Dennis E. Tobin on tue 16 nov 99
We are getting crystals on the surface of "Amber celadon" . Amber celadon
was used as a liner is a recent cone 9-10 salt firing. We seem to get more
on some pieces than others, location in the kiln didn't seem to matter. In
Rhodes, he says that this is devitrification, caused by to much silica or
too much metalic oxide in the glaze and the crystals grow in the cooling
of the kiln. Could the kiln be cooling too slow? Is there a significant
difference between Yellow Ochre and Yellow Iron? Could the mesh of the
silica be a factor?
Amber celadon
Custer 30
Whiting 25
Flint 25
OM4 10
Gerstley Borate 3
Yellow Ochre 7%
This only started happening recently. Any ideas?
Thanks in advance!
Dennis Tobin
Associate Professor
Art Department
Miami University
Oxford, Ohio 45056
(513) 529-1505
Craig Martell on wed 17 nov 99
Hello Dennis:
I ran this glaze through my computer and came up with the following.
It's pretty high in limestone, about .83 in a RO unity formula. I would
drop it down to about .7 or so. You could dump the gerstley borate which is
probably forming calcium borate crystals. The alumina and silica are under
the recommended limits for a cone 8-10 glaze. Lower alumina will promote
crystal growth. This glaze has about .25 alumina and it should be at least
.3 I would think. You can bring up the silica a bit too. The crystal
growth is taking place during the cool cycle. You can eliminate this or
lessen the problem with a quick cool to about 1800F, if you don't want to
alter the glaze. If this were a celedon that I wanted to use, I'd alter it
for sure. By my standards this glaze could use some work. It's not what I
would call a well balanced glaze. Those are my standards and as usual they
can be "round filed" if you and other folks have a different notion.
I'd be glad to adjust the glaze for you if Ron hasn't done it already! :>)
later, Craig Martell in Oregon
Stephen Grimmer on wed 17 nov 99
Dennis,
Your glaze is a pretty high on the calcia and a bit low on the silica
and alumina. That much alkaline earth combined with the low silica and
alumina can sometimes give a calcium matte when the cooling is slow enough.
I've adjusted the glaze, trying to keep the rest of the properties the same.
Try it and let us know.
I've seen the same thing happen with the Leach 1234 celadon in salt kilns.
It's high in calcia, too.
Steve Grimmer
Bowling Green State University
Bowling Green, OH
Two Recipes: Amber Celadon & Untitled Recipe 1
==================================================
CUSTER FELDSPAR..... 30.00 30.00% 36.65 36.65%
WHITING............. 25.00 25.00% 12.22 12.22%
FLINT............... 25.00 25.00% 27.94 27.94%
OM #4 BALL CLAY..... 10.00 10.00% 12.22 12.22%
GERSTLEY BORATE..... 3.00 3.00% 3.66 3.66%
YELLOW OCHRE........ 7.00 7.00% 7.31 7.31%
======== ========
100.00 100.00
CaO 0.81* 18.02% 0.67* 9.43%
MgO 0.00* 0.05% 0.01* 0.06%
K2O 0.10* 3.69% 0.18* 4.26%
Na2O 0.06* 1.37% 0.10* 1.58%
TiO2 0.00 0.14% 0.01 0.16%
Al2O3 0.27 10.89% 0.48 12.29%
B2O3 0.07 1.82% 0.12 2.10%
SiO2 2.61 62.06% 4.49 68.16%
Fe2O3 0.03* 1.96% 0.05* 1.96%
Cost/kg 0.51 0.56
Si:Al 9.67 9.41
SiB:Al 9.92 9.66
Expan 7.60 6.91
----------
>From: "Dennis E. Tobin"
>To: CLAYART@LSV.UKY.EDU
>Subject: glaze crystals / devitrification
>Date: Tue, Nov 16, 1999, 11:17 AM
>
>----------------------------Original message----------------------------
>We are getting crystals on the surface of "Amber celadon" . Amber celadon
>was used as a liner is a recent cone 9-10 salt firing. We seem to get more
>on some pieces than others, location in the kiln didn't seem to matter. In
>Rhodes, he says that this is devitrification, caused by to much silica or
>too much metalic oxide in the glaze and the crystals grow in the cooling
>of the kiln. Could the kiln be cooling too slow? Is there a significant
>difference between Yellow Ochre and Yellow Iron? Could the mesh of the
>silica be a factor?
>
>Amber celadon
>Custer 30
>Whiting 25
>Flint 25
>OM4 10
>Gerstley Borate 3
>Yellow Ochre 7%
>
>This only started happening recently. Any ideas?
>
>Thanks in advance!
>
>Dennis Tobin
>Associate Professor
>Art Department
>Miami University
>Oxford, Ohio 45056
>(513) 529-1505
Louis H.. Katz on wed 17 nov 99
Hi Dennis,
More questions than answers. I will leave the calc stuff up to others.
Yes Yellow Ochre is less pure than Yellow iron. I believe it has more silica
in it. It might be enough to change a glaze but I'll leave that determination
to someone else who has an analysis.
Gertsley is slightly soluble and might be the culprit. Boron, If I remember
correctly helps prevent devitrification.
With coarser silica, less silica will go into solution. Any change recently in
the glaze fit?
Is your whiting different? Whiting in some trades is a generic term refering
to powdered limestone of any type and the amount of Magnesium Carbonate in it
can vary greatly. Not sure if that would cause the crystals but Magnesium is
known for matt glazes but it seems like a small percentage to be much of a
problem.
Louis
Ron Roy on thu 18 nov 99
Hi Dennis,
All that CaO is going to have an affect on the devitrification - there is
too much.
I have reformulated to spred the fluxes out more - and make it more durable.
Custer - 40.0
Whiting - 21.0
Strontium Carb - 3.5
Silica - 27.0
OM #4 - 8.5
Gerstley borate - 3.0
Ochre - 7.0
Total - 110.0
Expansion and ratio are the same as the original.
Yes there is a big difference between ochre and iron oxide.
Red iron oxide is almost all Fe2O3 and my analysis for yellow ochre has
10.6% - ochres will vary - ask your supplier to give you an analysis for
both. There could be titanium in your new material - that will promote the
kind of crystals you describe.
If you try my revision - let us know how it works - RR
>----------------------------Original message----------------------------
>We are getting crystals on the surface of "Amber celadon" . Amber celadon
>was used as a liner is a recent cone 9-10 salt firing. We seem to get more
>on some pieces than others, location in the kiln didn't seem to matter. In
>Rhodes, he says that this is devitrification, caused by to much silica or
>too much metalic oxide in the glaze and the crystals grow in the cooling
>of the kiln. Could the kiln be cooling too slow? Is there a significant
>difference between Yellow Ochre and Yellow Iron? Could the mesh of the
>silica be a factor?
>
>Amber celadon
>Custer 30
>Whiting 25
>Flint 25
>OM4 10
>Gerstley Borate 3
>Yellow Ochre 7%
>
>This only started happening recently. Any ideas?
>
>Thanks in advance!
>
>Dennis Tobin
>Associate Professor
>Art Department
>Miami University
>Oxford, Ohio 45056
>(513) 529-1505
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
Ron Roy on thu 18 nov 99
Hi Steve,
You will see my solution tomorrow - well today I guess - Good to see you
doing this - takes some of the pressure off me - I'll get to your other
question tomorrow - RR
>----------------------------Original message----------------------------
>Dennis,
> Your glaze is a pretty high on the calcia and a bit low on the silica
>and alumina. That much alkaline earth combined with the low silica and
>alumina can sometimes give a calcium matte when the cooling is slow enough.
>I've adjusted the glaze, trying to keep the rest of the properties the same.
>Try it and let us know.
>I've seen the same thing happen with the Leach 1234 celadon in salt kilns.
>It's high in calcia, too.
>
>Steve Grimmer
>Bowling Green State University
>Bowling Green, OH
>
>Two Recipes: Amber Celadon & Untitled Recipe 1
>==================================================
> CUSTER FELDSPAR..... 30.00 30.00% 36.65 36.65%
> WHITING............. 25.00 25.00% 12.22 12.22%
> FLINT............... 25.00 25.00% 27.94 27.94%
> OM #4 BALL CLAY..... 10.00 10.00% 12.22 12.22%
> GERSTLEY BORATE..... 3.00 3.00% 3.66 3.66%
> YELLOW OCHRE........ 7.00 7.00% 7.31 7.31%
> ======== ========
> 100.00 100.00
>
> CaO 0.81* 18.02% 0.67* 9.43%
> MgO 0.00* 0.05% 0.01* 0.06%
> K2O 0.10* 3.69% 0.18* 4.26%
> Na2O 0.06* 1.37% 0.10* 1.58%
> TiO2 0.00 0.14% 0.01 0.16%
> Al2O3 0.27 10.89% 0.48 12.29%
> B2O3 0.07 1.82% 0.12 2.10%
> SiO2 2.61 62.06% 4.49 68.16%
> Fe2O3 0.03* 1.96% 0.05* 1.96%
>
> Cost/kg 0.51 0.56
> Si:Al 9.67 9.41
> SiB:Al 9.92 9.66
> Expan 7.60 6.91
>
>
>
>
>----------
>>From: "Dennis E. Tobin"
>>To: CLAYART@LSV.UKY.EDU
>>Subject: glaze crystals / devitrification
>>Date: Tue, Nov 16, 1999, 11:17 AM
>>
>
>>----------------------------Original message----------------------------
>>We are getting crystals on the surface of "Amber celadon" . Amber celadon
>>was used as a liner is a recent cone 9-10 salt firing. We seem to get more
>>on some pieces than others, location in the kiln didn't seem to matter. In
>>Rhodes, he says that this is devitrification, caused by to much silica or
>>too much metalic oxide in the glaze and the crystals grow in the cooling
>>of the kiln. Could the kiln be cooling too slow? Is there a significant
>>difference between Yellow Ochre and Yellow Iron? Could the mesh of the
>>silica be a factor?
>>
>>Amber celadon
>>Custer 30
>>Whiting 25
>>Flint 25
>>OM4 10
>>Gerstley Borate 3
>>Yellow Ochre 7%
>>
>>This only started happening recently. Any ideas?
>>
>>Thanks in advance!
>>
>>Dennis Tobin
>>Associate Professor
>>Art Department
>>Miami University
>>Oxford, Ohio 45056
>>(513) 529-1505
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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