Bob Johnson on thu 18 feb 10
Paul--
Here's the recipe for the iron red glaze that sometimes has a cracking and
crawling problem. But it is gorgeous--when it works. It's one of John Post'=
s
iron reds, which he calls Pete's Tomato Red #13. (See his web site at
http://www.johnpost.us/.) He lists it as a ^6 oxidation glaze, but I fire i=
t
to ^7 in reduction and then reoxidize until ^8 falls.
Frit 3134 10.8
Nepheline syenite 23.1
Bone ash 10.0
Magnesium carb 6.2
Whiting 7.7
EPK 21.0
Flint 21.2
Add:
Bentonite 2.0
Red iron oxide 10.0
When I spray it on, it rarely cracks--mainly when I dip or swirl it around
on the inside of a vessel--so I know that water is an issue. John Britt
noted that it is probably too flocculated, so I think I'll try the sodium
silicate trick he suggested. (Thanks, John!) I'll post how that works when =
I
do another glaze batch in a couple of weeks.
Bob
-----Original Message-----
Bob,
Could you post the recipe please? Sometimes, if there is a lot of clay
in there it will cause crawling, and you can do things about that,
like substituting calcined kaolin for raw kaolin. It would reduce the
dry shrinkage.
Best,
Paul Herman
Great Basin Pottery
Doyle, California US
www.greatbasinpottery.com/
Paul Herman on thu 18 feb 10
Bob,
Could you post the recipe please? Sometimes, if there is a lot of clay
in there it will cause crawling, and you can do things about that,
like substituting calcined kaolin for raw kaolin. It would reduce the
dry shrinkage.
Best,
Paul Herman
Great Basin Pottery
Doyle, California US
www.greatbasinpottery.com/
On Feb 18, 2010, at 3:04 PM, Bob Johnson wrote:
> What advice do you experienced potters have about an otherwise great
> glaze-Pete's Tomato Red-that sometimes cracks when drying and then
> crawls
> when fired? I know that too thick a coat will sometimes do this, but
> it
> needs some thickness to work well. I also know that, before firing,
> sometimes I can rub the cracks with my finger and get them to fill
> in, but
> this doesn't always seem to work.
>
>
>
> I've read in one source that adding a gum to the glaze will help, but
> another source says that can actually cause cracking. If it would
> solve the
> problem, I'd be happy to tweak the recipe a little the next time I
> mix it.
> The problem is, I currently have a fair-sized batch that I'd like to
> use up
> first.
>
>
>
> Any ideas?
>
>
>
> Bob Johnson
>
> Southern Oregon
Bob Johnson on thu 18 feb 10
What advice do you experienced potters have about an otherwise great
glaze-Pete's Tomato Red-that sometimes cracks when drying and then crawls
when fired? I know that too thick a coat will sometimes do this, but it
needs some thickness to work well. I also know that, before firing,
sometimes I can rub the cracks with my finger and get them to fill in, but
this doesn't always seem to work.
I've read in one source that adding a gum to the glaze will help, but
another source says that can actually cause cracking. If it would solve the
problem, I'd be happy to tweak the recipe a little the next time I mix it.
The problem is, I currently have a fair-sized batch that I'd like to use up
first.
Any ideas?
Bob Johnson
Southern Oregon
William & Susan Schran User on thu 18 feb 10
On 2/18/10 6:04 PM, "Bob Johnson" wrote:
> What advice do you experienced potters have about an otherwise great
> glaze-Pete's Tomato Red-that sometimes cracks when drying and then crawls
> when fired? I know that too thick a coat will sometimes do this, but it
> needs some thickness to work well. I also know that, before firing,
> sometimes I can rub the cracks with my finger and get them to fill in, bu=
t
> this doesn't always seem to work.
> I've read in one source that adding a gum to the glaze will help, but
> another source says that can actually cause cracking. If it would solve t=
he
> problem, I'd be happy to tweak the recipe a little the next time I mix it=
.
> The problem is, I currently have a fair-sized batch that I'd like to use =
up
> first.
Don't know what the recipe is for this glaze, but if it is a copper red it
is low on clay that would help to bind the glaze to the pot as it dries.
I face a similar thing with crystalline glazes that contain no clay at all
AND about 25% of the glaze being zinc that can cause all sorts of issues as
the glaze dries.
I mix with a CMC solution rather than water and have no problems, but
storage long term of such a glaze is not suggested as the gum is organic an=
d
will mold.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
David Woof on fri 19 feb 10
Hi all=3D2C A shot of chlorine bleach in the CmC mixing solution keeps the=
s=3D
olution and subsequent glaze mold and stench free for the life of the used =
=3D
and stored product. I know some folks use H. peroxide and other mold inhi=
=3D
biters with success as well.
=3D20
For my use=3D2C CMC soultion=3D2C in conjunction with Bentonite 1.5 to 5.0 =
% is=3D
a virtually bullet proof solution for many glaze problems. and has never c=
=3D
aused adverse effects. If there exceptions=3D3B please post=3D2C so no on=
e is=3D
misled.
=3D20
David
=3D20
1b. Re: cracking & crawling
Posted by: "William & Susan Schran User" wschran@COX.NET=3D20
Date: Thu Feb 18=3D2C 2010 4:33 pm ((PST))
=3D20
On 2/18/10 6:04 PM=3D2C "Bob Johnson" wrote:
=3D20
> What advice do you experienced potters have about an otherwise great
> glaze-Pete's Tomato Red-that sometimes cracks when drying and then crawls
> when fired? I know that too thick a coat will sometimes do this=3D2C but =
it
> needs some thickness to work well. I also know that=3D2C before firing=3D=
2C
> sometimes I can rub the cracks with my finger and get them to fill in=3D2=
C =3D
but
> this doesn't always seem to work.
> I've read in one source that adding a gum to the glaze will help=3D2C but
> another source says that can actually cause cracking. If it would solve t=
=3D
he
> problem=3D2C I'd be happy to tweak the recipe a little the next time I mi=
x =3D
it.
> The problem is=3D2C I currently have a fair-sized batch that I'd like to =
us=3D
e up
> first.
=3D20
Don't know what the recipe is for this glaze=3D2C but if it is a copper red=
i=3D
t
is low on clay that would help to bind the glaze to the pot as it dries.
=3D20
I face a similar thing with crystalline glazes that contain no clay at all
AND about 25% of the glaze being zinc that can cause all sorts of issues as
the glaze dries.
I mix with a CMC solution rather than water and have no problems=3D2C but
storage long term of such a glaze is not suggested as the gum is organic an=
=3D
d
will mold.
=3D20
Bill
--
William "Bill" Schran
wschran@cox.net
=3D20
=3D20
_________________________________________________________________
Your E-mail and More On-the-Go. Get Windows Live Hotmail Free.
http://clk.atdmt.com/GBL/go/201469229/direct/01/=3D
Ron Roy on fri 19 feb 10
Hi Bob,
Send me the recipe and I'll suggest some revisions to stop that
happening - just the kind of thing that calculation is good for.
What kind of water are you using?
RR
Quoting Bob Johnson :
> What advice do you experienced potters have about an otherwise great
> glaze-Pete's Tomato Red-that sometimes cracks when drying and then crawls
> when fired? I know that too thick a coat will sometimes do this, but it
> needs some thickness to work well. I also know that, before firing,
> sometimes I can rub the cracks with my finger and get them to fill in, bu=
t
> this doesn't always seem to work.
>
>
>
> I've read in one source that adding a gum to the glaze will help, but
> another source says that can actually cause cracking. If it would solve t=
he
> problem, I'd be happy to tweak the recipe a little the next time I mix it=
.
> The problem is, I currently have a fair-sized batch that I'd like to use =
up
> first.
>
>
>
> Any ideas?
>
>
>
> Bob Johnson
>
> Southern Oregon
>
Paul Herman on fri 19 feb 10
Hi Bob,
Two things in the recipe are possibly problematic. First there is 21%
EPK, which is a lot and might cause excessive shrinkage. Then there is
the magnesium carbonate which is a high firing-shrinkage material.
MgCO3 is used to make beaded and crawling glazes, when you WANT those
effects.
What I would do is this: Replace a third or half of the EPK with
calcined kaolin, and get your magnesia from some other source, like
dolomite or talc. I think it would be worth a try to simple replace
all the whiting and MgCO3 with dolomite.
Please let us know how any experiments work out, and if you cure the
problems.
Best,
Paul Herman
Great Basin Pottery
Doyle, California US
www.greatbasinpottery.com/
On Feb 18, 2010, at 10:18 PM, Bob Johnson wrote:
> Paul--
>
> Here's the recipe for the iron red glaze that sometimes has a
> cracking and
> crawling problem. But it is gorgeous--when it works. It's one of
> John Post's
> iron reds, which he calls Pete's Tomato Red #13. (See his web site at
> http://www.johnpost.us/.) He lists it as a ^6 oxidation glaze, but I
> fire it
> to ^7 in reduction and then reoxidize until ^8 falls.
>
> Frit 3134 10.8
> Nepheline syenite 23.1
> Bone ash 10.0
> Magnesium carb 6.2
> Whiting 7.7
> EPK 21.0
> Flint 21.2
> Add:
> Bentonite 2.0
> Red iron oxide 10.0
>
> When I spray it on, it rarely cracks--mainly when I dip or swirl it
> around
> on the inside of a vessel--so I know that water is an issue. John
> Britt
> noted that it is probably too flocculated, so I think I'll try the
> sodium
> silicate trick he suggested. (Thanks, John!) I'll post how that
> works when I
> do another glaze batch in a couple of weeks.
>
> Bob
>
>
>
> -----Original Message-----
> Bob,
>
> Could you post the recipe please? Sometimes, if there is a lot of clay
> in there it will cause crawling, and you can do things about that,
> like substituting calcined kaolin for raw kaolin. It would reduce the
> dry shrinkage.
>
> Best,
>
> Paul Herman
>
> Great Basin Pottery
> Doyle, California US
> www.greatbasinpottery.com/
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