Dave Finkelnburg on sat 19 may 01
Matt,
If this is truly observed inside a lidded jar, then volatile alkalis is
likely not the cause. ( That will be a cause on the outside of pots in wood
fire).
Most likely suspect is iron. Iron is not a flux in oxidation, but is in
reduction. Most natural ingredients have some iron, even though it may not
be noticeable to the naked eye.
Dave Finkelnburg in Idaho
Don & June MacDonald on sat 19 may 01
I fire some glazes in either Cone 6 reduction or Cone 6 oxidation.
Apart from the obvious changes caused by iron or copper in reduction, I
also notice a surface difference in some glazes, and I presume it is
more likely due to the slower cooling cycle in the gas kiln than the
difference between oxidation and reduction.
June from B.C.
Matt MacIntire wrote:
>
> I've been wondering why the same bucket of glaze can have such a different
> surface quality when fired to the same cone in different kiln environments.
> There is some environmental variable that seems to cause (some?) glazes to
> flux much more actively in a reducing environment. For example, a glaze
> that has a nice buttery surface in a gas kiln, comes out looking quite
> underfired in an electric kiln. And I notice that if I put that same glaze
> in a wood kiln, it can become almost glossy.
>
> What is the cause of this increased fluxing action in reducing environments?
>
> I am not talking about the effect of any fly ash. I observe that this
> difference happens even when the glaze is on the inside of a lidded jar.
> With these buttery to matte glazes [often high in Magnesia and with little
> or no colorants] the reduction environment seems to promote much more
> thorough melting, with increasing reduction causing increased melting.
>
> Other things being equal, I can think of three suspects:
>
> 1> maybe the reduction is changing certain glaze materials to another form
> that is more reactive
>
> 2> maybe the water vapor in a fuel fired kiln has some effect on the glaze
> melting
>
> 3> maybe there are some volatile alkalis that are affecting the glaze melt
>
> Can anyone help explain this phenomenon to me?
>
> Thanks in advance
>
> Matt
>
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Matt MacIntire on sat 19 may 01
I've been wondering why the same bucket of glaze can have such a different
surface quality when fired to the same cone in different kiln environments.
There is some environmental variable that seems to cause (some?) glazes to
flux much more actively in a reducing environment. For example, a glaze
that has a nice buttery surface in a gas kiln, comes out looking quite
underfired in an electric kiln. And I notice that if I put that same glaze
in a wood kiln, it can become almost glossy.
What is the cause of this increased fluxing action in reducing environments?
I am not talking about the effect of any fly ash. I observe that this
difference happens even when the glaze is on the inside of a lidded jar.
With these buttery to matte glazes [often high in Magnesia and with little
or no colorants] the reduction environment seems to promote much more
thorough melting, with increasing reduction causing increased melting.
Other things being equal, I can think of three suspects:
1> maybe the reduction is changing certain glaze materials to another form
that is more reactive
2> maybe the water vapor in a fuel fired kiln has some effect on the glaze
melting
3> maybe there are some volatile alkalis that are affecting the glaze melt
Can anyone help explain this phenomenon to me?
Thanks in advance
Matt
Matt MacIntire on sun 20 may 01
Thanks to Dave and June for their comments.
I had (perhaps mistakenly) dismissed the idea of the added fluxing action of
trace amounts of reduced iron. I had considered that the trace amounts
present would be insufficient to cause noticeable change. A couple people
have pointed out the fluxing power of reduced iron to me off list. Perhaps
the effect of these trace amounts may indeed cause this extra fluxing
action.
But that immediately raises an obvious question... If *trace* amounts of
reduced iron can cause such profound melting, why wouldn't a glaze with 10%
of iron run right off the pot!?? A glaze like a temmoku which has
substantial amounts of iron would (by this reasoning) seem to be unworkable
if reduced heavily...?? Yet I have observed that a temmoku glaze seems to
melt adequately in oxidation or reduction. It looks different, but melts
fairly similarly.
Perhaps there is something else going on. I don't know. The main type of
glaze I notice this effect on are high magnesia glazes with that soft,
buttery look. I don't actually notice much variability with glossy glazes.
I probably should have emphasized that more in my original post.
June pointed out a variable which I had neglected to consider. Different
heating and cooling cycles could be responsible for this difference.
Certainly the electric kiln fires and cools the fastest, while the hard
brick wood kiln is slower to heat and cool than the gas kiln made out of
IFB. If not directly causative, this is certainly consistent. Any other
opinions here...??
Ultimately this effect I notice is probably the cumulative result of a lot
of little differences. Slower firing cycle, more fluxing from trace amounts
of iron, some other unidentified changes, etc.
I appreciate everyone's comments.
Matt
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