Jess McKenzie on sun 18 oct 09
Bill Merrill Wrote, in part:
... The ancient Greeks got the red and black pots with a
complex reduction and oxidization process ( I can't remember
what is) ... Reduction in the body takes place at a low
temperature so the iron can be changed from red iron to black
iron. ...
..............................
You remember it well, Bill. What you wrote is almost exactly
what Joseph Veach Noble described in "The Techniques of
Painted Attic Pottery." (Attica, btw, is in the southeast part
of Greece.)
Simply put, the Greeks' method was to begin with a clay body
full of red iron oxide (ferric oxide), partially covered with
a terra sig ("Greek glaze"). Then they fired in reduction
until two things happened:
1. the iron converted to the black form (magnetite,
Fe(3)O(4)), and
2. the terra sig coating sintered, thus sealing the black
form from oxygen.
Then the potters reoxidized the kiln's atmosphere. The iron
*not* protected by terra sig turned back to red, and the
protected black iron stayed that way.
Those old pots are gorgeous. I'm surprised no one on the list
has tried the method.
Thanks, Bill....
~jess
Bill Merrill on sun 18 oct 09
Firing in reduction is not what industry standards do. Everything was
oxidization until artist potters came along and discovered what
reduction does to glaze and clay bodies. Ancient kilns that were wood
fired alternated between reduction and oxidization. Some of the ancient
potters discovered if the covered their pots in a saggar ( to keep
flyash off the glaze)and left some open spaces, it caused some amazing
blushing of the glazes, soft blue to copper red peach blum colors. The
ancient Greeks got the red and black pots with a complex reduction and
oxidization process ( I can't remember what is) There were many ways
to fire in the past as there ways to fire now.
=3D20
Reduction in the body takes place at a low temperature so the iron can
be changed from red iron to black iron. The toasty color on a stoneware
body is the color it is because it oxidizes as it cools. If the piece
is fluxed properly the interior color of the piece is grey, which is
reduced iron. There are lots of oxides that aren't affected by
reduction. For example Cobalt. It will stay blue in either oxidation
or reduction. Shino glazes take on the carbon trapped look if you
reduce heavily at 012-010. Reduction at the high temperature range is
also noticeable. =3D20
=3D20
Oxidization seems to make glazes more lucid if the end of the firings
are oxidized. Most reduction kilns will oxidize on cooling since most
kilns are not absolutely air tight. The blackness of the pots of Shiho
Kanzaki is caused by the pots being pulled into the coal bed and the
kiln opening is filled with wood at least 20 times and the entire kiln
is mudded up, every crack. The pots don't re oxidize. If a glaze runs
and completely shuts off the air in the underneath the foot of a pot,
the foot if unglazed will be grey to black.
=3D20
If you do no reduction until high temperature is reached, the reduction
will not penetrate a vitrified stoneware body and have the same effect
on a mat glaze as when reduced from 1650 degrees until finish. Try
using Rhodes 32 with 5% tin and fire it both ways( on stoneware)...quite
different.
=3D20
Porcelain has little iron in it and the reduction at early stages is
primarily so the oxides that need changing (copper, iron) are altered
to get copper red transmutation glazes and celadons etc.
=3D20
Slow cooling helps develop mat glazes especially well. Mat glazes have
a crystalline structure and they tend to be a more luscious surface than
it f they were cooled quickly. Many shiny glazes and glazes that
develop interior micro bubbles develop well if cooled quickly. Many
porcelain bodies that are translucent will benefit from faster firings
and coolings so the pyroplastic quality of the body doesn't warp or
slump. A very fluid porcelain body can distort, for example if a thin
cup has a handle on it the pot will deform and be elliptical after
cooling.
=3D20
I have some pictures on Flickr and you can see some of my glazes if you
are interested. =3D20
=3D20
Rules aren't any good unless the rule serves you. If you don't like
what you hear or read, try it your way....
=3D20
http://www.flickr.com/photos/25594880@N04/
=3D20
Please I am just passing on observations that have worked for me since I
started making pots in 1965. If you do things differently,
great...whatever works for you. =3D20
=3D20
=3D20
=3D20
Lee Love on mon 19 oct 09
Bill,
And modern gas/oil reduction is very different that
traditional cycling atmosphere in large woodkilns. Also, cool down
firing is not a panacea. It is good for certain effects. It is
counter indicated in high silica bodies because it promotes the
formation of cristobalite in the clay body.
--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/
"Ta tIr na n-=3DF3g ar chul an tI=3D97tIr dlainn trina ch=3DE9ile"=3D97tha=
t is, "T=3D
he
land of eternal youth is behind the house, a beautiful land fluent
within itself." -- John O'Donohue
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