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Celadon failures


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#1 Riorose

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Posted 16 April 2012 - 01:50 AM

We can't figure out what is going wrong. We have had successful celadons in the past ranging from the paleest blues to greens and greys depending on the formulas used and differnt ball clays or kaolins used. My ceramic partner here in Portugal insists the firings are the same as before. I think she is reducing too late in the game. Our kiln is about .75 square meters. Gas, Spanish made. Could you please make comments and suggestions? We fire to 600C over 6 hours then speed up to around 120-150 hour until we hit 1000C. Then she introduces reduction by tightening the air intake rings (we have 4 burners) and reducing the exit opening til we have a strong blue flame. 45minutes then we open it a bit til we see a greenish flame which we keep until we have hit 1240 or 1250. We try to not go over that temperature to reduce deformation in the porcelains. Especially since she does a lot of Limoges casting materials. the kiln slows down quite a bit when we start reducing. overall the firings last around 12.5 to 13 hours. I think we are starting the reduction too late in the game. According to John Britt cone 10 book he seems to suggest starting celadon reduction around 850 -900C. We dont have an oxygen meter. If we started that early our firing time would be extended to probably 14 hours because of the slow down it does. Also she has been warned by other Portuguese ceramists to keep the gas pressure no higher than 2.5 whereas I feel up to 3 would feed more gas and keep the temp moving faster.

#2 JBaymore

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Posted 16 April 2012 - 05:24 AM

1000C sounds too high for most celedons to start reduction.

If she thinks the firings are "the same" as before, maybe your pyrometer is off? Drift as the thermocouples age is quite common. You might have been starting reduction in the past right at the top edge of when the glazes you use were still gas permeable to the CO. The slight temp change,.. and POOF... no more greens but ugly yellows.

Try using cones to measure the starting point for reduction.... not the pyyrometer. It is more accurate anyway.

best,

.......................john
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#3 Marcia Selsor

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Posted 16 April 2012 - 05:55 AM

you can also gauge the reduction by holding a stick at open ports .. If it doesn't burn when in the flame, then you are reducing. It will burst into flames as you remove it from the port..
Marcia

#4 StefanAndersson

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Posted 16 April 2012 - 04:17 PM

I use limoge and find that I can reduce the body( turning it slightly grey) at 1000 degrees. If you have a good reduction this should be visible to you as well.

If you find that the kiln to slow I can suggest the way one of my teachers fired celadon (turning a nice blue). Start the reduction at 950 degrees and reduce your way up to 1150 (both temp depends on claze and can be lower or higher) at this point she switched to oxidation and finished the kiln. Maybe this way can cut times for you. The idea is to reduce the glaze until it closes and there by keeping its reduced appearance. An added bonus is sometimes a more brilliant/ shiny celadon.

#5 Mark C.

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Posted 16 April 2012 - 08:18 PM

1000C sounds too high for most celedons to start reduction.

If she thinks the firings are "the same" as before, maybe your pyrometer is off? Drift as the thermocouples age is quite common. You might have been starting reduction in the past right at the top edge of when the glazes you use were still gas permeable to the CO. The slight temp change,.. and POOF... no more greens but ugly yellows.

Try using cones to measure the starting point for reduction.... not the pyyrometer. It is more accurate anyway.

best,

.......................john


I agree 100 % with above-Reduction fires even when the thermocouple is in a protection tube (should be in reduction fires anyway) age and give wrong readings over time
I think cones are the cure and starting earlier with reduction.
Mark
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#6 neilestrick

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Posted 17 April 2012 - 01:51 PM

I like to reduce at cone 010 or so, maybe even 012, and completely stall the kiln for 45 minutes. It ensures that everything gets reduced before it gets too hot. Then I lighten up on the reduction and let it climb. At peak temperature, I give it lots of air and a wide damper and let it clean out for about 10 minutes before shutting it down. Nice bright celadons and reds. Make sure it gets hot enough. My celadon goes gray when slightly cool.
Neil Estrick
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