Jeff Tsai on sun 19 may 02
I think you will find more help available if you detail your question more.
You are using raku clay you say and you say you are firing in an electric
kiln, however it would help if you also provided us with:
The temperature you are firing your glaze fire to.
The glaze recipe which you are currently using, along with firing
Firing schedule of the kiln.
how you apply the glaze.
You also say you're having problems with grey patches...while I don't know
what could be causing this, I'm sure someone might be able to help you if
included the recipe. It might be application, copper reds in reduction can
clear when thin, and over a iron bearing clay body like the raku body
probably is, this could appear grey.
I highly doubt it has to do with carbon trapping which is a characteristic
more often associated with gas firing in which you can reduce the atmosphere
of the kiln. Unless you are throwing in pieces of wood into the electric
kiln, it shouldn't be reducing and no carbon trapping would occur.
You should repost with the needed information and others will be able to
accurately assess your dilema and help you.
Jeff Tsai on sun 19 may 02
I also just remembered, if you are working as cone 6, you might try
at Ron and John's "Mastering Cone 6 Glazes" I recall seeing a copper red=20
recipe for oxidation in the book.
If that is not a possibility, here is another cone 6 glaze that may help
jumping off point for further TESTS. Note the notes at the bottom of the=20
glaze recipe, and note that this is not one of MY glazes, I've never used
I found it in the CLayart Glaze database:
Glaze Name: Copper Red
43.88 Kona f-4 feldspar
13.27 Ferro frit 3134
4.08 zinc oxide
100.00 % Totals:
2.04 Tin Oxide
1.02 Black Copper Oxide
>From the ClayArt Glaze Recipe Database at SDSU.
If you test this glaze and find it useful=20
please consider resubmitting it to the GlazeBase database.=20
estimated thermal expansion: 78.82x10-7/=B0C
Unity Formula for Copper Red:
0.060 K2O 0.279 Al2O3 2.539 SiO2=20
0.182 Na2O 0.119 B2O3 9.1:1 Si:Al Ratio
Percentage Analysis for Copper Red:
60.70 % SiO2
11.30 % Al2O3
3.30 % B2O3
2.27 % K2O
4.50 % Na2O
12.58 % CaO
0.96 % MgO
4.42 % ZnO
Possible Health Hazards: Flint: free silica-wear a NIOSH approved dust
when handling dry material Talc: wear a NIOSH approved dust mask when=20
handling dry material=20
I am looking for a cone 6, reduction Copper Red glaze recipe, and would=20
appreciate any suggestions or ideas. The one that we are currently using
bit too runny at cone 6. We tried a line blend with a copper red that I
used at cone 10, but the color went to a pink as soon as we moved it up
scale enough to not make it so runny. Thanks, Rick Malmgren
Craig Martell on tue 21 may 02
After reading Carole Tripp's advice on electric reds I thought of something
that may pertain to this issue. Carole's advice was very good, I might add.
Copper Reds are hard enough to do in a gas or wood kiln. Firing copper
reds in an electric can be a very discouraging affair. Instead of trying
to use the electric for something other than what it will do best, try
another approach to reds. Mainly, chrome tin reds, pinks, mauves, and
whatever sort of hue you can weasel out of your glaze. Once the chemistry
is worked out and you have a good base for chrome tin reds, the results
will be light years ahead of copper in the electric and very consistent
from firing to firing.
Some basic advice on chrome tin reds is to totally eliminate zinc oxide
from the glaze and have plenty of calcium. Feldspar, whiting or
wollastonite, boron (use Ferro Frit 3134), plus clay and silica will make a
very good base for a chrome tin glaze.
regards, Craig Martell in Oregon with very little spare time these days