Bruce Girrell on sun 20 aug 00
We've been doing a lot of experimentation here lately and one of the thin=
gs
we tried was a copper matte. We have avoided this up to now, partly out o=
f a
fear of color loss and partly because the surface is, in some ways, almos=
t a
clich=E9. After seeing some spectacular pieces at a show recently, though=
, we
thought we'd give it a go.
We used a 90/10 copper carbonate/frit 3110 mix with spray application. I
fired in strong reduction and placed the piece in a bin with sawdust, whi=
ch
normally gives me my strongest blacks and other reduction effects. I burp=
ed
the can several times until it would no longer reignite. The first attemp=
t,
which was sprayed in a rather thin layer produced muted, dark colors - if
you could even call them colors.
The following week, I sprayed another coat on the same pot, this time mak=
ing
sure that I got a fairly heavy coat on. The first time I could see the
texture of the clay through the glaze. This time the coat was thick enoug=
h
to hide the clay texture. I also added CMC because the glaze would come o=
ff
the pot at the slightest touch the first time. I fired the same way - str=
ong
reduction, sawdust bin, burp until no more flame. This time I got a bumpy
(sort of overfired looking) copper surface - no other color at all.
In the archives I find plenty of discussion about the recipes for copper
matte, but nothing about the firing conditions or post-fire handling. So =
-
some questions:
1) Is a copper matte fired in reduction or oxidation, or doesn't it matte=
r?
2) How do you determine glaze maturity?
3) How should the pot be handled in post-fire reduction? How essential is
burping?
4) Do you see the colors as they develop or does the magic happen inside =
the
can?
Bruce "feeling like a complete novice again" Girrell
Lamar, Luke on mon 21 aug 00
Hi Bruce, hope this helps,
1) It depends on the look you are going for. If I want mostly copper I fire
in heavy reduction; rapidly drop piece in can of newspaper; put newspaper on
top and shut the lid. I don't open the can until the piece has cooled. If I
want a surface with coppers, reds, blues, and yellows, I fire in heavy
reduction. I wait a few seconds before post-fire reduction. After about 5 -
10 minutes I open the can. If I have achieved the colors I am looking for, I
quench the piece in a bucket of water. If the piece is still looks very
copper, I wait for the colors to change (re-oxidation) then soak in water.
If the piece doesn't turn out, I refire.
2) I have a shiny copper glaze (Copper Penny) that matures at the same rate
as my copper matt, so I put a Copper Penny test tile or piece near the
peephole with my copper matts. When the Copper Penny piece becomes smooth
and shiny I know the firing is done.
3) Answer 1 explains how I do it. Generally, I leave my coppers alone until
cool or ready to quench.
4) Sometimes when I open the can up too soon, I can see the copper changing
color (usually to a reddish brown then to a dark and dirty green).
A note about quenching pieces in water: try to submerge the entire piece as
quickly as possible. The water should fill the inside as well as surround
the outside. If the piece has a narrow opening, don't submerge - the piece
might crack or explode.
Luke Lamar
some questions:
1) Is a copper matte fired in reduction or oxidation, or doesn't it matter?
2) How do you determine glaze maturity?
3) How should the pot be handled in post-fire reduction? How essential is
burping?
4) Do you see the colors as they develop or does the magic happen inside the
can?
Bruce "feeling like a complete novice again" Girrell
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Fraser Forsythe on tue 22 aug 00
Hi Bruce.
Some of what I've found...
> 1) Is a copper matte fired in reduction or oxidation, or doesn't it
matter?
definitely do NOT fire in reduction. fire in oxidation. and fire to low
temp - 850
to 900c
> 2) How do you determine glaze maturity?
I use a pyrometer but if you're going by eye copper matts will appear to
look
slightly shiny. That's still probably too hot.
> 3) How should the pot be handled in post-fire reduction? How essential
is
> burping?
Some gurus out there swear by burping. I've never found it gives me
anything
consistent so I don't do it.
> 4) Do you see the colors as they develop or does the magic happen
inside the
> can?
>
in the can. Except copper reds at Raku temps actually develop during the
oxidation
period - out of the kiln before going in for slight post reduction
Here's my routine:
glaze: patina
gerstley borate 65
copper carb 20
bone ash 15
I dont spray I dip - water down the glaze if you want a thin
application. I think
it's safer too. I fire to 875-900c. (sorry but centigrade is what I
remember - nice
round numbers).
I use DRY DRY DRY sawdust. Was that too subtle?
I use a damp towel put over the garbage can to seal in most of the
smoking, and
leave the pots in for at least 30 minutes. Better for my lungs and I
think it
works.
Never have breakage - except platters sometimes!
The glaze you're using can produce some remarkable effects but it's not
easy.
Besides the 90/10 copper to frit glazes are pretty nasty in terms of
toxicity,
crawling, and concrete bucket syndrome. The glaze above gives really
nice colours
consistently.
Some other observations I've yet to fully understand:
1. sometimes I watch the pots burning in the can for a while until I see
bronze
colors forming on the surface esp. while flames are in contact. I then
cover up the
can with a towel and lid. gets good results - but not always.
2. sometimes the very first pot(s) of the day are amazing and things get
crappier
as the day goes on. May just be the copper accumulating in my brain and
distorting
my ability to be objective. but it might also have something to do with
the build
up of water in the sawdust in the can. - so change it regularly.
3. winter firings on sunny days with little wind are the best - which
adds to the
speculation that humidity may play a big role in colour on copper matts.
whatever
you do dont fire when it's really humid.
hope that helps. I have other glazes on my web site your free to check
out.
It's late, I'm tired and I hope this made sense.
Fraser Forsythe
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