Craig Martell on tue 6 jun 00
Chris sez:
>I just unloaded a high fire reduction firing and after pulling everthing out
>I started to look a little closer at a large platter that was on the top
>shelf, glazed with shino and Rob's green. The shino was disintegrating before
>my eyes flaking to powder on the center bottom of the platter and along the
>outer back edge. I had this same shino on many pieces in the kiln and no
>other piece had this problem.
Hi:
This is a tough one to answer but I'll venture a guess. It doesn't sound
like shivering at all. Shivering occurs when a glaze has melted and is
fusion bonded to the clay. The glaze is under too much compression from
poor fit and it pops off taking shards of the claybody with it.
Your shino is very high in spodumene. I looked at the seger formula and if
you use the ideal spodumene analysis, it comes up as .38 moles of lithium
in flux unity. That's about 1.8 moles too many but I don't think that's
the problem. Spodumene doesn't have a set fusion temperature, just a melt
"range". I've fired fusion buttons of several types of spodumene and it
can go from a dry unmelted button to a flat fused glass in less than a cone
difference. This is at about cone 10 to 11. You don't say what kind of
spod you are using or what your top temperature is so it's hard to make a
real qualified guess. Anyway, with the amount of spodumene you have in
this glaze my guess is that your platter didn't get the heat necessary to
fuse the glaze in places and that's what caused the powdery flaking. If
you look at a lot of shinos in flux unity, you'll see that the alumina is
really high. .5 moles of alumina is given as the limit and shinos usually
have about .9 moles or more. Yours has more than 1.0 moles of alumina and
as you probably know alumina will make a glaze more refractory. There is a
eutectic ratio with soda and lithium that gives a very strong melt and
makes these glazes work pretty well with a lot of alumina and silica. I
think that this shino is a bit unbalanced in the soda lithium dept and that
may account for dryness and flaking in a cooler part of the kiln. The
glaze does work in other parts of the kiln but the melt range may be a bit
tight.
The tough thing about sleuthing out a problem like this is that language
doesn't communicate as well as visual experience. If I could see the
glazes and the differences between the two, it would be a lot more telling,
and easier to troubleshoot. My feeling is that this glaze would be better
and more behaved if you lowered the spodumene and upped the soda.
regards, Craig Martell in Oregon
KellDogn@AOL.COM on tue 6 jun 00
Hi all,
I just unloaded a high fire reduction firing and after pulling everthing out
I started to look a little closer at a large platter that was on the top
shelf, glazed with shino and Rob's green. The shino was disintegrating before
my eyes flaking to powder on the center bottom of the platter and along the
outer back edge. I had this same shino on many pieces in the kiln and no
other piece had this problem. The kiln was reduced heavily at about 013 kept
in reduction until about cone six when I need to allow more air to even out
top and bottom. Then at end of firing heavy reduction for about 15-20 min.
The shino is :Peach to Red Shino
Neph Sy 39.3
Spod. 30.3
OM4 17.2
Soda Ash 8. plus some
EPK 4.8
I will be refiring this platter with some added shino on the flaked parts but
would like to know what happened. I have had this problem before but its been
a while. Any thoughts?
Chris
and if you have time send rain........
kelldogn@aol.com
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