winston taylor on wed 30 aug 00
I'm seeking info about firing temp and procedures for copper matt raku
glazes. The recipe I have is 150 grams copper oxide, 10 grams iron oxide, 15
grams frit 3110. I'm not getting those rainbow colors of reds, violets and
blues etc. HELP.
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Bruce Girrell on wed 30 aug 00
Winston Taylor wrote:
>I'm seeking info about firing temp and procedures for copper matt raku
>glazes. The recipe I have is 150 grams copper oxide, 10 grams iron oxide,
15
>grams frit 3110. I'm not getting those rainbow colors of reds, violets and
>blues etc. HELP.
Well, Winston, I just asked the same question about a week ago, Check the
archives (August 2000) for the subject "A question about copper matte".
Thank you to all those who replied to me on- and off-list.
Here is what I have learned in the ensuing time:
1) Be careful not to overfire this glaze. It gets all bumpy and ugly.
2) Put it in the can and leave it in the can until it is cool enough that it
won't reoxidize.
3) The application should be relatively thin. The most successful results
that I have gotten so far (which is not all that successful yet, but loads
better than my first attempts) were dipped, but I think that spraying will
produce a better coverage. If it's on thick enough to run, it's on too
thick. Be careful of overspray. I glazed the interior of some pieces with
white and sprayed the outside with copper matte. The tiny amount of
overspray that got inside the pot (invisible to the naked eye) turned the
entire inside of the pot red.
4) Use some CMC or other hardener in the glaze. If you don't, merely
touching the pot will remove the glaze.
I got a couple of off-list replies besides what you see in the archives. I
have not had time to evaluate the suggestions that I received there. One
person warned me that this glaze is quite old and other better, less costly
versions have been created. This makes sense to me. The large quantity of
copper, with nearly no glass former, is almost certain to make colors that
fade after a disappointingly short period of time.
Bruce "still learning too" Girrell
Frances Pusateri on thu 31 aug 00
Winston,
The raku copper matt glaze we use is Copper Sand Matt from Robert
Piepenburg's book Raku Pottery. The formula is 8 parts gerstly borate,
2 parts bone ash, =BD part copper carbonate, =BC part cobalt oxide. We =
are
lucky to have several hundred pounds of gerstly borate available until
reliable substitutes are available. If you don't have access to =
gerstly
borate try http://gerstleyborate.com for
available substitutes. When I first started experimenting with Raku, I
tested several different copper matt glazes and Copper Sand Matt was by
far the most reliable in terms of developing the blue, green, magenta, =
&
copper red flashing during the post fire reduction. I teach in a
community clay program and we use this glaze successfully with all our
students, beginners to advanced, kids and adults. In terms of firing, =
I
don't fire to a specific cone. We use several different glazes,
including the Copper Sand Matt, Soldner's Raku Base 80/20 (80% Gerstly
Borate, 20% Neph Sy), Black Raku (4% Borax, 43% Gerstly Borate, 21% =
Soda
Ash, 21% Barnard Clay, 11% Neph Sy, 4% Cobalt Carb, 2% Copper Carb),
and White Crackle (80% Gerstly Borate, 20% Neph Sy, 5% EPK, 4%
Zircopax). Most of these glazes melt at approx. cone 05, although we =
do
have some glazes that melt at approx. cone 08. I always have one of =
the
non matt glazes in each firing and I use this pot as the indicator that
temperature has been reached. In my experience with Copper Sand Matt,
it has a wide range for its firing temperature and this hasn't impacted
the development of color during the post fire reduction. What I have
noticed in terms of color development is that glaze application is
important. If the glaze is too thin, it will be dark blue with no =
color
and you will be able to see the clay underneath. If the glaze is too
thick, it will craze. When I fire the kiln after the first firing that
includes heating up the brick base of the kiln, I fire to temperature
quickly (15 - 20 minutes) in an oxidizing atmosphere, then reduce for
the last 2 - 3 minutes. We pull the pots quickly out of the kiln and
place them in the reduction materials. We use shredded paper, sawdust,
wood chips, and straw. We reduce under metal paint cans to metal trash
cans or wash tubs depending on the size of the piece. I personally
prefer straw and wood chips as a reduction material. We make a "nest"
of the reduction materials and then place an inverted can over it to
reduce. An observation I have made is that if you loosely arrange the
reduction materials as high as your pot, you get a good flame licking
over the piece and you get good color development over the entire =
piece.
If you watch the pot carefully as the flame licks over it, you can see
the color development and can direct it somewhat by using a can lid to
move the fire over the piece. If you do this, you need to be careful
not to let too much of the reduction materials burn away or you won't
get good reduction when you smother the fire with the can. For larger
pieces, I have made a "charcoal chimney" out of cardboard (cardboard
tube at least 4 - 5 inches wider than the widest part of the pot with
several holes in it in a ring about 1 inch from the bottom and slightly
taller than the shoulder of the pot). This device pulls the flame up
from the bottom of the pot and distributes it more evenly over the
shoulder. You need to be careful though, with a large pot, a lot of
heat can be generated and flame may shoot 10 feet in the air. I let =
the
pot cool until the atmosphere in the can is mostly clear, about the =
time
it takes for the next load to fire. I then remove the pot and douse in
water to "freeze" the color. The Copper Sand Matt is particularly nice
used in combination with the Black Raku glaze.
Also, I wanted to thank Steven Branfman, Ken Kang, Richard Jeffery, and
others who responded to my problem with our Raku Kiln. I am still
having problems with one burner (I think it must be partially clogged
somewhere above the burner orifice), but using your suggestions about
increasing the secondary air flow, etc I was able to fire the kiln to
temperature in 15 minutes or so with only minor problems with the one
burner. We had our first firing last night for my workshop, with
successful results and many happy students with nice pots. Thanks for
your support.
Francie =20
Frances M. Pusateri
Colorado Division of Wildlife
317 W. Prospect
Fort Collins, CO 80526
(970)472-4336
(970)472-4458 FAX
francie.pusateri@state.co.us
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