you fire
it to, I understand the bloat actually happens in the bisque. We
strongly suspect that is why we got so much bloating when I fired it
to the lower bisque.
Our puzzlement is why it caused so much more damage to the high-gloss glazes?
Thanks,
Joe & Rita
--
shaw pottery
1513 South Thornburg Street
Santa Maria, CA 93458
805-922-5507
805-720-2146 (cell)
(mailto:shawpottery@earthlink.net)
http://home.earthlink.net/~shawpottery/shaw_pottery.html
joe shaw on sat 11 sep 04
Hi Folks,
We need your help in solving a mystery. For the last four years I
have been firing with an older Cress 10 cu. ft. electric kiln using
^5 B mix from Laguna, but firing to ^6, with bisque to ^04. Over that
time we have frequently experienced bloats.
We recently purchased a Bailey 16 cu. ft. electric kiln. One of the
first bisque firings was to ^08, and the following to ^06. Both loads
were glazed fired together to ^6, using a variety of Laguna glazes
and some I have made myself with a range of colors, both high gloss
and matt. (Sapphire, Forest Green, Italian Straw, Peacock, Fake
Shino, Weathered Bronze, Ocean's Edge, and Beach Bottle) Some of the
larger were sprayed and the rest dipped.
It was a large firing, bottom shelf a mix of med. and small bowls,
small jars, and small mugs, second shelf a 19" shallow bowl, lids to
all the jars, and three very large jars. Two half shelves followed
with 18" and 17" shallow bowls over the 19".
Almost every single piece that was glazed in a high gloss has severe
bloating. The bloating ranged from raised bubbles about 1/2" to
hundreds of small bloats covering the bowls. The only glazed pieces
that were not bloated were the lids to the jars which were on the
higher shelves. The lids ranged from 3 to 5" in diameter.
Some of the matt glazed pieces had only a few larger bloats, some
escaped entirely.
The cone pacs were evenly melted, the firing took 10 hours and 48
minutes with a 30 minute soak, using the Bailey program for slow
glaze.
While we are sure the lower bisque firings exacerbated the bloating,
we are puzzled why the high gloss was affected to a higher degree,
far more than the matt.
This was our very first totally full firing in the new kiln.
Unfortunately, we now have very few pieces to take to the Santa
Barbara Art Show tomorrow... but we shall persevere and live to fire
again.
Thank you for your kind assistance,
Joe and Rita Shaw
(photos of the bloated pots available upon request)
--
shaw pottery
1513 South Thornburg Street
Santa Maria, CA 93458
805-922-5507
805-720-2146 (cell)
(mailto:shawpottery@earthlink.net)
http://home.earthlink.net/~shawpottery/shaw_pottery.html
John Britt on sat 11 sep 04
The first thing I would ask is what is the clay body you are using. (What
is its range?)I have used an earthenred clay body that was a the peak of
its range at cone 5 and if you exceeded that it would start to bloat.
John Britt
www.johnbrittpottery.com
Snail Scott on sun 12 sep 04
At 04:25 PM 9/11/2004 -0700, you wrote:
>Almost every single piece that was glazed in a high gloss has severe
>bloating...
>Some of the matt glazed pieces had only a few larger bloats, some
>escaped entirely.
Sounds to me like the initial bisque was too
fast or too cool, or both. This seems to have been
exacerbated by the glaze. Apparently the glossy
glazes melt and skin over at a lower temperature
than the matte glazes, trapping the escaping
gasses that remained and preventing the final
re-firing from acting as a 'second bisque' during
the lower temperature phases as it might otherwise
do. Does the new kiln fire quicker than the old one?
-Snail Scott
Earl Krueger on sun 12 sep 04
On Sep 12, 2004, at 6:07 AM, MarjB wrote:
> Some air movement/ top peep open would help.
I have found that just pulling the top peep hole plug
doesn't result in much air flow through my kiln.
However, if I pull both the top and bottom I can feel
the air coming out the top hole when the kiln gets warm.
I don't have a kiln vent so I just leave both plugs out
until the kiln gets close to my bisque temperature,
then I put them both in to help even out the temp.
YMMV but that's how I do it and I haven't had any
bloating or pinholing problems.
Earl K...
Bothell, WA, USA
MarjB on sun 12 sep 04
I can imagine the horrible feelings you experienced when unloading the kiln.
I suggest that you review your bisque firing. Are you going slow/holding
during carbon burn out stage ? The B-Mix bodies have alot of ball clay/s in
them therefore more organic material to burn out. Slow down/hold the bisque
between 700-900 C. Do you have any venting ? Some air movement/ top peep
open would help. You probably should do a absorption test on your clay
batch to be sure that there has not been changes. MarjB
----- Original Message -----
From: "joe shaw"
To:
Sent: Saturday, September 11, 2004 7:25 PM
Subject: Mystery firing
>
> --
> shaw pottery
> 1513 South Thornburg Street
> Santa Maria, CA 93458
> 805-922-5507
> 805-720-2146 (cell)
> (mailto:shawpottery@earthlink.net)
> http://home.earthlink.net/~shawpottery/shaw_pottery.html
>
>
____________________________________________________________________________
__
> 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 Britt on sun 12 sep 04
Joe and Rita,
I don't know much about this clay body, although I am sure someone will
step forward who does.
I would guess that the high gloss glazes you are using contain frits
and/or gerstley borate (or substitutes). These materials melt very early
(1500 F - 1700 F) and probably sealed over the surface of the pot inpeding
the gases from escaping resulting in more bloating.
Hope that helps,
John Britt
www.johnbrittpottery.com
Jon Pacini on mon 13 sep 04
HI Joe and Rita, Greetings All,
This type of bloat is pretty common with dense/white firing bodies. It is
usually associated with organics that don't get burned out of the body
during bisque.
What happens is carbon is trapped in the body after the glaze has started to
fuse, then when the body becomes soft during vitrification, the gas expands
and creates the bloats. More than likely the high gloss glazes are fusing at
a lower temperature than the non-bloating glazes. This is trapping the
carbon under the clay/glaze interface. The non-bloating glazes are still
porous to gasses and allowing them to escape.
Something to keep in mind when firing with an electric kiln, particularly a
large diameter kiln, is that the heat given off by the elements is radiant.
That is, the heat starts from the outside edge and works it way in. What can
occur many times is the outer pots or even the sides of individual pots
facing toward the elements will bisque hotter and as you move to the center,
the pots are cooler. It's an occurrence called shadowing. Try putting cones
on the outside edge of the shelves nearest the elements and bury another set
in the center behind all the pots. Do this on a number of shelves. See if
you're getting a variation in temperature during the bisque
If you are there are a couple of ways to resolve the situation. First is to
slow the firing down, this can best be done by increasing the soak period at
the end of the bisque firing. I like to sneak up on the end of bisque firing
so that the kiln has a chance to equalize in heat. You may want to reprogram
the kiln to fire the last 25* of the bisque in 60 minutes. Then soak for 20
minutes. This will reduce the shadowing problem. The other is to bisque to a
hotter temperature. Normally I recommend a ^04 firing when this type of
problem persists.
Hope that will help out with future firings.
Best regards
Jon Pacini
Clay Manager
Laguna Clay Co.
Headed for the heartland of America and the Woodfire Conference
-----Original Message-----
From: joe shaw [mailto:shawpottery@earthlink.net]
Sent: Saturday, September 11, 2004 4:25 PM
To: ppinnelle1@unl.edu; ronroy@pop.ca.inter.net; John Hesselberth; Jon
Pacini
Subject: Mystery firing
Hi Gentlemen,
We need your help in solving a mystery. For the last four years I
have been firing with an older Cress 10 cu. ft. electric kiln using
^5 B mix from Laguna, but firing to ^6, with bisque to ^04. Over that
time we have frequently experienced bloats.
We recently purchased a Bailey 16 cu. ft. electric kiln. One of the
first bisque firings was to ^08, and the following to ^06. Both loads
were glazed fired together to ^6, using a variety of Laguna glazes
and some I have made myself with a range of colors, both high gloss
and matt. (Sapphire, Forest Green, Italian Straw, Peacock, Fake
Shino, Weathered Bronze, Ocean's Edge, and Beach Bottle) Some of the
larger were sprayed and the rest dipped.
It was a large firing, bottom shelf a mix of med. and small bowls,
small jars, and small mugs, second shelf a 19" shallow bowl, lids to
all the jars, and three very large jars. Two half shelves followed
with 18" and 17" shallow bowls over the 19".
Almost every single piece that was glazed in a high gloss has severe
bloating. The bloating ranged from raised bubbles about 1/2" to
hundreds of small bloats covering the bowls. The only glazed pieces
that were not bloated were the lids to the jars which were on the
higher shelves. The lids ranged from 3 to 5" in diameter.
Some of the matt glazed pieces had only a few larger bloats, some
escaped entirely.
The cone pacs were evenly melted, the firing took 10 hours and 48
minutes with a 30 minute soak, using the Bailey program for slow
glaze.
While we are sure the lower bisque firings exacerbated the bloating,
we are puzzled why the high gloss was affected to a higher degree,
far more than the matt.
As you can see by the final picture, the Weathered Bronze came
through like a trouper, no problems at all!
This was our very first totally full firing in the new kiln.
Unfortunately, we now have very few pieces to take to the Santa
Barbara Art Show tomorrow... but we shall persevere and live to fire
again.
Thank you for your kind assistance,
Joe and Rita Shaw
PS We are also posing this question to ClayArt in hopes of further
assistance.
--
shaw pottery
1513 South Thornburg Street
Santa Maria, CA 93458
805-922-5507
805-720-2146 (cell)
(mailto:shawpottery@earthlink.net)
http://home.earthlink.net/~shawpottery/shaw_pottery.html
joe shaw on mon 13 sep 04
>Sounds to me like the initial bisque was too fast or too cool, or
>both. This seems to have been exacerbated by the glaze. Apparently
>the glossy glazes melt and skin over at a lower temperature than the
>matte glazes, trapping the escaping gasses that remained and
>preventing the final re-firing from acting as a 'second bisque'
>during the lower temperature phases as it might otherwise do. Does
>the new kiln fire quicker than the old one?
>
>-Snail Scott
Consensus: The mystery problem of "high-gloss severe bloating vs matt
glaze minor bloating" seems to be the high gloss glazes are fusing at
a lower temperature than the non-bloating (matte) glazes, which many
have suggested.
The bisque fire takes only about 12 to 13 hours, using the Bailey
slow bisque program. In our old kiln, we fired about 18 to 20 hours
on bisque, sometimes longer. No vent back then. We just left the peep
holes open, an unfortunately expensive solution.
However the glaze firing is much quicker, it only took 10 hours and
49 minutes, using the Bailey slow glaze program. The witness cones
went down properly.
And, of course, I forgot to mention that we do now have a Bailey kiln
vent, and leave the top vent open. While it has not eliminated the
bloats entirely, they have decreased somewhat.
John Hesselberth had a very good point, in that this was a very
solidly packed kiln. He suggested we use a longer soak, about 15 to
20 minutes. While the program calls for (I believe) about 10 minutes,
at this point I am considering about 2 to 3 days (wry grin here).
*******
Onward discussion on an old topic:
While I know the discussion of bloating with Laguna ^5 B mix has
been ongoing, has anyone found a way to truly cure it once and for
all? Again, we fire bisque fire (normally) at ^04 with a soak at the
end, and glaze ^6 with another soak.
But has anyone ever eliminated the bloats entirely on a long term
basis? If so, pray tell how?
While I enjoy the throwing quality of ^5 B mix, the frequency of
bloats (of course always in a nicer piece) is enough to cause me to
start trying out other clays. If I find one I truly trust and enjoy,
that works with my standard glazes, you will be the first to know.
Our sincere thanks to each and all who have responded to our queries.
Joe & Rita
--
shaw pottery
1513 South Thornburg Street
Santa Maria, CA 93458
805-922-5507
805-720-2146 (cell)
(mailto:shawpottery@earthlink.net)
http://home.earthlink.net/~shawpottery/shaw_pottery.html
Ron Roy on tue 14 sep 04
Hi Joe,
It would help if we knew - was the clay the same batch as before - or a new
batch?
It does look like the clay is being overfired - this can be due to the
variability of materials used to mix the body, a mistake in weighing the
materials out and also improper bisque firing - or a combination of
factors.
We are assuming you are using cones by the way - every pyrometer is
different and they all have to be monitored with cones - there is no way
around that.
New kilns fire faster than old kilns - how fast was the bisque firing in
the new kiln compared to the old kiln?
New kilns are tighter than old kilns - read - old kilns allow more air
inside during firing. If you are stacking pots together during the bisque -
you may be preventing proper oxidation (burn out of carbon) during the
critical period - 700C to 900C - make sure there is plenty of make up
oxygen and don't fire too fast - it's hard to say how fast - so much
depends on how much organic material is in the clay - and there can be lots
- and it can vary from batch to batch - a lot -depending on the clays used.
I have always fired bisque to cone 04 - I am convinced it helps with the
carbon burn out and also helps avoid quartz induced dunting during bisque
cool down.
If the problem is unburned carbon - you may be able to see it - break some
ware and compare the sections with no glaze and glazed - is there a colour
difference.
I am guessing now but - if Laguna recommends cone 5 - it may mean you are
pushing that body to start with - it would not take much to over fire it -
a lot can be learned by testing each new shipment of clay - if the supplier
is not testing - it will save you at lot of pain in the long run.
If you need to know how to test clay let me know.
RR
>We need your help in solving a mystery. For the last four years I
>have been firing with an older Cress 10 cu. ft. electric kiln using
>^5 B mix from Laguna, but firing to ^6, with bisque to ^04. Over that
>time we have frequently experienced bloats.
>
>We recently purchased a Bailey 16 cu. ft. electric kiln. One of the
>first bisque firings was to ^08, and the following to ^06. Both loads
>were glazed fired together to ^6, using a variety of Laguna glazes
>and some I have made myself with a range of colors, both high gloss
>and matt. (Sapphire, Forest Green, Italian Straw, Peacock, Fake
>Shino, Weathered Bronze, Ocean's Edge, and Beach Bottle) Some of the
>larger were sprayed and the rest dipped.
>
>It was a large firing, bottom shelf a mix of med. and small bowls,
>small jars, and small mugs, second shelf a 19" shallow bowl, lids to
>all the jars, and three very large jars. Two half shelves followed
>with 18" and 17" shallow bowls over the 19".
>
>Almost every single piece that was glazed in a high gloss has severe
>bloating. The bloating ranged from raised bubbles about 1/2" to
>hundreds of small bloats covering the bowls. The only glazed pieces
>that were not bloated were the lids to the jars which were on the
>higher shelves. The lids ranged from 3 to 5" in diameter.
>
>Some of the matt glazed pieces had only a few larger bloats, some
>escaped entirely.
>
>The cone pacs were evenly melted, the firing took 10 hours and 48
>minutes with a 30 minute soak, using the Bailey program for slow
>glaze.
>
>While we are sure the lower bisque firings exacerbated the bloating,
>we are puzzled why the high gloss was affected to a higher degree,
>far more than the matt.
>
>This was our very first totally full firing in the new kiln.
>Unfortunately, we now have very few pieces to take to the Santa
>Barbara Art Show tomorrow... but we shall persevere and live to fire
>again.
>
>Thank you for your kind assistance,
>Joe and Rita Shaw
>(photos of the bloated pots available upon request)
>
>--
>shaw pottery
>1513 South Thornburg Street
>Santa Maria, CA 93458
>805-922-5507
>805-720-2146 (cell)
>(mailto:shawpottery@earthlink.net)
>http://home.earthlink.net/~shawpottery/shaw_pottery.html
>
>______________________________________________________________________________
>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
Ron Roy on fri 17 sep 04
Hi Joe,
If the problem with cone 5 B mix is because it is just not refractory
enough - and you have a way to mix plastic clay - you will find that mixing
in a certain proportion of a more refractory clay will help the situation.
When someone finds a solution using that technique - using Tuckers or
Axners clays - I reformulate the new body and it can be ordered as a
special mix.
Calculation software is - again - a great tool for doing that sort of
thing. I have done it many times and have always been able to get the right
results the first time.
RR
>Onward discussion on an old topic:
>
>While I know the discussion of bloating with Laguna ^5 B mix has
>been ongoing, has anyone found a way to truly cure it once and for
>all? Again, we fire bisque fire (normally) at ^04 with a soak at the
>end, and glaze ^6 with another soak.
>
>But has anyone ever eliminated the bloats entirely on a long term
>basis? If so, pray tell how?
>
>While I enjoy the throwing quality of ^5 B mix, the frequency of
>bloats (of course always in a nicer piece) is enough to cause me to
>start trying out other clays. If I find one I truly trust and enjoy,
>that works with my standard glazes, you will be the first to know.
>
>Our sincere thanks to each and all who have responded to our queries.
>Joe & Rita
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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