Teresa Testa on wed 29 sep 04
Hello Everyone,
Something very interesting happened this week. I was getting ready for a big
show this weekend and had the last load firing on Sunday. I use Coleman
porcelain and cone 10 glazes. Well, the Bartlet display read error #1 at around
2100 F. I reprogramed and tried again. I got to around 2192 F and it did
the same thing. So I stopped the firing and figured I had to change the coils,
( I knew this was coming so I had new coils on hand). I thought I would have
to re-fire all my work. I was very stressed trying to figure out how to get
it done before the show. Monday I opened the door and WOW, all the pieces in
the kiln came out great.
Here is my question, I used a wide variety of glazes in the kiln and all of
them came out almost exactly the same as they would have in a regular firing to
2345 F. Do all cone 10 glazes mature at lower temperatures? Is the high
firing temperature just to mature the cone 10 clay body? If this is true could I
use the cone 10 glazes I have on a cone 6 body and save myself some wear and
tear on my kiln? The only difference I noticed was a a thin layer of white
dust like substance all over the pieces, which came off easily.
If anyone has any thoughts on this I would appreciate your input.
Teresa
Windancer Studio
Las Vegas, NV
William Melstrom on wed 29 sep 04
Did you have any cones in your kiln? I'll bet that you did the heat-work
equivalent of a cone 10 firing.
-- William
----- Original Message -----
From: "Teresa Testa"
To:
Sent: Wednesday, September 29, 2004 9:31 AM
Subject: Under fired kiln: cone 10 glazes, WOW!
> Hello Everyone,
>
> Something very interesting happened this week. I was getting ready for a
big
> show this weekend and had the last load firing on Sunday. I use Coleman
> porcelain and cone 10 glazes. Well, the Bartlet display read error #1 at
around
> 2100 F. I reprogramed and tried again. I got to around 2192 F and it did
> the same thing. So I stopped the firing and figured I had to change the
coils,
> ( I knew this was coming so I had new coils on hand). I thought I would
have
> to re-fire all my work. I was very stressed trying to figure out how to
get
> it done before the show. Monday I opened the door and WOW, all the pieces
in
> the kiln came out great.
>
> Here is my question, I used a wide variety of glazes in the kiln and all
of
> them came out almost exactly the same as they would have in a regular
firing to
> 2345 F. Do all cone 10 glazes mature at lower temperatures? Is the high
> firing temperature just to mature the cone 10 clay body? If this is true
could I
> use the cone 10 glazes I have on a cone 6 body and save myself some wear
and
> tear on my kiln? The only difference I noticed was a a thin layer of
white
> dust like substance all over the pieces, which came off easily.
>
> If anyone has any thoughts on this I would appreciate your input.
>
> Teresa
>
> Windancer Studio
> Las Vegas, NV
>
>
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John Hesselberth on wed 29 sep 04
On Wednesday, September 29, 2004, at 10:31 AM, Teresa Testa wrote:
> Do all cone 10 glazes mature at lower temperatures? Is the high
> firing temperature just to mature the cone 10 clay body? If this is
> true could I
> use the cone 10 glazes I have on a cone 6 body and save myself some
> wear and
> tear on my kiln? The only difference I noticed was a a thin layer of
> white
> dust like substance all over the pieces, which came off easily.
Hi Teresa,
No, all cone 10 glazes don't mature at lower temperatures and yours may
not be mature either--particularly if they are mattes. With matte
glazes you can get almost the same appearance by underfiring them as
you get by firing them to maturity and cooling them slowly. But the
glazes will perform very differently. The underfired ones will not be
as stable or durable, particularly to leaching by acidic foods or basic
dishwasher detergents.
So you have to be careful. If they are glossy glazes and look just as
glossy as they do at cone 10, the odds are higher that they are
sufficiently mature. In my experimental work I found some glossy glazes
that performed almost the same from cones 4 to 10 -- but not all will.
You really have to test them to be sure.
Firing to cone 6 rather than 10 in an an electric kiln, though, makes a
lot of sense. Your element life will probably be 3 or 4 times longer.
But please do make sure your glazes not only look the same, but perform
the same at the lower temperature. If you want to send me a couple of
the recipes off-list I would be happy to offer an opinion--but it will
be only that--an opinion. Test, test, test to be sure.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Teresa Testa on thu 30 sep 04
Hi John,
All of the glazes, except one were bright and glossy. I make sculptural
pieces, so the leaching really isn't a problem. There were a couple base glazes
from Laguna, (#520 clear gloss and crystal palace base), and a few from Cushing
and Chappel. I also fired some rotten stone glazes, which are always matt.
The rotten stone came out looking more dirt brown, than red brown like it
usually does. Here's one of the recipes from Cushing:
Wild Wonder cone 10
gerstley borate 48
talc 22
titanium dioxide 14
zinc oxide 10
lithium carb 6
At higher temps this glaze leaves craters where thick, but this time it
matured just fine. Looks great in oxidation, runs a lot. I only use it on the
inside. It likes to be fired in the bottom back of my kiln like the other
crystal glazes I use.
Let me know what you think.
Teresa
Windancer Studio
www.windancerstudio.com
Ivor and Olive Lewis on thu 30 sep 04
Dear Teresa Testa,
I think there are a couple of points of note here though they do not
answer your question,
The first is that if your Coleman cone 10 porcelain has only been up
to 2192 it may be underfired. This could mean that it has not achieved
its optimum properties of strength and translucency.
The second is that you are assuming that "almost identical visual
qualities" equate to "identical properties".
Your interesting white dust seems to indicate that there is an
ingredient in the glazes which has not been taken into solution during
the melting phase. That it can be readily removed may show that your
melting solvents need more heat to help dissolve this ingredient and
achieve maturity. If this is the case, then that material may be
present and unchanged deep within the glaze as well. Whatever this
substance is, it may be an indication of potential instability.
Only scientific tests would prove or disprove my proposition, but I
would not change a good routine which produces a successful product
just to make a minimal saving without being quite sure that I am
getting identical results.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
John Hesselberth on thu 30 sep 04
Hi Teresa,
Your White Wonder "glaze" is so far outside normal compositional limits
that I would call it a 'surface treatment' rather than a glaze. If it
gives you the look you want on sculptural pieces (and you don't plan
to put them outside where they might be exposed to acid rain or bird
poop) go ahead and use it at whatever temperature. However it has an
extremely low level of silica and almost no alumina so it would be
completely unsatisfactory on functional work.
If a composition like this one can have a maturing temperature it is
probably closer to cone 6 than cone 10. It has lots of boron and zinc
which act as active fluxes and are usually not needed nor seen in cone
10 glazes--at least not at levels this high. The cratering you
sometimes get at cone 10 would also say you are probably overfiring it
at cone 10.
You would seem to be a prime candidate for an easy switch to cone 6.
Get yourself a clay body that gives you the degree of maturity you want
at cone 6 and use the glazes that give you the effects you want. Just
please don't share them with your functional potter friends. Good luck.
Regards,
John
On Thursday, September 30, 2004, at 09:38 AM, Teresa Testa wrote:
>
> Wild Wonder cone 10
>
> gerstley borate 48
> talc 22
> titanium dioxide 14
> zinc oxide 10
> lithium carb 6
>
> At higher temps this glaze leaves craters where thick, but this time it
> matured just fine. Looks great in oxidation, runs a lot.
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Teresa Testa on fri 1 oct 04
Hi Ivor,
I found the glaze in Cushing's Handbook. It is "A Compilation of Papers
Concerning Glazes, Glaze Calculation & Formulation, Clay Bodies, Raw Materials
with Clay Body, Slip and Glaze Formulas, Firing, Pottery Making Tips and Other
Useful Information," by Val M. Cushing, Professor of Ceramic Art at Alfred
University in New York. This is an excellent handbook and I would highly recommend
it. It can only be ordered through the author. It was supposed to be used in
reduction, but it works in oxidation firing as well. If you are interested I
can e-mail you a picture.
Teresa Testa
Windancer Studio
Las Vegas, NV
Ivor and Olive Lewis on fri 1 oct 04
Dear Teresa Testa,
You post a recipe
gerstley borate 48
talc 22
titanium dioxide 14
zinc oxide 10
lithium carb 6 >
This is an interesting species of ceramic coating, but I would find it
hard to classify as a glaze.
Even though you tell us it melts freely and flows well it lacks the
main ingredients which we traditionally include in a glaze, Silica and
Alumina. Reliance on Boron as a Glass Former may leave it susceptible
to dissolution by water.
But if it meets your aesthetic needs I see no reason why it should
not be used indoors for decorative purposes.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Ron Roy on fri 1 oct 04
Hi Teresa,
While it is not surprising that some of the glazes do well at a lower
temperature - this may indicate that those glazes could use more silica and
alumina - if durability is a factor.
the more important aspect is the vitrification of the clay. It certainly is
possible that the clay may leak and absorb water when fired lower - making
it unfit for microwave use for instance.
You should always have at least one cone in your firings - firing longer
but not as high may still give enough heat work to do the job - a cone will
show how close you actually got to top temperature.
RR
>Something very interesting happened this week. I was getting ready for a big
>show this weekend and had the last load firing on Sunday. I use Coleman
>porcelain and cone 10 glazes. Well, the Bartlet display read error #1 at
>around
>2100 F. I reprogramed and tried again. I got to around 2192 F and it did
>the same thing. So I stopped the firing and figured I had to change the coils,
>( I knew this was coming so I had new coils on hand). I thought I would have
>to re-fire all my work. I was very stressed trying to figure out how to get
>it done before the show. Monday I opened the door and WOW, all the pieces in
>the kiln came out great.
>
>Here is my question, I used a wide variety of glazes in the kiln and all of
>them came out almost exactly the same as they would have in a regular firing to
>2345 F. Do all cone 10 glazes mature at lower temperatures? Is the high
>firing temperature just to mature the cone 10 clay body? If this is true
>could I
>use the cone 10 glazes I have on a cone 6 body and save myself some wear and
>tear on my kiln? The only difference I noticed was a a thin layer of white
>dust like substance all over the pieces, which came off easily.
>
>If anyone has any thoughts on this I would appreciate your input.
>
>Teresa
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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