Kathy McDonald on wed 6 dec 06
I am glad you wrote Lily:
As I stated in my earlier post I certainly
don't have the technical language or the
expertise to describe what is happening
in quite the same way as others on the list do.
I liked the fact that you posted some ways to
look at the problem that are actually right up my alley,
and most likely are quite scientific.
You suggest:
"If it were my problem I would check out the whole situation
in layers. A:
the c.10 glaze alone.
B.the c 10 glaze with rutile/iron
C. the c.10 glaze with rutile/iron and overglaze.
D. The whole nine
yards--c.10., rutile/iron, overglaze. c.6 glaze
E.. The cone 10 glaze with
just the c. 6 glaze atop.
F. The cone 10 glaze with just underglaze on top.
And for good measure,
G. the underglaze atop the clay, with c.6 glaze over
it."......
Ok ...I've done A, B , ,,,all have been successful at cone
10...
my bread and butter for years.
C seems to work great if I just fire to 10....its in the
refire to C6 E that
all hell breaks loose.,,,iron and rutile from my original c
10 firing come out all
blistered after the refire to c 6 E.
D E and F ...seem OK ... after a cone 10 firing....it's
that refire process
that seems to be the problem.
G I need to try yet, just to see what happens at both c 6
and at 10
I have also one more thing I want to try we'll call that
H...fire to cone 6 first
...then fire to cone 10 as refire and see what that does.
Thanks Lily..this helped me break it down
The combos do show promise so I'm going to keep working.
Kathy
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of
Lili
Krakowski
Sent: Wednesday, December 06, 2006 6:00 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: blistering
Kathy McDonald wrote:
"[Mugs glazed only with CM White] looked rather boring so I
thought
I'll jazz them up a bit, and see what happens if I
refire them to C6 with some colorful overglazes (Mayco) and
a clear coat over the outside of the mug (also Mayco c6).
I have done this in the past
with mugs glazed only in the CM White and they were fine.
That's the "control" part of this experiment.
"They were fine with the exception of areas that had the
iron/rutile
glaze decoration. It was a crusty bubbled mess . ...
"All other areas are fine. It is highly unlikely that the
clay caused it because
it was fired to vitrification, and was a nice semi gloss
surface in all areas except those with the iron underglaze.
" I broke a few just to see...there was that characteristic
blackened area almost to the core of the clay right where
the iron decoration was.
I also broke a piece fired a few firings ago in the same
glaze combo but NOT refired
and it showed no evidence of any blackened areas in the
clay."
So. An always successful c.10 white glaze with some
rutile and iron painted atop, had underglazes and c.6 glaze
applied.
Refiring to c.6 created nasty blisters on the surface, but,
examination showed, they occurred only where the overglaze/
c.6 glaze was
atop the rutile/iron decoration.
The question is : what happened?
Well, of recent, I have--through correspondence with a
friend/ expert--begun
to think a lot more about the vertical behavior of glaze.
We generally
focus on the horizontal behavior, and sort of ignore what I
long have
suspected: that the vertical behavior is an equally
important mover and
shaker.
What I am guessing is this: At c.6, the c.10 glaze
experienced some
slight softening which allowed the release of some gases
which may have been
simply ones that had not quite finished their escape in the
original
firing. These gases made their escape safely where there
was the c.10 base
and the c.6 overcoat. This because the interface reactions
between the two
glazes was copasetic.
But where the gases were either under the rutile/iron layer,
or inside it,
the gases could not elbow their way through the underglaze
and the c.6
glaze. I am guessing the blackness is from the reduction of
whatever created the gases produced
It also might be possible that there was a reaction between
the rutile/iron
and the overglazes, creating gases which could not make
their escape
totally.
I think we tend to think of escaping gases as tornadoes, or
gusts...I
somehow do not think so. I think they are very slight and
gentle, just
enough to disturb the glaze surface.
Lili Krakowski
Be of good courage
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