Sabri Ben-Achour on sun 9 sep 07
So I guess we've all had particularly bad shino results, but I was
wondering if anyone had an explanation for some particular disasterous
outcomes I've been getting:
I have consistently been getting hideous light green and sometimes bubbly
results from a cone 10 gas fired malcolm shino on porcelain. Where waxed,
I get the usual orange color. It's only where it hasn't been waxed that I
get this nasty dirty green color.
Refiring to cone 10 in an electric kiln resulted in a much lighter, mostly
clear glaze result, but still ugly.
I'm going to just reglaze them with a low fire matt glaze, but can anyone
explain this horrible, horrible green?
My theory is: insufficient reduction early on (explaining why it's light
green instead of black or blackish green), and insufficient oxidation later
(John Britt's book says that carbon trap shinos can get a greenish tinge if
over-reduced in the later stages - does anyone know why this is?).
Unfortunately I don't have much control over the firing schedule so I
haven't been able to test these ideas.
Anyone have some insights?
Sabri
Dave Finkelnburg on mon 10 sep 07
Sabri,
You are on the right track with your thinking.
An excessively thick glaze application can be
problematic for producing "snot" green in American
Shinos. Lack of carbon trapping is always either due
to starting "heavy" reduction to produce carbon too
late (after the glaze has started melting) OR letting
the kiln slip or creep into oxidation and burning off
the carbon BEFORE the glaze melts. Forming the carbon
and keeping it there until the glaze melts requires
very consistent control of the kiln atmosphere. The
carbon can burn off in a VERY short time.
I hope this helps you to think about this
frustrating problem that has afflicted virtually all
of us who love carbon trapping.
Dave Finkelnburg
From: Sabri Ben-Achour
I have consistently been getting hideous light green
and sometimes bubbly
results from a cone 10 gas fired malcolm shino on
porcelain. Where waxed,
I get the usual orange color. It's only where it
hasn't been waxed that I
get this nasty dirty green color.
...can anyone
explain this horrible, horrible green?
My theory is: insufficient reduction early on
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Bruce Girrell on mon 10 sep 07
Sabri Ben-Achour wrote:
> My theory is: insufficient reduction early on (explaining why it's light
> green instead of black or blackish green), and insufficient oxidation
later
To me, green in an iron glaze is an indication of a reduction environment so
I doubt that insufficient reduction is your problem.
On the other hand, I would wholeheartedly agree that insufficient oxidation
later on is strongly associated with your problem. The color development of
shinos happens during the cooldown. If you don't believe me, make some draw
rings and try it yourself. Start pulling rings at peak temperature and then
about every 300 degrees C or every 500 degrees F on the way down. I usually
also leave one in that doesn't come out until the kiln is unloaded.
Another thought occurred to me also: Any chance that load happened to get
hotter than normal? Reoxidation is more difficult once a glaze seals over
and shinos, with all that alumina, don't melt particularly well. If the load
got too hot, then maybe the glaze melted more and didn't reoxidize properly
as a result of that.
Bruce "just my opinion" Girrell
Sabri Ben-Achour on thu 13 sep 07
Thanks guys for your help and suggestions!
I will definitely try thinning the shino and see if that prevents the green
from showing up, and I'll let you know how that goes.
Bruce - it's not the green that I thought might be the result of
insufficient reduction early on, just the lightness of the green (if it had
been dark blackish green it would have been a sign that carbon trapping -
and so reduction - had occurred early enough).
Would you suggest adding more air for oxidation around cone 9, just before
(presumably) the glaze seals over?
I just wish someone knew the chemical principles behind the green, orange,
and black. I'll keep looking into it and post whatever, if anything, I come
up with.
Sabri
On Mon, 10 Sep 2007 10:56:50 -0400, Bruce Girrell
wrote:
>Sabri Ben-Achour wrote:
>
>> My theory is: insufficient reduction early on (explaining why it's light
>> green instead of black or blackish green), and insufficient oxidation
>later
>
>To me, green in an iron glaze is an indication of a reduction environment so
>I doubt that insufficient reduction is your problem.
>
>On the other hand, I would wholeheartedly agree that insufficient oxidation
>later on is strongly associated with your problem. The color development of
>shinos happens during the cooldown. If you don't believe me, make some draw
>rings and try it yourself. Start pulling rings at peak temperature and then
>about every 300 degrees C or every 500 degrees F on the way down. I usually
>also leave one in that doesn't come out until the kiln is unloaded.
>
>Another thought occurred to me also: Any chance that load happened to get
>hotter than normal? Reoxidation is more difficult once a glaze seals over
>and shinos, with all that alumina, don't melt particularly well. If the load
>got too hot, then maybe the glaze melted more and didn't reoxidize properly
>as a result of that.
>
>Bruce "just my opinion" Girrell
Lee Love on thu 13 sep 07
On 9/9/07, Sabri Ben-Achour wrote:
> Anyone have some insights?
Which recipe? Are you adding iron for applying to porcelain? How do
you test your thickness, once the glaze is on the pot?
You get the snot green on high soda ash carbon trap glazes when
they are appied too thickly.
--
Lee in Minneapolis, Minnesota USA
"Making pots should not be a struggle.
It should be like walking down a hill
in a gentle breeze." --Shoji Hamada
http://mashikopots.blogspot.com/
"For a democracy of excellence, the goal is not to reduce things to a
common denominator but to raise things to a shared worth."
--Paolo Soleri
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