Craig Martell on wed 31 oct 01
Hi:
I've worked with blue celadons a bit and have been able to make a few good
ones. The firing schedule is only part of achieving blues with iron
oxide. The glaze base is very important as well as the claybody. You
don't give any info about your clay or glazes so I can't comment on that
stuff. I use porcelain and get the best sky blue colors on a grolleg body
or on my domestic porcelain with grolleg slips.
The firing schedule is as follows. Oxidize until cone 06. At 06 set the
kiln in reduction with the probe reading 0.7. Let the kiln climb in
reduction with no further adjustment until cone 5. At cone 5, adjust the
damper to get a reading of 0.55 and let the kiln climb to cone 8. At cone
8, put the kiln into heavier reduction with the probe reading 0.65. No
further adjustment until cone 10 is down, just maintain the oxy reading of
0.65. Cut the btu input to the kiln by about 66% when cone 10 is down and
adjust the damper to get an oxy reading of 0.50 and soak the kiln for one
hour. Then shut down and close the ports and damper to let the kiln cool
slowly. I should mention that a 66% decrease in btu input at cone 10 will
hold the temp steady in my kiln for about a half hour and then I start to
drop very slowly. At the end of the hour soak, the kiln is at about cone
9. Also, from the time cone 8 starts to soften to the point that cone 10
is down is about a three hour span of time. High silica blue celadons will
be very fat and deep if you traverse this temp span slowly.
This works for my blue celadons and my other glazes on my claybodies. I
don't guarantee it will work for anyone else. You never know though.
regards, Craig Martell in Oregon
Roger Korn on thu 1 nov 01
Hi Craig,
Do you use a flow meter to monitor BTU input, or just drop the pressure to .66^2 =
44% of the previous pressure? I'm trying to decide whether I need to measure actual
flow or if the assumption that flow through an orfice is proportional to the square
of the pressure is "close enough".
Many thanks, and say 'Hi' to Mel for me. He demoed how to teach a complete beginner
to throw and I've shamelessly ripped off his method with great success.
Roger Korn
McKay Creek Ceramics
rogerk1941@earthlink.net
presently in AZ: 520-567-5699
back in OR, around 10 December: 503-647-5464
cell: 503-312-2781
Craig Martell wrote:
> Hi:
>
> I've worked with blue celadons a bit and have been able to make a few good
> ones. The firing schedule is only part of achieving blues with iron
> oxide. The glaze base is very important as well as the claybody. You
> don't give any info about your clay or glazes so I can't comment on that
> stuff. I use porcelain and get the best sky blue colors on a grolleg body
> or on my domestic porcelain with grolleg slips.
>
> The firing schedule is as follows. Oxidize until cone 06. At 06 set the
> kiln in reduction with the probe reading 0.7. Let the kiln climb in
> reduction with no further adjustment until cone 5. At cone 5, adjust the
> damper to get a reading of 0.55 and let the kiln climb to cone 8. At cone
> 8, put the kiln into heavier reduction with the probe reading 0.65. No
> further adjustment until cone 10 is down, just maintain the oxy reading of
> 0.65. Cut the btu input to the kiln by about 66% when cone 10 is down and
> adjust the damper to get an oxy reading of 0.50 and soak the kiln for one
> hour. Then shut down and close the ports and damper to let the kiln cool
> slowly. I should mention that a 66% decrease in btu input at cone 10 will
> hold the temp steady in my kiln for about a half hour and then I start to
> drop very slowly. At the end of the hour soak, the kiln is at about cone
> 9. Also, from the time cone 8 starts to soften to the point that cone 10
> is down is about a three hour span of time. High silica blue celadons will
> be very fat and deep if you traverse this temp span slowly.
>
> This works for my blue celadons and my other glazes on my claybodies. I
> don't guarantee it will work for anyone else. You never know though.
>
> regards, Craig Martell in Oregon
>
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Craig Martell on thu 1 nov 01
Roger axed me if:
>Do you use a flow meter to monitor BTU input, or just drop the pressure to
>.66^2 =
>44% of the previous pressure?
Hello Roger:
I was talking about reduction of gas pressure. You make a good point
because the difference in % of gas pressure and % of btu input would be
different. I'm just concerned with holding the temp for a bit and then
letting it drop off slowly. More heat work.
later, Craig Martell in Oregon
Jeff Lawrence on thu 1 nov 01
Craig Martell was talking about his blue celadons:
I use porcelain and get the best sky blue colors on a grolleg body
or on my domestic porcelain with grolleg slips.
Hello Craig,
I'm puzzling over some blue tests and am curious about your
blues
- Are your blues less blue on brown claybodies? what color
do you get on dark clay?
- do you stick with grolleg body or slips to avoid titanium
greening?
- Have you ever tried to determine if any of your blue is
caused by silica opalescence vs. reduced iron? (e.g. by
tossing a sample into a similar electric firing.)
Thanks in advance for any data points,
Jeff
Jeff Lawrence
jml@cybermesa.com
Craig Martell on thu 1 nov 01
Jeff asked:
>- Are your blues less blue on brown claybodies? what color
> do you get on dark clay?
Hello Jeff:
I don't know. I've never tried them on anything but porcelain. I know
it's wise to test glazes on more than one type of clay but I really don't
plan on doing anything other than porcelain so I don't bother. I would
think that the glazes would be less blue and more towards green and grey
celadons on iron and titanium bearing clays.
>- do you stick with grolleg body or slips to avoid titanium
> greening?
Yes. The grolleg body that I use produces the brightest blue celadons. My
other body, which is domestic kaolins with a small amount of ball clay
pushes the glazes a bit towards a grey blue but not green.
>Have you ever tried to determine if any of your blue is
> caused by silica opalescence vs. reduced iron? (e.g. by
> tossing a sample into a similar electric firing.)
I'm reasonably certain that the blues are not due to opalescence as in the
"chun blue" effect. This is hard to describe with words but the blue is
more "solid" looking and does not appear to be due to an opalescent
spectral sort of thing. The chemistry of the glazes are such that the blue
is produced by a good amount of potassium. I've run the same glazes with
lower amounts of potassium and more sodium against the same silica/alumina
ratio and they have produced green with less potassium. Everything else is
about the same. A good point for the Seger values of the fluxes for blues
is: 0.3 potassium and 0.7 calcium against a silica/alumina ratio of about
12 to 1. This is impossible to do because there aren't any potash
feldspars that have all potassium and no sodium. We get as close as we
can. A small amount of barium helps make the blues more lively. About
0.09 BaO is what I use in my best blue celadon. This is about 4% barium
carbonate in the recipe.
The potash feldspar from Idaho that Michael Wendt has talked about via
Clayart is perfect for blue celadons. It has about 13% potassium and a
minute amount of barium. Custer and G-200 are both around 10% potassium
and glazes will be hard to fit to porcelain because of the higher amount of
soda present. I've had to raise the % of Custer and G-200 to about 60% for
blue celadons and the glazes usually craze. Fortunately, I still have a
couple hundred pounds of Kingman left and it has about 12% potassium and I
can get blues that fit my bodies.
regards, Craig Martell in Oregon
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