Maggie Woodhead on wed 7 nov 01
Hello Clayarters and Kia Ora to friends I met at Denver,
I am prompted to write to this subject as I have had some success with getting
good reds but it did not come easily. A real learning curve.
Firstly we fire in a home made kiln built by Derek (husband) no plans, drawing
on the garage wall, his own ideas put into practice. It works! Beautifully!
One of the changes we made was to use a milli voltmeter instead of the usual
analogue meters used by most potters. I found some years ago that the readings
using this meter gave a quicker reading of what was happening in the kiln,
changes in air or gas or any other manipulation would register immediately.
Initially I used a chart that came from Nils Lou's book giving the temperature
equivalents for the millivolt readings. However I am so familiar with these
readings now that I tend not to think of the temperatures and have to look them
up if people ask.
We preheat the night before the firing for about an hour at a low heat, @ 2Kp.
The kiln fires to cone 9 -10 from start to finish in between 5 and 1/2 hrs and
7hrs, depending on the loading, weather and so on, with very even firing, the
bottom cone going down about 20 to 30 mins before the top.
The trick for going into reduction is to remove 2 bungs at the base of the
chimney which reduces the flow through the kiln and gives us a good light
reduction without reducing primary air or increasing the gas which we keep at
about 10 Kp.
The glaze recipe is Pete Pinnell's translated into materials available in New
Zealand with only one exception, I am using cuprous copper instead of copper
carbonate. My own idea, and I would be interested in the opinions of our
chemically astute members about this!
The other thing Derek and I agreed to try from the beginning was to "strike
fire" and using these methods have had improving colour with each firing. The
last one being the best yet.
We fire till the cones are down i.e. top cone 10 to '3'. The kiln is allowed
to cool with the damper covered takes about 2 to 3 hours, again weather
depending.
When the meter reads 9.3 millivolts, that is just over 900c, we relight one of
the two burners to a very low heat and watching the meter carefully allow the
kiln to cool through to 7.8 millivolts that is just below 800c making sure we
drop no faster than .3 of a millivolt in 15 minutes. I still want to experiment
and try draw rings for the second time (first time the glaze was too thin) to
capture the temp when there is the greatest increase in the colour. I think it
might be about 8.4 mv.
There are a few other points which are important. The glaze must be well made
and mixed to disperse the very small amounts of oxides, also one must glaze the
pots very thickly, more than for any other glaze. To this end I fire bisque to
about 920c in an electric kiln. SLOWLY. I had one disastrous firing because the
glaze was too thin. The pots give best colours if protected by other pots from
direct flame, i.e. stacked together in the centre of the kiln, still find there
are variations in the shading and occasionally a sort of crystalline effect that
was unexpected.
Thanks to Dale Mark and Tom Buck who gave me great encouragement last year when
I was asking for advice about how to find this the most prized glaze for a
potter. Not so popular with the buyers but we love it when the difficulties are
overcome and those luscious reds emerge from the kiln.
Thanks to Mel for telling me about I.T.C. it has made a big difference, to David
Hendley and Joyce Lee for odd scraps of information. You are all on the wall of
my pottery in the Rogues Gallery! Love you all.
----Best Wishes Maggie----
maggiew@clear.net.nz
David Hendley on thu 8 nov 01
Hi Maggie and other copper red junkies,
I have also been doing some strike firing this fall. I often hesitate
to do it because I have 'orders' in the kiln, and people are expecting
a certain look
Although I do everything ' by the seat of the pants', my firing
schedule, though approximate, is similar:
A fast firing to cone 10 (with heavy reduction in the cone 010 to
cone 06 range),
Slow cooling (all air blocked off as much as possible),
Reduction introduced again in 2 or 3 hours, in the 06 to 010 range.
More slow cooling.
My first problem was ashes. I simply started wood-firing again
during the cool down. The problem was that the glazes were
still 'sticky', trapping bits of ash, but not molten enough to
engulf and melt the ash.
These pieces are not ruined, but the glossy glazes have a rough
feel, more like a matt glaze.
I have no gas, so for my last try I kept the kiln closed up except for
small openings in the fireboxes and sprayed motor oil into the
fireboxes.
This seemed to work pretty good; upon removing the spy brick,
characteristic waves of reducing flame could be seen in the kiln.
The copper reds from this firing were perfect, but I sometimes
get great copper reds without reducing again during cooling, so
I'll have to try it over the course of several firings.
Some of my other glazes changed from strike firing.
Most interesting is my high titanium semi-matt. In my regular
firings, it tends to be ivory, breaking to blue.
In the strike firings, it turns yellow!
It's actually kind of yellow crystals over a pale blue base.
I think this may be a result of slower cooling, rather than
reduction.
Also interesting is my rutile blue glaze, which is actually my
copper red glaze, with added rutile.
It is a great glaze because it is definitely blue, but there is
a hint of red in it. If you break a piece with this glaze, you will
see that the glaze is actually red, with a thin layer of blue on
the surface.
In the strike firing, where a piece received maximum flame,
the blue does not develop, and the glaze is red throughout.
I got some bowls that are red on one side, going through
all shades of purple, and then blue on the other side.
My current thinking about firing copper reds, subject, of course,
to retractions and revisions at any time, is that you want to
reduce like heck in the 800-900 degree, cone 012-06, range,
either during heating or cooling, or both. Above 900 degrees,
too much reduction will just muddy up the color. You don't
want oxidation, but just slight reduction.
Contrary to Joyce's observations, I don't think anything
happens once you are cooler than dull red heat.
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
----- Original Message -----
From: "Maggie Woodhead"
To:
Sent: Tuesday, November 06, 2001 2:11 PM
Subject: Copper Reds and Strike firing
> Hello Clayarters and Kia Ora to friends I met at Denver,
>
> I am prompted to write to this subject as I have had some success with
getting
> good reds but it did not come easily. A real learning curve.
>
> Firstly we fire in a home made kiln built by Derek (husband) no plans,
drawing
> on the garage wall, his own ideas put into practice. It works!
Beautifully!
> One of the changes we made was to use a milli voltmeter instead of the
usual
> analogue meters used by most potters. I found some years ago that the
readings
> using this meter gave a quicker reading of what was happening in the kiln,
> changes in air or gas or any other manipulation would register
immediately.
> Initially I used a chart that came from Nils Lou's book giving the
temperature
> equivalents for the millivolt readings. However I am so familiar with
these
> readings now that I tend not to think of the temperatures and have to look
them
> up if people ask.
> We preheat the night before the firing for about an hour at a low heat, @
2Kp.
> The kiln fires to cone 9 -10 from start to finish in between 5 and 1/2 hrs
and
> 7hrs, depending on the loading, weather and so on, with very even firing,
the
> bottom cone going down about 20 to 30 mins before the top.
> The trick for going into reduction is to remove 2 bungs at the base of the
> chimney which reduces the flow through the kiln and gives us a good light
> reduction without reducing primary air or increasing the gas which we keep
at
> about 10 Kp.
>
> The glaze recipe is Pete Pinnell's translated into materials available in
New
> Zealand with only one exception, I am using cuprous copper instead of
copper
> carbonate. My own idea, and I would be interested in the opinions of our
> chemically astute members about this!
>
> The other thing Derek and I agreed to try from the beginning was to
"strike
> fire" and using these methods have had improving colour with each firing.
The
> last one being the best yet.
>
> We fire till the cones are down i.e. top cone 10 to '3'. The kiln is
allowed
> to cool with the damper covered takes about 2 to 3 hours, again weather
> depending.
> When the meter reads 9.3 millivolts, that is just over 900c, we relight
one of
> the two burners to a very low heat and watching the meter carefully allow
the
> kiln to cool through to 7.8 millivolts that is just below 800c making sure
we
> drop no faster than .3 of a millivolt in 15 minutes. I still want to
experiment
> and try draw rings for the second time (first time the glaze was too thin)
to
> capture the temp when there is the greatest increase in the colour. I
think it
> might be about 8.4 mv.
>
> There are a few other points which are important. The glaze must be well
made
> and mixed to disperse the very small amounts of oxides, also one must
glaze the
> pots very thickly, more than for any other glaze. To this end I fire
bisque to
> about 920c in an electric kiln. SLOWLY. I had one disastrous firing
because the
> glaze was too thin. The pots give best colours if protected by other pots
from
> direct flame, i.e. stacked together in the centre of the kiln, still find
there
> are variations in the shading and occasionally a sort of crystalline
effect that
> was unexpected.
>
> Thanks to Dale Mark and Tom Buck who gave me great encouragement last year
when
> I was asking for advice about how to find this the most prized glaze for a
> potter. Not so popular with the buyers but we love it when the
difficulties are
> overcome and those luscious reds emerge from the kiln.
>
> Thanks to Mel for telling me about I.T.C. it has made a big difference, to
David
> Hendley and Joyce Lee for odd scraps of information. You are all on the
wall of
> my pottery in the Rogues Gallery! Love you all.
> ----Best Wishes Maggie----
> maggiew@clear.net.nz
>
Maggie Woodhead on fri 9 nov 01
Hi David,
I was pleased to hear your firing is much like ours although we
have a small gas kiln and do not have the problems of ash etc. Shows we are on
the right road!
"A fast firing to cone 10 (with heavy reduction in the cone 010 to
cone 06 range),
Slow cooling (all air blocked off as much as possible),
Reduction introduced again in 2 or 3 hours, in the 06 to 010 range.
More slow cooling."
I am not yet sure that reduction has all that much effect on the colour.
The reduction we introduce does not seem all that heavy to me from my experience
firing a diesel kiln, (have only fired in a wood kiln a few times) where we
frequently had some members firing with clouds of smoke emerging from the he
chimney. What a waste?
An oxyprobe would no doubt tell us more but we have not dared the expense of
that to date. I too gain the reduction by the 'look and see' method and we have
reduction now that is comparatively light in that by merely removing the two
bungs at the bottom of the chimney we hold back the flow through the kiln and
reduce the oxygen in the body of the kiln. During the cooling down period,
when we relight the kiln we do not introduce any reduction at all, just what you
would get without introducing more gas and minimum air.
Not very scientific but getting the colour.
I have had some interesting results with glaze on glaze decoration using rutile
glazes under the copper glaze, so am interested in that part of your post. Some
really lovely and elusive effects. If I can get the pictures from Derek's
computer I will send you one off line, hope you can get them.
Have a few queries still to sort out. What causes the slightly smoky shades on
the odd pot? Could be thinner dipping. A bit of orange peel occasionally, why?
Very happy with what we get so far.
One more question, do you saggar or protect your pots in any way from direct
flame?
Thanks for your insight into this. ----Best Wishes Maggie----
maggiew@clear.net.nz
Jeff Lawrence on tue 13 nov 01
David Hendley wrote:
you want to reduce like heck in the 800-900 degree, cone 012-06,
range, either during heating or cooling, or both.
Hi David,
I don't know anything about wood firing -- does stoking always
entail reduction? I'm curious because the reds I've gotten
blushed brighter with an oxidation soak about 500F below top
temp (per suggestions from Dannon Rhudy and Hank Murrow). I
wonder if the atmosphere matters as long as you mosey rather
than free-fall down the temperature gradient. I never did try
a reduction soak on the way down.
I also had an attempted copper red bowl that came out transparent
tan on the outside with something ugly inside. I sentenced it
to kiln furniture duty, filled with sand and supporting large
rounded shapes. After 2-3 oxidation firings at cone 04 (1925-50F
in that kiln) the clear tan developed a beautiful ruby (but the
inside was even uglier!)
The bowl makes me wonder about the utility of a reduction
atmosphere on the way down.
Jeff Lawrence
jml@cybermesa.com
PS to Maggie: an orange peel copper red my friend Ray peered at
with his scope was due to phase separations -- lenses of glass
of different composition than the matrix, dimpling the surface.
The edges were sharply defined and the lenses looked like clear
amoebae or oil droplets. When we looked at them with and without
polarized light, the compositional difference was clear by the
different indices of refraction.
Dennis E. Tobin on tue 13 nov 01
Folks,
I remember Pete Pinnel at a worksop talking about copper reds. He did a
lot of research for Victor Babu while he was at KC on copper reds. He found
that if he did a post glaze firing in the elctric kiln to cone 08 of the
reduced copper reds the reds would come out brighter. He even told of
firing copper reds that had no color once they came out of the reduction
kiln and when they fired them to 08 in the electric kiln then they came out
red from he electric. Haven't tried it myself as our kilns produce reds
without a problem. They are updraft softbirck kilns with Johnson burners.
At the end of the firing we shut off the gass then keep the fans on and the
damper open for an hour, that keeps crystals from forming on our temoku.
Even though they are updrafts the Johnson burners create a great deal of
backpressure. My 2 cents
Dennis Tobin
Dennis Tobin
Admissions Coordinator
Associate Professor
Art Department
Miami University
Oxford, Ohio 45056
(513) 529-1505
vince pitelka on wed 14 nov 01
> So, to sum up everything we know:
> To achieve the best copper red glazes,
> 1. Fire in either oxidation or reduction during the heating phase.
> 2. Fire in either oxidation or reduction at top temperatures.
> 3. Fire either quickly or slowly.
> 4. Cool down from peak temperature either slowly or quickly.
> 5. Introduce reduction, or not, during cooling.
> 6. Remove pots from the kiln while they are still hot, or let them
> cool completely.
> 7. Refire in an electric kiln until you get the color you like, or not.
Thank heavens someone has finally summed this up in a clear and concise
manner. Thank you David Hendley!
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
David Hendley on wed 14 nov 01
Well, this is getting interesting, there are so many conflicting
methods for firing copper reds.
I am amazed that people are getting reds that change color
after removal from the kiln. NEVER seen anything like that.
Of course, I am the patient type, and let my kiln cool for 2
days until I can remove pots with an ungloved hand.
I don't understand how an oxidation firing to cone 08 could
bring out red from a clear glaze, either.
My experience concurs with yours, Jeff, regarding an oxidation
atmosphere during the hottest part of the firing.
In my wood kiln, I try to keep it in reduction the whole time
from red to orange heat, and then, above about cone 06,
range from mild reduction to oxidation. In my kiln, you will
always go into reduction for a few seconds when you first
stoke. In the later part of the firing, I let the atmosphere clear
completely before stoking again.
BTW, some people think you SHOULD free-fall the temperature
gradient on the way down.
So, to sum up everything we know:
To achieve the best copper red glazes,
1. Fire in either oxidation or reduction during the heating phase.
2. Fire in either oxidation or reduction at top temperatures.
3. Fire either quickly or slowly.
4. Cool down from peak temperature either slowly or quickly.
5. Introduce reduction, or not, during cooling.
6. Remove pots from the kiln while they are still hot, or let them
cool completely.
7. Refire in an electric kiln until you get the color you like, or not.
Any questions?
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
----- Original Message -----
From: "Jeff Lawrence"
To:
Sent: Tuesday, November 13, 2001 6:45 AM
Subject: Copper Reds and Strike firing
> David Hendley wrote:
> you want to reduce like heck in the 800-900 degree, cone 012-06,
> range, either during heating or cooling, or both.
>
> Hi David,
>
> I don't know anything about wood firing -- does stoking always
> entail reduction? I'm curious because the reds I've gotten
> blushed brighter with an oxidation soak about 500F below top
> temp (per suggestions from Dannon Rhudy and Hank Murrow). I
> wonder if the atmosphere matters as long as you mosey rather
> than free-fall down the temperature gradient. I never did try
> a reduction soak on the way down.
>
> I also had an attempted copper red bowl that came out transparent
> tan on the outside with something ugly inside. I sentenced it
> to kiln furniture duty, filled with sand and supporting large
> rounded shapes. After 2-3 oxidation firings at cone 04 (1925-50F
> in that kiln) the clear tan developed a beautiful ruby (but the
> inside was even uglier!)
>
> The bowl makes me wonder about the utility of a reduction
> atmosphere on the way down.
>
> Jeff Lawrence
> jml@cybermesa.com
>
> PS to Maggie: an orange peel copper red my friend Ray peered at
> with his scope was due to phase separations -- lenses of glass
> of different composition than the matrix, dimpling the surface.
> The edges were sharply defined and the lenses looked like clear
> amoebae or oil droplets. When we looked at them with and without
> polarized light, the compositional difference was clear by the
> different indices of refraction.
>
Edouard Bastarache on wed 14 nov 01
Hello Des & David,
Do you guys belong to
"The league of honest skeptical ones"?
(Hehehehe)
Later,
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
http://sorel-tracy.qc.ca/~edouardb/
http://www.absolutearts.com/portfolios/e/edouardb/
----- Original Message -----
From: Des & Jan Howard
To:
Sent: Wednesday, November 14, 2001 5:38 PM
Subject: Re: Copper Reds and Strike firing
> David
> Yes & No
> Des
>
> David Hendley wrote:
>
> > So, to sum up everything we know:
> > To achieve the best copper red glazes,
> > 1. Fire in either oxidation or reduction during the heating phase.
> > 2. Fire in either oxidation or reduction at top temperatures.
> > 3. Fire either quickly or slowly.
> > 4. Cool down from peak temperature either slowly or quickly.
> > 5. Introduce reduction, or not, during cooling.
> > 6. Remove pots from the kiln while they are still hot, or let them
> > cool completely.
> > 7. Refire in an electric kiln until you get the color you like, or not.
> >
> > Any questions?
>
> --
>
> Des & Jan Howard
> Lue Pottery
> LUE NSW 2850
> Australia
> Ph/Fax 02 6373 6419
> http://www.luepottery.hwy.com.au
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Des & Jan Howard on thu 15 nov 01
David
Yes & No
Des
David Hendley wrote:
> So, to sum up everything we know:
> To achieve the best copper red glazes,
> 1. Fire in either oxidation or reduction during the heating phase.
> 2. Fire in either oxidation or reduction at top temperatures.
> 3. Fire either quickly or slowly.
> 4. Cool down from peak temperature either slowly or quickly.
> 5. Introduce reduction, or not, during cooling.
> 6. Remove pots from the kiln while they are still hot, or let them
> cool completely.
> 7. Refire in an electric kiln until you get the color you like, or not.
>
> Any questions?
--
Des & Jan Howard
Lue Pottery
LUE NSW 2850
Australia
Ph/Fax 02 6373 6419
http://www.luepottery.hwy.com.au
Imzadi D. on sat 17 nov 01
Someone recently wrote, (unfortunately I deleted the email) that they add:
.25% cobalt carb to their red so they get blue instead of those ugly clear
patches, with which I am all too familiar with. (I don't control the glaze
firings where I work out of.)
How does this affect the RED color? Is it still the same shade, or overall,
more purple?
Imzadi
Ron Roy on sun 18 nov 01
When colour is the result of crystal formation - the re-firing simply gives
the crystals another time period to grow - the trick is to not begin to
melt the crystals again - just get the glaze melted enough to let em get
bigger. The crystals form on the way up (unless they start to melt) and on
the way down by the way.
Same thing for most glazes - some shiny glazes will get less shiny when
struck. Super cooled glass wants to rearrange itself into crystals to
satisfy empty valences - all they need is more time during the crucial
temperatures when this process is going on. Crystal growth is limited only
by available material and time during the crucial temperature range.
Slowing cooling between these crucial temperatures will have the same
effect as striking by the way.
Kiln controllers are ideal for holding a glaze at any given temperature or
range.
Wood fired kilns cool more slowly - due to the heat not being turned off
suddenly - which perhaps explains why David does not have a problem with
his reds.
Good section on crystals in the Hamer book.
RR
David Hendley said -
>I don't understand how an oxidation firing to cone 08 could
>bring out red from a clear glaze, either.
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Marianne Lombardo on sun 18 nov 01
What do you mean by "striking"? Can you elaborate on this? I have not
heard of this term.
Marianne
> Slowing cooling between these crucial temperatures will have the same
> effect as striking by the way.
>
>
Des & Jan Howard on mon 19 nov 01
Ron
A test pot glazed with a Cone 10 copper red came out of our
kiln clear in colour, quite substantial crazing.
When refired in a bisque firing to 980C the glaze was still clear,
but, the craze lines were in red with slightly raised
edges crystalline in appearance ????????
Des
Ron Roy wrote:
> When colour is the result of crystal formation - the re-firing simply gives
> the crystals another time period to grow - the trick is to not begin to
> melt the crystals again - just get the glaze melted enough to let em get
> bigger. The crystals form on the way up (unless they start to melt) and on
> the way down by the way.
>
> Same thing for most glazes - some shiny glazes will get less shiny when
> struck. Super cooled glass wants to rearrange itself into crystals to
> satisfy empty valences - all they need is more time during the crucial
> temperatures when this process is going on. Crystal growth is limited only
> by available material and time during the crucial temperature range.
--
Des & Jan Howard
Lue Pottery
LUE NSW 2850
Australia
Ph/Fax 02 6373 6419
http://www.luepottery.hwy.com.au
Ron Roy on tue 20 nov 01
Hi Marrianne,
Just looked through all my books and only found two references - Currie's
first book P 189 and 192 - he refers to Parmelee & Harman - page 466. It is
the process of reheating a glaze to the temperatures where crystals will
start growing again. This temperature range varies from glaze to glaze and
glaze type to glaze type. The trick would be to find what temperature
worked for each glaze you would want to grow crystals in - or make sure you
cool slow enough for it to happen as the glaze cools in the initial glaze
firing.
Some potters report wonderful results - Liz Willoughby strikes her Shino
glazes if she is not happy with them. Gets what I think are very good
results.
RR
>What do you mean by "striking"? Can you elaborate on this? I have not
>heard of this term.
>
>Marianne
>
>> Slowing cooling between these crucial temperatures will have the same
>> effect as striking by the way.
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
| |
|
