Ian Jaffray on thu 23 may 96
Re ^ cone 6 Ketchup red recipe which appeared on Clayart recently
(EPKaolin 5, Silica 30, Cust .Felds 20, GBorate 32, Fe2O3 15 ).
I made a test batch using local South African materials ( which
compare very closely to the American ones) but using Colemanite as I
did not have Gerstley Borate. The result was brown with greenish
tinges & small red dots/patches - far from a ketchup red.
Can anyone comment on why the red should have appeared in patches as
it did ?
Regards
Ian
PJLewing@aol.com on fri 24 may 96
You don't say whether you're doing oxidation or reduction here,but I'll
assume you're doing oxidation, since that's what most people do at ^6. Also
because it's what I've done quite a bit of research on iron reds in.
First of all, the problem may be your iron. Different brands and qualities
of iron make a HUGE difference in this color. Try every kind you can get
your hands on. Also, I think you may just have too much iron in this thing.
Second, I think you've got too much calcium. According to my Insight
analysis, you've got .88 molecular equivalents, which is way over the limit
formula. Most of the successlul iron reds I've gotten have abot .50 calcium,
and almost as much, but not quite as much magnesium. A simple switch to
Gerstley Borate would get you much closer to that. Then you may want to add
some talc or magnesium carbonate. That would also lower your potassium and
sodium, which might help, too.
Third, your silica/alumina ratio is too low. You need to be at least up
around 18. You're about 14.5 now. This, of course, will make this runnier.
Fourth, this may work just as it is if you fire it hotter. Or,if it's
practical, try refiring to ^04.
June Perry on fri 24 may 96
Dear Ian:
Many iron red glazes go green if they are applied on the thick side and red
when applied a a bit thinner. Make sure that your batch is very well mixed
and sieved and apply the glaze a bit thinner than you did and then double dip
part of your test tile. This will give you a better indication.
Other than that, there maybe a difference in the purity of materials in
South Africa.. In the past when colemanite was available in this country
there could be a great variation from batch to batch even from the same mine.
From what you say I suspect that thinning the glaze a bit more might yield a
bit more of the red color.
Perhaps your will get some more feed back and other suggestions from other
list members.
Good luck!
June Perry
EMail: Gurushakti@aol.com
JoAnn Axford on fri 24 may 96
I think you are missing an ingredient in your glaze. I have a Ketchup Red
formula the same as yours but with 14% Talc. See if this helps. Also, this
glaze will produce a beautiful iron red only on a clay body with lots of
iron. On porcelain or any other light clay, it is an ugly brown. Check your
formula and try it on a brown clay, JoAnn Axford near Albany, N.Y.
Brian P. Jones x2712 on fri 24 may 96
The color only develops on cooling. If you cool too
fast, you will end up with medium brown with spotting
as you said. You can refire your existing peices to
cone 06-04 for more color development. You will probably
end up with more "red" and less brown.
The glaze works by having a saturation of red iron oxide.
On cooling, the iron precipitates out of the glass lattice
and collects on the surface. I believe the color formation
from this process begins at about 1000C with slow cooling
from there. The longer you hold out the cooling process,
the darker the color.
Good luck.
Brian
Don Kopyscinski on fri 24 may 96
The first try bounced back, I hope it's not a duplication.
Hi Ian,
You wrote regarding your iron red glaze:
> The result was brown with greenish tinges & small red dots/patches - far from
a ketchup red.
Can anyone comment on why the red should have appeared in patches as it did ?<
Just yesterday I was looking at some test tiles under a 10X loop. In examining
an old Kaki glaze tile I noticed that in part the verigated rusty red color was
due to crystalization on the surface of the glaze. There were actually about
three distinct components to the glaze. A brown background, some rather
strikingly red crystals, and some that were a kind of rusty brown. As is seen
with zinc crystalline glazes, crystals have an interesting capacity to be
selectively colored (I have had brilliant blue in a golden yellow glaze and
several other interesting combinations - this was 10+ years ago, I'm a
production functional potter these days.). My conclusion is that the distinct
red can be had but, a control of the cooling cycle is important.
Each glaze which is prone to crystallization, has a distinct "zone of
crystallization" (Norton breifly touches on this in "Elements of Ceramics,
Parmelee discusses crystallization in depth in 'Ceramic Glazes" P. 204-214, and
shooting from the hip, I would expect that Cardew mentions it in Pioneer
Pottery.). In addition, a varied type, size, number, and perhaps color of
crystal may be had by varying the time spent in a particular zone. It doesn't
matter to the glaze if it is going up or down in the zone, as long as it is
above the set point of the glaze, and within the zone crystals will form (that
is, after a full melt). I believe this is why there were a variety of reports of
enhanced color response to re-firing iron red glazes in a bisque kiln quite some
time back. Let's say the zone which is preferential to the red color crystal is
between 1400-1700 degrees (hypethetical assumption to illustrate the point). The
re-fired pots to cone 06 were given significant addtional time in the target
range to allow more red to form (both on the way up and down).
I have a glaze which I can make blue, or pink, or blue with pink patches,
depending on the rate of cooling. In experimenting with cooling cycles I once
also hit on a distinct yellow crystal from the same glaze in addition to the
pink (the only difference in the cycle being a slightly extended 1500*F to
1200*F range). In my normal cooling cycle, these (bordering on micro)crystals
will be be pink, the background glaze blue, and the percentage of pink can be
adjusted by controlling the cooling rate and in so doing, the amount of time in
it's particular zone of crystallization. I would also add that this may be
partially responsible for so many gas fired glazes being considerably more matt.
In addition to other factors, the kilns usually have a greater thermal mass and
thus cool more slowly, allowing more time in the zones where mattness is
developed often through microcrystalline development. I have a glossy blue glaze
that I can turn into a satin matt by varying the rate of cooling (most
pronounced in magnesium bearing glazes). I will add that I think that a lot of
problems people encounter with glazes that lack depth, are a result of low
thermal mass kilns cooling much too quickly.
A monitoring of kiln cooling behavior and it's relationship to crystalline
growth in glazes will most likely let you hit on the zone that is most
preferential to the red color. I have had varying results with different Iron
Oxides as well (best was had with "Dark Red Iron Oxide" once available from the
now defunct Cutter Ceramics. (Anyone know what grade of iron this was and where
a supply may be obtained?....Please drop me a note.) Try all the variations you
can get your hands on and see which one best promotes the color you're after).
Off to glaze,
Don Kopyscinski
Bear Hills Pottery
Newtown, CT
Don Kopyscinski, 74134.2672@compuserve.com
or
DonKopy@aol.com
John Baymore on sun 26 may 96
RE: Some more thoughts on saturated iron reds.
The microcrystaline surface of kaki (persimmon) type "red" glazes is extremely
dependent both on cooling rate, and on the atmosphere during cooling. The
vairiations in the glaze surface are also greatly affected by the level of
reduction and the iron content of the underlying clay body.
Don Kopyscinski's post yesterday was PACKED full of very accurate observations
on the development of microcrystal (and macrocrystal) surface effects. Those
with computer controlled kilns have a great tool to play with in developing
these types of glazes. The critical factors here are glaze
composition........... temperature..... duration...... and atmosphere.
In my wood fired noborigama (very FAR from a computer controlled kiln ), I
use a single kaki over an iron bearing stoneware that gets greatly different
results depending on which chamber I stack it in. This is true not only because
of the different atmospheres that I use in the chambers on the heating cycle,
but also on the effect of the cooling rates and the composition of gases flowing
through the chambers. The difference between chamber 1 and chamber 2 is
dramatic.
First of all, there will be heavy iron spotting from the body bleeding into the
glaze in chamber 1. This is because the first chamber is fired in heavy
reduction from about cone 04 to cone 10.
In chamber 1, just after the main firebox chamber, the kaki will go a dark
tessha with a more crystalized, matt'er surface. This is because of the fact
that when the main firebox is shut down there are a lot of coals still under the
grate, and the main firebox is at about cone 14. Since most of the air supply
through the main firebox is cut off (but not all) when stoking is switched to
the second chamber, the early cooling cycle in chamber 1 is slightly reducing.
There is also a lot of heat still being produced by the coals burning off in the
main firebox, so the early cooling cycle in chamber 1 is greatly retarded. (In
fact, you have to be careful to not have the temperature INCREASE after you stop
stoking the main firebox!)
This causes enhanced development of larger sized crystals in my particular kaki,
and some of the iron percipitating into the crystal structure floating on the
surface of the glaze is in the reduced black state. Hence the darker tessha
coloration. This atmosphere and temperature climb/drop on cooling can (and must
be) be controlled by the amount of air being allowed to flow through the primary
air ports on the main firebox.
The second chamber is usually fired on the heating cycle only slightly reducing
to neutral. There is slightly heavier reduction at about cone 04 to cone 2 to
get the iron involved and disbursed in an early glaze (and body matrix) melt,
but then the atmosphere is cleared out a bit up to cone 9. The iron in the
body is much less active with the glazes because of this. While iron spotting
is evident, it is much less pronounced.
The second chamber firebox is internal to the chamber, and does not have the
large pile of coals at the end of the stoking. By the time the second chamber
has been fired off and is into the cooling cycle, the coals in the large main
firebox are long gone. The air flow through the still hot main firebox and
first chamber preheats the primary for the third and fourth chambers. So
chamber two is getting a flow of very hot (reactive) oxygenated air through it.
The cooling cycle here is greatly oxidising, but a bit faster in the early
stages than in chamber one. It is then greatly drawn out due to the huge
thermal mass that the main firebox and chamber 1 provide to the flow of air.
The result is a tomatoe red with a lustorous sheen created by very small
crystals on the surface. These smaller crystals form in my glaze because of the
sustained temperature at a lower temperature than in chamber 1. This chamber
cools very slowly because of the huge thermal mass on either side of it (two hot
chambers on either side.)
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