Joseph Herbert on thu 14 aug 97
For Ric, and others, worried about my writing staff: I am using the proceeds
from my various ClayArt postings to employ (part time, like UPS) a number of
comedy writers formerly employed by Dennis Miller, Chevy Chase, and the show
"Moonlighting". They generally feel that writing slightly humorous comments
about toxic or hazardous materials preferable to producing "pertinent
Sociopolitical, cogent, topical humor". I am certainly grateful for their
help, which enhances my image as a curmudgonly purveyor of pithy
observations, rather than allowing the pale wraith of a contract trainer,
living away from home, with too much time for the computer screen, to seep
Some of these things are really hard to punctuate.
Attempting an answer for Nils about the cone 06 glaze as a protectant for
copper red glazes: the low temperature glaze fuses at a low temperature, far
below the fusion temperature of the copper red glaze, and forms a barrier
that protects the copper red glaze until the higher fire glaze melts and the
two glazes mix. This should assure that the high fire glaze composition at
the time of melting is the same as when applied. Kiln gases and other
volatile elements would be held away from the glaze until temperature is
reached. The small amount of copper in the glaze as formulated would not
have a chance to leave the glaze easily before the base glaze materials fuse
and incorporate it. This seems like a very good, simple idea that has a
sound physical basis.
To continue the thought, most of the time, I never even consider the
possibilities of applying glazes of more than one maturing temperature. The
only context that occurs to me, normally, is that of decoration where you
fire the ware high, and reduce the maturing temperature of the glazes that
are applied sequentially to the original high fired ware. The last glaze is
the cone 022 gold luster I sign my name with. That way, if the colors fade
in the dishwasher, my name is the first thing to go and they can t
Anyway, I noticed that Pfatzgraf, famous stoneware dinnerware makers, covers
stilt marks with applications of lower fire glazes. Never thought of that
but it must be standard industry practice. That and this idea of Nils
indicates to me that there may be a whole range of possible uses for
different temperature maturing glazes, beyond decoration, in the flower
painting sense. Something like a high temperature resist, for example, where
a partial application of a lower temperature glaze keeps a vapor borne
material, like soda or nickel, from reacting with some part of a glaze piece.
There is not a lot new in the pottery biz so other uses of different
temperature glazes for non-obvious purposes may abound. Just wish they would
bound over here.
Alex, Aurora and Leah Solla on fri 15 aug 97
At 10:44 AM 8/14/97 -0400, you wrote:
>copper red glazes: the low temperature glaze fuses at a low temperature, far
>below the fusion temperature of the copper red glaze, and forms a barrier
>that protects the copper red glaze until the higher fire glaze melts and the
>two glazes mix.
One thing I have found with Copper Red glazes is that they need a fairly
early "body" reduction. Usually below c08. At this point the glaze isnt
sintered yet, but I wonder how a c04 glaze over the copper red would be
behaving? I think, though I would be curious to hear other ideas, that the
real physical phenominon happening here is that the c04 clear is simply yet
another layer of glaze, thus allowing the glaze greater thickness for the
formation of colloidial copper to form hense the clear red color. See
Robert Tichane's book to the same effect.
I am anxious to try this and to see what effect it has on my reds. Let you
know in a week or so.
former copper red fanatic (now rehabilitated)
Richard Burkett on sat 16 aug 97
Joseph Herbert says:
>Attempting an answer for Nils about the cone 06 glaze as a protectant for
>copper red glazes: the low temperature glaze fuses at a low temperature,
>below the fusion temperature of the copper red glaze, and forms a barrier
>that protects the copper red glaze until the higher fire glaze melts and
>two glazes mix. This should assure that the high fire glaze composition
>the time of melting is the same as when applied. Kiln gases and other
>volatile elements would be held away from the glaze until temperature is
>reached. The small amount of copper in the glaze as formulated would not
>have a chance to leave the glaze easily before the base glaze materials
>and incorporate it. This seems like a very good, simple idea that has a
>sound physical basis.
It sounds good, although I'm not really sure you want to "protect" copper
from reduction by sealing the surface. And I'd watch carefully that using
a lowfire glaze doesn't produce carbon trapping within the copper red
glaze and result in a dull gray red. This is essentially what happens with
carbon trap glazes, as best I can tell: the soluble crust on the surface
of the raw glaze (most of these glazes contain soda ash in sizable
quantities) melts well before the rest of the glaze sealing the surface,
allowing carbon to be 'trapped' behind this sealed layer, protected from
The copper red color forms largely during the cooling cycle. Pete Pinell
posted a marvelous draw trial test of this to ClayArt a few years back and
it's worth reposting.
Pete's results using a known working copper red glaze:
>The rings were pulled form the top peephole at various times during the
>firing, and the results were as follows:
>Cone 012 (before reduction) - Color: white. Surface: dry
>Cone 04 (after body reduction) - Color: gray with just a trace of pink.
>Surface: a bit less dry
>Cone 2 (light reduction) Color: light gray. Surface : satin matt.
>Cone 5 (light reduction) - Color: light gray. Surface: semi-gloss.
>Cone 91/2 (end of reduction, begin 1 Hr. of re-oxidation) -Color: Clear
>with a very slight gray tint. Surface: Glossy.
The fired result, allowed to cool normally, was bright red. Interestingly,
Pete refired the draw trials above in a cone 06 oxidation firing with
normal cooling and got good reds AFTER refiring.
Glaze thickness and speed of cooling seem to be among the more critical
factors in the reoxidation and nucleation of the copper red color. Perhaps
the successful use of an overcoat of clear over copper red by some potters
has more to do with these factors? Robert Tichane has written a voluminous
tract "Reds, Reds, Copper Reds" on this topic. I recommend it for a
greater understanding of how these somewhat fugitive glazes form.
Richard Burkett - School of Art, SDSU, San Diego, CA 92182-4805
E-mail: firstname.lastname@example.org <-> Voice mail: (619) 594-6201
Home Page: http://rohan.sdsu.edu/dept/rburkett/www/burkett.html
HyperGlaze@aol.com & http://members.aol.com/hyperglaze/
Daniel W Kirkland on sun 17 aug 97
Unfortunately, I started my subscription to Clayart today. So I did not
receive the original message of "Copper Red Raincoat", but I did receive
Richard Burkett's response. So if I am repeating something which has
already been stated, I apologize.
I really have no idea regarding a low fire glaze protectant for copper
reds, nor have I had any need for them. The reason being that I develop
rich cone 10 copper reds by firing either in oxidation or in neutral, and
then I reduce the kiln when the kiln is cooling. So after I reach cone
10, I close the damper and then let the kiln cool to a very dull red
color which is somewhere around cone 018 to cone 016. At this point I
turn the gas back on and reduce heavily for anywhere between 20 minutes
and 1 hour. In the past I would reduce the kiln extremely hard by only
opening the damper a crack and putting the gas on full. But I have
discovered that it does not require such a hard reduction. I think that
it would be possible to get a peach blossom effect by reducing very
lightly at cone 018 to 016. This type of firing has been termed "strike"
firing, and that is how I refer to it. But I have heard this term being
used to describe a number of different firing techniques.
I experimented by reducing the kiln at a higher temperature. I let the
kiln cool to cone 06 and then reduced. The problem that I had was that I
was not getting reds. Instead most everything was coming out rather
oxidized. So I think what was happening was that it was re-oxidizing on
the way down after the cone 06 reduction.
There was also the possibility of letting the kiln cool to a point below
cone 018. So the kiln would be dark when I reduced; sometimes this would
work, and other times it wouldn't. I have no idea of how low of a
temperature copper stops reacting to reduction, nor do I really
understand why the glazes would re-oxidize after I reduced at cone 06.
If any one has any information regarding this I would like to hear it.
A few of the copper red glazes that I used were:
Copper Red #25 (cone 10)
Custer Feldspar 78.7
Gerstley Borate 9.1
Copper Carbonate 1.0
Tin Oxide 1.0
#25 consistently came out as a very rich deep red. One of the real
advantages of strike firing for copper reds is consistency -- I was
consistently getting kiln loads of copper reds.
PV Red (cone 5)
PV Clay 50
Gerstley Borate 50
Copper Carbonate 3.5
PV Red becomes an intense copper red luster when strike fired. The
method of firing this glaze is the same as above, but instead this is
fired to only cone 5; then it is reduced in the same fashion. At this
point, I need to give credit to Steve Horn, since strike firing to cone 5
is the method that he uses, and he is a person who taught me a great deal
regarding these techniques.
I also worked with strike firing for developing high fire lusters,
because striking seemed to develop a kind of sheen to the pieces. In
particular I used it on shino glazes. The glaze that I used for this was
a shino from Fergus Stuart in Australia:
Shino (cone 10 - 15):
Nepheline Syenite 16 lbs
Kaolin 2.5 lbs
Ball Clay 2.5 lbs
Tin Oxide 1 lbs
Salt 1 lbs
Red Clay 1 lbs
The red clay that I use is a clay that I dig myself, but any red clay
And finally the latest glaze that I discovered to work in strike firing
is the base glaze for Coleman Purple, and instead of the regular
colorants, I used 5% Copper Carbonate. The glaze is as follows.
Custer feldspar 50%
Gerstley Borate 5%
Barium Carbonate 5%
Zinc Oxide 2%
Copper Carbonate 5%
This becomes a rich deep plum red luster when fired to cone 10 and then
reduced at cone 018 to 016. I also fired this glaze in a typical
reduction firing without reducing in the cooling phase, and the glaze was
a rather unattractive green.
There seems to be a great possibility for developing wonderful high
temperature lusters using high levels of copper (maybe 10 to 25 percent
copper carbonate). This is something that I will be exploring in the
future. The only real danger that I see is the possibility of melting
the pieces in the firing, because of the solubility of copper carbonate.
I hope that this information is of help to someone.