Chris Trabka on thu 8 mar 07
I've noted that in most copper red glazes have a copper carbonate to tin
oxide ratio of 1 to 2. I also note the alumina is low and there is some
boron.
My questions:
How much alumina can be present and still get a good copper red?
How much boron (if any) is needed?
How does ratio of copper carbonate to tin affect the color?
Has anyone performed testing to answer these questions?
Chris
David Hendley on thu 8 mar 07
----- Original Message -----
> My questions:
> How much alumina can be present and still get a good copper red?
> How much boron (if any) is needed?
> How does ratio of copper carbonate to tin affect the color?
> Has anyone performed testing to answer these questions?
Lots of research has been done concerning copper red glazes.
Here is the bibliography for my article "Simply Red", published
in Ceramics Monthly, October 1999:
Currie, Ian. Stoneware Glazes, A Systematic Approach, second edition.
Maryvale, Queensland, Australia: Bootstrap Press, 1986.
Hamer, Frank. The Potter’s Dictionary of Materials and Techniques. New York:
Watson-Guptill, 1975.
Rhodes, Daniel. Clay and Glazes for the Potter, revised edition. Randor,
Pa.: Chilton, 1973.
Sanders, Herbert H. Glazes for Special Effects. New York:Watson-Guptill,
1974
Tichane, Robert. Reds, Reds Copper Reds. Painted Post, N. Y.: New York Glaze
Institute, 1985.
Various authors. Studio Potter, volume 8, number 1. Goffstown, N. H.: Daniel
Clark Foundation, 1979.
Of course the Tichane book, being a whole book on the subject, probably
has the most information.
As for your specific questions, I don't think there are any precise
numbers.
For example, as you increase the alumina in the glaze, the quality of
the color slowly degrades. I like to formulate cone 10 copper red
glazes with .30 molecular equivalents of alumina because that puts
the alumina just within the limits formula of generally accepted stable
glazes. Really bright red glazes, however are usually "alumina deficient"
in functional and stable glaze terms - this means they will craze and
run. As you increase the alumina above .30, the color becomes less
brilliant. You, the potter, have to decide where in the scale you want
your glaze to be.
No boron is needed to make a copper red glaze. In fact, boron tends
to give glazes a bluish tint and is thus detrimental to the red color.
But again, the potter has to balance red color and functionality.
Boron is added because it is a flux with a low coefficient of expansion
and thus helps to control crazing. A red glaze that is loaded up with
sodium and/or potassium as the fluxes can be a very bright red, but
it will craze and run. Boron helps fix these problems.
I also add magnesium to copper red glazes. Again, substituting some
of the sodium/potassium with magnesium is actually detrimental to
the color, but the lower COE of magnesium makes it a useful addition
to red glazes.
Most copper red glazes also include barium and zinc as fluxes. The
purpose of my article, "Simply Red", was to determine if they are
really necessary to the development of good copper reds, and I
concluded that, no, they are not. The overwhelming conclusion of my
study was that kiln firing, not glaze formulation, is the most important
variable in producing good copper red glazes.
David Hendley
Maydelle, Texas
david(at)farmpots(dot)com
http://www.farmpots.com
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