mel jacobson on sun 5 dec 10
i would start with my favorite base cone 6 recipe.
add 5-7 percent iron ox, and then add bone ash to taste.
make about 500 pots and test with a variety of the above
glaze idea.
bingo. nothing to it.
think how proud you will be when you open the
kiln to bright red.
mel
from: minnetonka, mn
website: http://www.visi.com/~melpots/
clayart link: http://www.visi.com/~melpots/clayart.html
new book: http://www.21stcenturykilns.com
alternate: melpots7575@gmail.com
David Woof on mon 6 dec 10
Well I always did think Mel had a Santa fixation (what with his generosity =
=3D
and work with Iron Reds) =3D20
=3D20
He is right on with the simplicity of getting started. My first semester =
=3D
students go searching for colors using an established base clear. They lear=
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n much they didn't start out to learn along the way because soon they are a=
=3D
dding or subbing other stuff in addition to oxides and figuring out why wha=
=3D
t changed that much with a particular addition. Some I have to pry out of t=
=3D
he glaze lab so I can go home at night. =3D20
=3D20
Open ended assignments (or those regarded as points of departure) lead to d=
=3D
iscovery while still getting me the learning outcomes I seek.
=3D20
I present my assignment as a problem to solve and if I have 15 students I l=
=3D
et them know I want at least 15 solutions.
=3D20
David Woof.......Clarkdale=3D2C Arizona.......the trees are bare of leaves =
bu=3D
t the grass and wild mustard are growing tall. We are 2.5 months into our =
=3D
8 month spring time!!!=3D20
**********************************
=3D20
1. if i wanted cone 6 iron red
Posted by: "mel jacobson" melpots2@VISI.COM=3D20
Date: Sun Dec 5=3D2C 2010 7:36 pm ((PST))
i would start with my favorite base cone 6 recipe.
add 5-7 percent iron ox=3D2C and then add bone ash to taste.
make about 500 pots and test with a variety of the above
glaze idea.
bingo. nothing to it.
think how proud you will be when you open the
kiln to bright red.
mel
from: minnetonka=3D2C mn
website: http://www.visi.com/~melpots/
clayart link: http://www.visi.com/~melpots/clayart.html
new book: http://www.21stcenturykilns.com
alternate: melpots7575@gmail.com
=3D20
=3D20
=3D
John Hesselberth on mon 6 dec 10
I haven't seen anyone mention it yet--I might have missed it--but very =3D
slow cooling is a good way to get the various iron red oxidation recipes =
=3D
to be more reliably and brighter red. Try slowly dropping the =3D
temperature to about 1600F and holding it there for 2 hours. =3D
Alternatively refire pots that came out brown in a bisque firing. It =3D
will do the same thing. Iron reds are not much dependent on kiln =3D
atmosphere, but rather on recrystallization.
Regards,
John=3D
Neon-Cat on wed 8 dec 10
=3D93Recrystallization=3D94, the term John Hesselberth used in reference to
slow-cooling Iron Red glazes is a bit of a catch-all word in common
usage. Although fancy enough when used on its own, it does beg
completion, as in =3D91recrystallization of hematite=3D92 (alpha-Fe2O3, fer=
ric
iron, aka red iron oxide).
What really happens when firing an Iron Red glaze are processes that
lead to good aggregation of the hematite particles. They don=3D92t
crystallize or need to recrystallize but stay as they are throughout
the firing. What we want to do is encourage and make it easy for the
hematite particles to come together to make larger color centers. We
also want to make sure we have good dispersion of these color centers
and, most importantly, we want to make sure we do not obscure these
red islands of color once we do make them.
Hematite aggregation is accomplished on the way up in a firing
sequence. It can occur, too, during a peak temperature hold or during
a portion of a slow cooling cycle. Personally I=3D92d rather aggregate
hematite on the way up =3D96 slow cooling is fraught with peril.
To make this thread more inclusive for those working with native clay,
umber, ochre, and other materials that introduce iron into a glaze,
I=3D92ll mention that all iron compounds in clays or those used as
additives become transformed by thermochemical reactions to hematite
between 800-900 C in an oxidation atmosphere. An oxidation atmosphere
is crucial to success. Under certain conditions additives or
ingredients in the glaze recipe (magnesium and potassium bentonite are
examples) can prevent, inhibit or delay oxidation of Fe(II) iron, but
by 1000 C, given enough time to react, it=3D92s all hematite. (I fire my
native clay bodies slowly during this phase so that all my
non-hematite iron is transformed to hematite for great red-red color.)
Hematite does not enter into melt solution as firing progresses in
oxidation =3D96 it remains as discrete particles that can be thought of as
being embedded in the finished glaze later. (Some colorants, like
copper oxide and cobalt oxide can dissolved in the glaze solution;
they tend to form diffuse bleeding into the glaze that is seen as
non-crisp linear decorations or halos at color edges.) Iron that has
been released out of a clay lattice during the dehydroxylation of the
clay can become dissolved in the melt causing the glassy matrix itself
to become slightly colored, or this iron can be oxidized to hematite
particles that can then go on to aggregate happily with all the rest
of the hematite that may have been added to the recipe.
The particle size of hematite is important. Although somewhat
counter-intuitive, small particles actually aggregate more quickly. A
mix of particle sizes in non-hematite iron compounds, especially when
working at low temperatures, can yield streaky or molted surfaces in a
variety of pretty, but not necessarily desired hues =3D96 large particles
may oxidize more slowly and sometimes not completely.
The purity of the hematite itself can affect color. For example, in
nature aluminum may substitute for iron in the hematite crystal
structure; processes are sometimes manipulated during synthesis to
encourage this substitution in the interest of producing brighter
colors when hematite is later used.
One of the risks or rewards of slow cooling to produce
recrystallization (also spelled recrystallisation) is that we may form
other crystalline phases and their polymorphs during the process
besides the desired hematite color centers. There are whole families
of calcium silicates, magnesium silicates, and lithium silicates such
as wollastonite, pseudowollastonite, anorthite, gehlenite, dolomite
and diopside crystalline phases that might be created by slow cooling
after mid-range temperature firing. Many of these will accept some
iron (as released from a clay lattice) into their structures so iron
silicates may form and so change the color of the final glaze. Glaze
surfaces may become more interesting with crystal development but
there are trade-offs. For one, glaze durability is negatively
impacted. Another important downside =3D96 red color can become obscured
by the very crystals created. Even if one used enough hematite to
satisfy the dietary demands of an entire neighborhood for months but
ended up making too many other crystalline phases in the glassy glaze
matrix during firing and cooling, one could end up with a bright white
or yellow glaze with absolutely no hint of red. Refiring to bisque
temperature will not help either glaze durability or color if the
glaze has formed too many spurious crystals. Then there=3D92s that matter
of extra time and effort spent in this futile pursuit. What we want is
red color and this comes from the aggregation of hematite through
proper glaze ingredient selection, preparation, mixing, formulation,
and firing.
Anyway, trying to master all cone 6 glazes using just a few (slow
cooling and refiring) of the many craft techniques available to us
seems a bit odd to me. Every glaze is of a different composition, each
producing a unique finished fired glaze, and each requiring its own
special start-to-finish handling by the potter or clay artist.
If you love colors, glazes, and a look at how things work, =3D93Ceramic
Masterpieces=3D94 by W. David Kingery and Pamela B. Vandiver, is a great
book =3D96 it=3D92s not a text and is easily to read and understand. It
details at a comfortable science level just how ancient works and
glazes were made and fired. It=3D92s out of print but still available
through used book dealers. I recently got a nice, almost new copy for
about 20 dollars and I=3D92m really enjoying reading it now.
Happy glazing and good luck with those Iron Reds!
Marian Gooding
Neon-Cat Ceramics
www.neon-cat.com
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