Karen Sullivan on sat 4 nov 00
I am in no way an authority with regard to firing...but when asked about
firing for red, it had been awhile, so I did a kiln load to reinforce
whether or not it worked, and other than falling asleep while firing and
going to cone 12, I got lots of red. Bright, bold, copper red.
My fingers are split from the cold and wet and so bear with me while I type
a lot...
The glaze I used was Pete Pinnell's copper red, cone 10
Custer spar 7380
gerstley borate 1020
whiting 1110
flint 490
copper carb 30
tin oxide 100
A note of interest, I mixed a batch at school with the Laguna gerstley
substitute and it was fine.
So my firing logic....
In my kiln, a 12 cu.ft. Halvorsen updraft gas kiln, I need to reduce for
body early...so I reduce at 1200 degrees...or my stoneware stays yellow.
So I reduce for about 10 minutes. At school, we fire a 30 cu.ft. Olsen, and
do not reduce that early...There is a concern for the clay becoming brittle
with reduction to early and too much.
Then I reduce at cone 1, for about 10 minutes (if I have a shino, I may
increase the reduction time for 5-10 more minutes).
The logic for reduction at cone 1 is that the clay is only beginning to
vitrify, and I don't want carbon coring, but I want some affect on the clay
body. I was always told that short quick reduction is better than long/hard
reduction.
Then I reduce at cone 5, for about 10 minutes. At this point the glaze is
molten and the reduction will affect the glaze.
All other times of the firing I watch the two peepholes, there is usually
backpressure coming from the top peep, and the flame is barely flicking out
of the bottom peep. That is my judgement of reading for a neutral
atmosphere. I fire to cone 11, or 10 1/2 at least, when I reach temp, I keep
the damper open until I see the flame disappear out of the top, perhaps
about 1 minute, I close up and am done.
For copper red, I return when the kiln is around 1500 degrees, or a dull
red, and I open the damper a bit, turn the gas on and reduce for about 5-10
minutes. At first there is a cracking/popping sound which settles and quiets
a bit. Afterwards, I turn off the kiln, close the kiln up and wait for it to
cool.
The logic is, I think, that the kiln is re-oxidizing as it cools, any kiln,
and since the copper is so volatile (fugitive?) you want to expose the glaze
to a reducing atmosphere so the outside surface will go red on the cooling
cycle. The gurus may have a much more specific description of why it works,
but it works beautifully every time. No moose liver brown but bright, cherry
red.
I typically use the glaze over porcelain...
I need to let you know that I haven't done copper reds for several years,
and revisited them this past month, I wanted to see if I still could do it.
Something I remember P.Pinnell saying was that heavy reduction was like
trying to get a basketball through a window screen and an ever shrinking
window screen. Pinnell's brilliance is his use of metaphors to illustrate
complex concepts that use visual imagery to make the idea memorable.
Hope this helps, bamboo karen
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