Bill Arnold on wed 27 feb 02
The old standard way of dropping a cone ten glaze to cone six was to add 15
% gerstley borate to original batch. I suppose that Boraq or some other G.B.
substitute should do the job.
----- Original Message -----
From: "Paul Lewing"
To:
Sent: Wednesday, February 27, 2002 5:54 PM
Subject: Re: Altering Maturing Temp for Glaze
> on 2/27/02 7:18 AM, Dupre Mr Marcy M at dupremm@MSTP.QUANTICO.USMC.MIL
> wrote:
>
> > What can I do to make that same glaze, with a new maturing temperature
of
> > ^6, in an oxidizing atmosphere?
>
> > I have read that replacing the potash feldspar with nepheline syenite
MIGHT
> > do the trick.
>
> That might work, but it might not be enough to get you all the way to cone
> 6. The reason that works is that Neph Sy is not exactly a feldspar, but
> it's similar. It has more sodium and less silica than a feldspar, so it
> melts at a lower temperature. However, this substitution will also raise
> the coefficient of expansion, so it may no longer fit your clay body. It
> might also help in fluxing it down to take some of that magnesia out,
which
> you're getting from the talc and the dolomite. Magnesia is almost not a
> flux down at cone 6- you're right at the lower end of its effective
> temperature range. You could replace it easily with calcium by replacing
> the talc with wollastonite. However, this substitution will also raise
the
> COE, and will affect some colors.
> Those are both pretty crude but commonsense kinds of things that might
work.
> If you have calculation software, a way of lowering the firing temperature
> of a glaze is to (1) keep the fluxes exactly the same; (2) lower both the
> SiO2 and the Al2O3 in such a proportion as to (3) keep the Si:Al ratio the
> same. So how much to lower the silica and alumina? Look at a set of
limit
> formulas. If the SiO2 is in the middle (let's say) of the range for a
cone
> 9 glaze, drop it to a value in the middle of the cone 6 range, and drop
the
> Al2O3 to such a value that the Si:Al ratio is the same as before. I hope
> that makes sense to you.
> One reason that this often doesn't work is that you get the Si and the Al
> down to such a level that it either isn't a stable glaze any more, or it
> doesn't even melt. And then you also get the fit problem. So you're
almost
> always forced to introduce a new oxide, usually boron or lithium, to flux
it
> more and lower the expansion. At this point, you may have a usable glaze,
> but it isn't much like the one you started with. So this can all be a
> useful exercise and produce good glazes, but three cones is a long way to
> change a glaze and expect an exact replica of the higher-fire glaze.
> Remember- when you change the glaze, you get a different glaze.
> Happy testing,
> Paul Lewing, Seattle
>
>
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Dupre Mr Marcy M on wed 27 feb 02
Dear all,
I found a marvelous recipe for a satin white glaze in Mimi Obstler's book,
"Out of the Earth, Into the Fire." It is a ^9 oxidizing glaze, firing to a
gray-white satiny "pick-me-up-and-feel-me" looking glaze. Now, here's the
question:
What can I do to make that same glaze, with a new maturing temperature of
^6, in an oxidizing atmosphere?
I use an L&L Jupiter electric kiln, capable of firing to ^10
I have read that replacing the potash feldspar with nepheline syenite MIGHT
do the trick. Before I spend a lot of time and money on materials, I
figured I'd pass it by the Clayart Committee first.
The recipe is called "Charlie D.:"
Custer spar 20
Kona F4 20
Flint 20
Dolomite 15
Talc 13
Whiting 2
Ball clay 10 (I intend to use OM#4)
-----------
100
By the way, are any of you potters in the Springfield, VA area? Would sure
like to meet you.
Thanks for any and all suggestions,
Tig Dupre
Get Dirty!
Paul Lewing on wed 27 feb 02
on 2/27/02 7:18 AM, Dupre Mr Marcy M at dupremm@MSTP.QUANTICO.USMC.MIL
wrote:
> What can I do to make that same glaze, with a new maturing temperature of
> ^6, in an oxidizing atmosphere?
> I have read that replacing the potash feldspar with nepheline syenite MIGHT
> do the trick.
That might work, but it might not be enough to get you all the way to cone
6. The reason that works is that Neph Sy is not exactly a feldspar, but
it's similar. It has more sodium and less silica than a feldspar, so it
melts at a lower temperature. However, this substitution will also raise
the coefficient of expansion, so it may no longer fit your clay body. It
might also help in fluxing it down to take some of that magnesia out, which
you're getting from the talc and the dolomite. Magnesia is almost not a
flux down at cone 6- you're right at the lower end of its effective
temperature range. You could replace it easily with calcium by replacing
the talc with wollastonite. However, this substitution will also raise the
COE, and will affect some colors.
Those are both pretty crude but commonsense kinds of things that might work.
If you have calculation software, a way of lowering the firing temperature
of a glaze is to (1) keep the fluxes exactly the same; (2) lower both the
SiO2 and the Al2O3 in such a proportion as to (3) keep the Si:Al ratio the
same. So how much to lower the silica and alumina? Look at a set of limit
formulas. If the SiO2 is in the middle (let's say) of the range for a cone
9 glaze, drop it to a value in the middle of the cone 6 range, and drop the
Al2O3 to such a value that the Si:Al ratio is the same as before. I hope
that makes sense to you.
One reason that this often doesn't work is that you get the Si and the Al
down to such a level that it either isn't a stable glaze any more, or it
doesn't even melt. And then you also get the fit problem. So you're almost
always forced to introduce a new oxide, usually boron or lithium, to flux it
more and lower the expansion. At this point, you may have a usable glaze,
but it isn't much like the one you started with. So this can all be a
useful exercise and produce good glazes, but three cones is a long way to
change a glaze and expect an exact replica of the higher-fire glaze.
Remember- when you change the glaze, you get a different glaze.
Happy testing,
Paul Lewing, Seattle
Dupre Mr Marcy M on fri 1 mar 02
All,
Many thanks for all the helpful suggestions. I'll be doing a lot of
weighing and mixing this weekend. Have to get out my ol' chemistry book, a
pack of stubby #2 pencils, a graph tablet, and my trusty slide rule. (Ya'll
still use a slide rule, doncha?)
Actually, I'm pulling your collective leg. I use software almost
exclusively these days. It's faster and more accurate than I could ever
hope to be! Just would like to have a button on the menu that says for Temp:__>, then another that says .
I wish you all a dynamite weekend. Get dirty!
Tig
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