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Fired copper carbinate rubbed off! Why?


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#1 Isculpt

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Posted 16 November 2012 - 12:49 AM

I often use copper carbonate painted on & then sponged off after bisque firing to emphasize detail in my sculptures, with fairly predictable results whether firing at 06 or 6. While 2nd-firing some bisqueware finished with copper carbonate, my pyrometer stopped working mid-fire. After the kiln had been firing for several hours at a setting that usually produces 010 to 07 temperatures, I turned it off. When I opened the kiln the next day, I was shocked to see that instead of the usual white ceramics with brown/black copper carbonate in the crevices, EVERYTHING was completely jet black and very carbon-y looking. The carbon rubbed off on my skin while unloading, so I washed everything. It took repeatedly scrubbing with a fingernail brush and then rinsing in torrents of water to remove all the 'loose' carbon, leaving a lot of uniformly (ugly) grey work. What happened here? Did I guess wrong on the temperature, and the kiln didn't get hot enough?
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#2 Isculpt

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Posted 16 November 2012 - 01:05 AM

I often use copper carbonate painted on & then sponged off after bisque firing to emphasize detail in my sculptures, with fairly predictable results whether firing at 06 or 6. While 2nd-firing some bisqueware finished with copper carbonate, my pyrometer stopped working mid-fire. After the kiln had been firing for several hours at a setting that usually produces 010 to 07 temperatures, I turned it off. When I opened the kiln the next day, I was shocked to see that instead of the usual white ceramics with brown/black copper carbonate in the crevices, EVERYTHING was completely jet black and very carbon-y looking. The carbon rubbed off on my skin while unloading, so I washed everything. It took repeatedly scrubbing with a fingernail brush and then rinsing in torrents of water to remove all the 'loose' carbon, leaving a lot of uniformly (ugly) grey work. What happened here? Did I guess wrong on the temperature, and the kiln didn't get hot enough?
Jayne


Let me phrase that question better: Did the kiln not get hot enough to -- what? What happens in the kiln that turns copper carbonate into a permanent coloration as opposed to a sooty black mess?

#3 Marcia Selsor

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Posted 16 November 2012 - 02:23 PM

Was there any wax in there?
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#4 weeble

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Posted 17 November 2012 - 04:46 AM

Electric kiln or gas kiln? It SOUNDS like you actually did have soot, which should just burn out if you refire properly. I've only played a bit with copper carbonate, but that really sounds like you didn't get hot enough.
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#5 Lucille Oka

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Posted 17 November 2012 - 05:34 AM

Sounds like improper mixing; try ball milling the ingredients.
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#6 Isculpt

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Posted 17 November 2012 - 07:55 AM

Was there any wax in there?
Marcia


No, but your question intrigues me and leaves me wondering about what role wax would play.

#7 Isculpt

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Posted 17 November 2012 - 08:00 AM

Electric kiln or gas kiln? It SOUNDS like you actually did have soot, which should just burn out if you refire properly. I've only played a bit with copper carbonate, but that really sounds like you didn't get hot enough.


It was an electric kiln, and I've washed off all the soot. which left behind lightly dyed "grey" ceramic. I think I'm going to try refiring with more cc, although I still don't have my pyrometer back from the repair guy. This sculpture needs to make the 3-hour drive to the gallery next Wednesday. Wish me luck!

#8 Isculpt

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Posted 17 November 2012 - 08:04 AM

Sounds like improper mixing; try ball milling the ingredients.


Lucille, I always simply add water to a tablespoon or two of copper carbonate. Thinking back, I did make this mixture a little thicker than usual. I'm ashamed to admit that I don't measure; I just judge its correctness by the thickness of the mixture. Hmmm. I wonder if too much cc in ratio to water, instead of the temperature, could be the culprit.

#9 Marcia Selsor

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Posted 17 November 2012 - 10:37 AM

I think you need to add the cc to something as a binder. The most simple in my mind and what I use is copper carb. with a frit 3110 and epk. 1/3, 1/3, 1/3
This is based on Val Cushing's underglaze ^04 base. It is good for making a wash to accent the texture.

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#10 Isculpt

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Posted 18 November 2012 - 01:06 AM

I think you need to add the cc to something as a binder. The most simple in my mind and what I use is copper carb. with a grit 3110 and epk.
This is based on Val Cushing's underglaze ^04 base. It is good for making a wash to accent the texture.

Marcia


Thanks, Marcia. I'll try that.

#11 perkolator

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Posted 19 November 2012 - 06:11 PM

sounds like there was no binder in your wash. unless you add something to the wash, or layer it on top of something that will help to fuse the material onto the piece (like a flux or a glaze) - then it won't be bonded after firing - which is what happened here.

i also suggest using it as underglaze (or close to/different ratios) to help fuse it to your clay. remember, you can always change the frit also if you're looking for different colorways since copper is easily influenced by its flux.




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