Eva Gallagher on mon 30 jan 12
Hi,
Has anyone else had any problems with Bmix cone 10 recently? In our Guild a
member's flat plates cracked after they were out of the kiln and a few week=
s
ago a tall vase of mine cracked. The plates were not glazed on the bottom s=
o
glaze fit/thickness could have been the problem, but my piece was glazed
with Wirtz shino inside and M Davis outside. The MD is well crazed and I
assume the Wirtz as well though hard to tell. About an hour after being
taken out I heard a loud crack an several smaller ones and two long cracks
appeared on either side of my handbuilt slab vase. Never had that happen
before with either handbuilt or thrown Bmix - it's always been a very
trouble free clay for us.
Eva Gallagher
Deep River Ontario
http://newfoundoutpotter.blogspot.com/
ronroy@CA.INTER.NET on tue 31 jan 12
HI Eve,
Could be cristobalite - if you can send me a couple of fired clay bars
we will know.
If you want to do that I'll send instructions on how to make the bars
- it's easy.
RR
Quoting Eva Gallagher :
> Hi,
> Has anyone else had any problems with Bmix cone 10 recently? In our Guild=
a
> member's flat plates cracked after they were out of the kiln and a few we=
eks
> ago a tall vase of mine cracked. The plates were not glazed on the bottom=
so
> glaze fit/thickness could have been the problem, but my piece was glazed
> with Wirtz shino inside and M Davis outside. The MD is well crazed and I
> assume the Wirtz as well though hard to tell. About an hour after being
> taken out I heard a loud crack an several smaller ones and two long crack=
s
> appeared on either side of my handbuilt slab vase. Never had that happen
> before with either handbuilt or thrown Bmix - it's always been a very
> trouble free clay for us.
> Eva Gallagher
> Deep River Ontario
> http://newfoundoutpotter.blogspot.com/
>
Vince Pitelka on tue 31 jan 12
Eva Gallagher wrote:
Has anyone else had any problems with Bmix cone 10 recently? In our Guild a
member's flat plates cracked after they were out of the kiln and a few week=
s
ago a tall vase of mine cracked. The plates were not glazed on the bottom s=
o
glaze fit/thickness could have been the problem, but my piece was glazed
with Wirtz shino inside and M Davis outside. The MD is well crazed and I
assume the Wirtz as well though hard to tell. About an hour after being
taken out I heard a loud crack an several smaller ones and two long cracks
appeared on either side of my handbuilt slab vase. Never had that happen
before with either handbuilt or thrown Bmix - it's always been a very
trouble free clay for us.
Hi Eva -
This is just a theory based on some experiences we had several years ago.
Is there any chance that the shino glaze was force-dried on the problem
pieces, or were they allowed to dry slowly in order to let the soda ash
migrate to the high spots? In my experience, when shinos are force-dried,
some of the soda ash stays inside the claybody. I think that B-Mix is
pushed pretty close to its limit when fired to maturity, so a little extra
flux could cause it to overvitrify and dunt. Dunt cracks are so
unpredictable. I have seen dunted pots crack along the throwing spiral all
the way down the pot. With slab work, the dunt usually occurs right across
the slab rather than along the joints, because of course flat surfaces are
the least able to absorb expansion and contraction and stresses build up.
Sometimes dunting occurs in the latter stages of cooling during the
annealing stage, especially at cristobalite inversion, sometimes it happens
right after the wares are removed from the kiln, and worst of all, sometime=
s
days or weeks later. You're a wood-firer, so I expect you have had plenty
of experiences with dunting.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net
http://iweb.tntech.edu/wpitelka/
Eva Gallagher on tue 31 jan 12
That's an interesting theory - I never thought that fast drying might leave
some soda ash behind.. However in this case I sprayed the piece after
pouring the inside. It sat around for about two months - became all nicely
covered with white bloom. I then decided before firing it, that the glaze
was probably too thin so I sprayed on more MD shino glaze and then it sat
around a few more days, but I may have put it on the radiator to dry -
though not sure. The glaze after all that still ended up looking rather
thin.
I did have problems with the MD shino spalling (if that is the correct
term - glaze pulling off with a thick layer of clay with it) which occured
when I resprayed some of my underfired MD shino pots from the wood kiln in
the gas. I heated up the under fired wood fired pots in the electric kiln
and then sprayed them with more MD shino and then they went to cone 10 in
the gas. There I figured that the soda ash migrated through the crazing
lines into the underfired clay and overfired and fluxed the top layer of
clay and so separated from the rest of the clay wall. The glaze was sort of
puffed up and crackled when you pressed down on it - I guess it also had
shivered?, But the layer was not just glaze but had lots of clay with it.
Anyway lotsof interesting things to keep a potter busy trying to figure it
all out.
Eva Gallagher
Deep River, Ontario
http://newfoundoutpotter.blogspot.com/
----- Original Message -----
From: "Vince Pitelka"
To:
Sent: Tuesday, January 31, 2012 7:22 PM
Subject: Re: problems with cone 10 Bmix dunting?/cracking
> Eva Gallagher wrote:
> Has anyone else had any problems with Bmix cone 10 recently? In our Guild
> a
> member's flat plates cracked after they were out of the kiln and a few
> weeks
> ago a tall vase of mine cracked. The plates were not glazed on the bottom
> so
> glaze fit/thickness could have been the problem, but my piece was glazed
> with Wirtz shino inside and M Davis outside. The MD is well crazed and I
> assume the Wirtz as well though hard to tell. About an hour after being
> taken out I heard a loud crack an several smaller ones and two long crack=
s
> appeared on either side of my handbuilt slab vase. Never had that happen
> before with either handbuilt or thrown Bmix - it's always been a very
> trouble free clay for us.
>
> Hi Eva -
> This is just a theory based on some experiences we had several years ago.
> Is there any chance that the shino glaze was force-dried on the problem
> pieces, or were they allowed to dry slowly in order to let the soda ash
> migrate to the high spots? In my experience, when shinos are force-dried=
,
> some of the soda ash stays inside the claybody. I think that B-Mix is
> pushed pretty close to its limit when fired to maturity, so a little extr=
a
> flux could cause it to overvitrify and dunt. Dunt cracks are so
> unpredictable. I have seen dunted pots crack along the throwing spiral
> all
> the way down the pot. With slab work, the dunt usually occurs right
> across
> the slab rather than along the joints, because of course flat surfaces ar=
e
> the least able to absorb expansion and contraction and stresses build up.
> Sometimes dunting occurs in the latter stages of cooling during the
> annealing stage, especially at cristobalite inversion, sometimes it
> happens
> right after the wares are removed from the kiln, and worst of all,
> sometimes
> days or weeks later. You're a wood-firer, so I expect you have had plent=
y
> of experiences with dunting.
> - Vince
>
> Vince Pitelka
> Appalachian Center for Craft
> Tennessee Tech University
> vpitelka@dtccom.net
> http://iweb.tntech.edu/wpitelka/
>
>
John Britt on wed 1 feb 12
Eva,
I have seen this with very heavy reduction.=3D20
Also, if the pieces are very thin with heavy reduction. So there may be n=
=3D
o=3D20
problems with thicker pieces but the thin ones crack (not enough structur=
=3D
e to=3D20
support the glaze tension).
Multi-firing fast can cause dunting (1063 F) and spalling.
John Britt Pottery
Eva Gallagher on wed 1 feb 12
Thanks for the suggestions. Just wondering why heavy reduction would result
in dunting?
Well we did try for heavy carbon trapping in the early stages - 012 and the=
n
regular redux until cone 8 and then oxidation for the last two hours unitl
cone 10.
Eva Gallagher
Deep River, Ontario
http://newfoundoutpotter.blogspot.com/
----- Original Message -----
From: "John Britt"
To:
Sent: Wednesday, February 01, 2012 8:15 AM
Subject: Re: problems with cone 10 Bmix dunting?/cracking
Eva,
I have seen this with very heavy reduction.
Also, if the pieces are very thin with heavy reduction. So there may be no
problems with thicker pieces but the thin ones crack (not enough structure
to
support the glaze tension).
Multi-firing fast can cause dunting (1063 F) and spalling.
John Britt Pottery
ronroy@CA.INTER.NET on wed 1 feb 12
Hi Eve,
Any lithium carb in those glazes?
RR
Quoting Eva Gallagher :
> That's an interesting theory - I never thought that fast drying might lea=
ve
> some soda ash behind.. However in this case I sprayed the piece after
> pouring the inside. It sat around for about two months - became all nicel=
y
> covered with white bloom. I then decided before firing it, that the glaze
> was probably too thin so I sprayed on more MD shino glaze and then it sat
> around a few more days, but I may have put it on the radiator to dry -
> though not sure. The glaze after all that still ended up looking rather
> thin.
> I did have problems with the MD shino spalling (if that is the correct
> term - glaze pulling off with a thick layer of clay with it) which occure=
d
> when I resprayed some of my underfired MD shino pots from the wood kiln i=
n
> the gas. I heated up the under fired wood fired pots in the electric kiln
> and then sprayed them with more MD shino and then they went to cone 10 in
> the gas. There I figured that the soda ash migrated through the crazing
> lines into the underfired clay and overfired and fluxed the top layer of
> clay and so separated from the rest of the clay wall. The glaze was sort =
of
> puffed up and crackled when you pressed down on it - I guess it also had
> shivered?, But the layer was not just glaze but had lots of clay with it.
> Anyway lotsof interesting things to keep a potter busy trying to figure i=
t
> all out.
> Eva Gallagher
> Deep River, Ontario
> http://newfoundoutpotter.blogspot.com/
>
>
> ----- Original Message -----
> From: "Vince Pitelka"
> To:
> Sent: Tuesday, January 31, 2012 7:22 PM
> Subject: Re: problems with cone 10 Bmix dunting?/cracking
>
>
>> Eva Gallagher wrote:
>> Has anyone else had any problems with Bmix cone 10 recently? In our Guil=
d
>> a
>> member's flat plates cracked after they were out of the kiln and a few
>> weeks
>> ago a tall vase of mine cracked. The plates were not glazed on the botto=
m
>> so
>> glaze fit/thickness could have been the problem, but my piece was glazed
>> with Wirtz shino inside and M Davis outside. The MD is well crazed and I
>> assume the Wirtz as well though hard to tell. About an hour after being
>> taken out I heard a loud crack an several smaller ones and two long crac=
ks
>> appeared on either side of my handbuilt slab vase. Never had that happen
>> before with either handbuilt or thrown Bmix - it's always been a very
>> trouble free clay for us.
>>
>> Hi Eva -
>> This is just a theory based on some experiences we had several years ago=
.
>> Is there any chance that the shino glaze was force-dried on the problem
>> pieces, or were they allowed to dry slowly in order to let the soda ash
>> migrate to the high spots? In my experience, when shinos are force-drie=
d,
>> some of the soda ash stays inside the claybody. I think that B-Mix is
>> pushed pretty close to its limit when fired to maturity, so a little ext=
ra
>> flux could cause it to overvitrify and dunt. Dunt cracks are so
>> unpredictable. I have seen dunted pots crack along the throwing spiral
>> all
>> the way down the pot. With slab work, the dunt usually occurs right
>> across
>> the slab rather than along the joints, because of course flat surfaces a=
re
>> the least able to absorb expansion and contraction and stresses build up=
.
>> Sometimes dunting occurs in the latter stages of cooling during the
>> annealing stage, especially at cristobalite inversion, sometimes it
>> happens
>> right after the wares are removed from the kiln, and worst of all,
>> sometimes
>> days or weeks later. You're a wood-firer, so I expect you have had plen=
ty
>> of experiences with dunting.
>> - Vince
>>
>> Vince Pitelka
>> Appalachian Center for Craft
>> Tennessee Tech University
>> vpitelka@dtccom.net
>> http://iweb.tntech.edu/wpitelka/
>>
>>
>
Dale Neese on wed 1 feb 12
As best as I can remember Malcolm Davis carbon trapping Shino glaze has a
huge percentage of soda ash. More than some other Shino recipes that I've
seen. Only time I remember there were instances when this high percentage o=
f
soda ash Shino was applied to raw clay and fired the pots cracked and some
were in crumbles. The high soda ash concentration was the only thing we
could contribute these flaws to. Bisque ware was not affected.
Dale Tex
"across the Alley from the Alamo"
Helotes, Texas USA
www.daleneese.com
Sharon Wetherby on thu 2 feb 12
Dale,
I attended a Malcolm Davis workshop in Houston a few years back. I asked
him about applying his carbon trap shino to pieces for single fire. He
didn't give a reason but told us not to do this. He did tell us that he
bisque fires his pieces for carbon trap shino to cone 010.
Sharon Wetherby
Fort Worth, TX
=3D=3D=3D=3D=3D=3D
Date: Wed, 1 Feb 2012 17:03:53 -0600
From: Dale Neese dneese@SATX.RR.COM
Subject: Re: problems with cone 10 Bmix dunting?/cracking
As best as I can remember Malcolm Davis carbon trapping Shino glaze has a
huge percentage of soda ash. More than some other Shino recipes that I've
seen. Only time I remember there were instances when this high percentage o=
f
soda ash Shino was applied to raw clay and fired the pots cracked and some
were in crumbles. The high soda ash concentration was the only thing we
could contribute these flaws to. Bisque ware was not affected.
Dale Tex
"across the Alley from the Alamo"
Helotes, Texas USA
www.daleneese.com
wileyhill@COMCAST.NET on fri 3 feb 12
Carbon trap shinos and unwashed ash glazes will cause issues in single-fire=
. In testing, I had many pots "spontaneously disassemble" in front of me sh=
ortly after glazing. While wetting with any glaze may cause cracks etc, the=
se caustics really do the trick- whether it's deflocculation, improved wett=
ing/penetration, etc.
Paul Haigh
Wiley Hill Mudworks
Web: http://wileyhill.com
Facebook: http://www.facebook.com/pages/Wiley-Hill-Mudworks/108145139230652
etsy: http://www.etsy.com/shop/WileyHillMudworks
Snail Scott on fri 3 feb 12
On Feb 2, 2012, at 9:58 AM, Sharon Wetherby wrote:
> ...I asked
> him about applying his carbon trap shino to pieces for single fire. He
> didn't give a reason but told us not to do this...
Hard to get a good clean burnout on the clay
if you have to start reduction for carbon trapping
at low temps. Carbon trapped in glaze is a matter
of taste; carbon trapped in clay leads to Bad Things.
-Snail
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