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Sculptural Teapot's Clear Matte Glaze Turned Green In The Firing


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#1 Isculpt

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Posted 05 May 2014 - 12:55 PM

I posted earlier about making sculptural teapots out of raku clay that I hoped would be functional.  I'm happy to report that one of the two sculptural teapots actually holds liquid and pours reasonably well.  The other teapot developed a crack in the base, so it's strictly non-functional. 

 

The weirdest part of this teapot experiment has been the result of the final glaze firing.  I've never known a clear glaze to turn green when fired! 

 

As usual, I bisque-fired to 06, then applied a copper carbonate wash and fired to cone 4. Next came underglazes fired to cone 6 followed by a clear matte glaze fired to cone 05. The glaze had thickened since I last used it, but I wrongly thought it would thin out in the firing. Consequently the "boat" teapot came out with a disappointingly thick glaze.  However, the greenish hue that developed in the firing is the most disturbing outcome.  I'm guessing the copper carbonate is the culprit, but I've followed the same steps before without this problem.

 

In a separate load I followed the same process but instead of firing between the underglaze coat and the topglaze, I applied them both and then fired the round "rabbit box".  The box was painted in pale watered-down underglazes, then top glazed. That bright green box in the photos is what came out of the kiln!!  

 

It has been recommended that I switch to manganese wash in place of the copper carbonate, which I will do.  But I'm still curious about why this happened. The images attached show the "mountain" teapot which developed a crack in the bottom, so I didn't bother to glaze fire it....which is why it's the only thing that isn't greenish!  The "boat" teapot is shown in its orginal colors after the underglaze firing but before the top glaze firing.  And then it is shown as it came out of the final glaze firing with a distinctly greenish cast.  The round box was originally painted in off white with pale green leaves but after firing, it came out bright green.

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#2 Pres

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Posted 05 May 2014 - 01:11 PM

I believe your copper wash is the culprit here. No matter how much you wash it off, there will be a lot of residue to show through the clear glaze.


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#3 TJR

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Posted 05 May 2014 - 02:11 PM

It's your copper wash. Copper tends to fly around the kiln as well, so even if you wash it off, it still gets all over the place. Manganese is a bit more stable but very dark. What about a wash of black stain?

TJR

Great looking pieces. Love the mermaid.

T.



#4 Benzine

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Posted 05 May 2014 - 03:14 PM

First off, I love those pieces. The fact they are functional, makes them even better.

In regards to the glaze, I can't speak on manganese, but I know black iron oxide will turn a dark green, when covered with Amaco Clear Transparent.
"Anything worth believing, is worth questioning"

#5 jrgpots

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Posted 05 May 2014 - 03:39 PM

very cool pieces!  artistic and functional...

 

Jed



#6 Babs

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Posted 05 May 2014 - 03:41 PM

Truly beautiful!

Ditto the above, copper the culprit.

Black stain would be most reliable, manganese wold also be worth a try.

Why are youo firing the underglaze colours and then another glaze firing?

Adore your sculptures.

Loved seeing them again.

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#7 Isculpt

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Posted 05 May 2014 - 10:29 PM

I know that you're all correct in that the copper carbonate is the problem. But what I don't understand is why the copper carbonate fires a nice metallic black and stays that way until the matte glaze goes on, when it decides to turn green. 

 

Benzine, your comment that "black iron oxide turns green with Amaco clear transparent" leads me to think that the answer to my question is that a chemical reaction occurs between some component of my matte glaze and the carbonate.  Is that a somewhat common quirk that I'll have to test for in clear glazes? 

 

Babs and TJR, what do you mean when you say a black stain?  I haven't used stains.  What exactly would I ask for?  I'm using the copper carbonate to get a metallic black to enhance the texture, as in the photo attached below of a sculpture that, at this stage, only has copper carbonate, brushed on and wiped off, then fired to cone 6.  I've been pretty happy with the copper carbonate until now, but it's obvious that if I'm going to be using clear glazes I'm going to have to find something else to give me that metallic black without ending up with green sculptures!

 

Babs, I didn't apply top glaze to the underglazes until after they'd been fired for fear of smearing them.  Is that an unfounded fear?

 

And TJR:  "copper tends to fly around the kiln" -- does that mean there is a residue of copper carbonate in the kiln now and it could jump on the next thing I decide to fire???  And my intention with the manganese is to wash off all that doesn't settle into the crevices.  Should I expect that manganese will leave behind a brown stain even where it's been washed off?  Yes, I will test it first, but I'm just wondering if I could maybe end up with a nice light brown stain on the high spots and a dark stain in the crevices..... That would be pretty cool.....

 

And, um, TJR, I'd like to accept the compliment on the mermaid, but honey, there's no mermaid in this post...

 

And thanks for the help and compliments...  Jayne

 

 

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#8 Babs

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Posted 06 May 2014 - 03:19 AM

Re firing inderglazes first then glazing, I don't know as I haven't done this.

I usually apply underglazes to raw ware and bisque though there was a recent article in CAD about underglazes being applied to bisque whether the artist went on to fire and then glaze I do not recall.  Maybe do a search in this forum

Stains are commercially, usually, made colours used by ceramists, some glaze stain and others body stains, don't know if they are chemically different.. the black will be black so if you like subtle shadings you may not get it. What you see is mainly what you get after firing with a couple of exceptions.

Applying the manganese to bisque and wiping off will give you a staining to the body and of course deeper where it remains unwiped in the recesses..

I've got black metallic finish using copper in a glaze but not the desired effect, caused by an overload of copper settling  in the bottom of bowls!!!

Loved seeing bear again!



#9 TJR

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Posted 06 May 2014 - 07:57 AM

Whoops, I apologize about the mermaid.Nobody has called me honey in a long time. Darling.

Black stain can be purchased at your local clay supplier. Many artists use it to enhance the texture. Apply as a wash with water and then sponge it.

Copper carb. tend to move around at high temperatures in a gas kiln. It is a volatile oxide, and people like its unpredictability. May be stable at lower temperatures.

I use it in stoneware/porcelain firings.

TJR.



#10 Marcia Selsor

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Posted 06 May 2014 - 08:03 AM

I would recommend the black stain or a black stained underglaze rather than manganese. Handling that much manganese in that way would not be healthy in my opinion.Also I think you would get a muddy color rather than a black.

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#11 PeterH

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Posted 07 May 2014 - 02:29 PM

>But what I don't understand is why the copper carbonate fires a nice metallic black and stays that way until the matte glaze goes on, when it decides to turn green.

 

As I haven't seen an answer to this question posted yet ...

 

Copper carbonate fires black (in an oxidized firing) because it turns into "black copper oxide". CuCO3 -> CuO + CO2

 

When copper is in a glaze (fired in oxidation) it typically gives green colours, although in alkaline glazes it can give turquoise/cyan.

 

It looks like your glaze is dissolving some of the copper oxide, giving copper-in-glaze colours.

 

Regards, Peter

 



#12 Isculpt

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Posted 07 May 2014 - 04:21 PM

Thanks, Peter.  That makes complete sense.

Enquiring minds just want to know, y'know?

Jayne



#13 perkolator

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Posted 07 May 2014 - 05:19 PM

like everyone's said, it's the copper wash giving you the issue.  had it been something like a copper-based underglaze, my guess is it would be less of an extent but you'd still get some migration of the copper into the clear.  underglaze has flux and clay to help keep/fuse it in place, whereas a wash doesn't have this.  mason stains would be good since those materials are already fired and don't tend to bleed as much/transfer like you've experienced.  i would not use manganese for this unless wearing gloves - IIRC manganese has a small enough particle size to be absorbed through skin and will stain you, and it's a heavy metal which in general is a bad idea to play with like this - it would also likely be more of a brown/purple vs black.

 

as for the question about why copper will be black without glaze, but green with...it's because copper is one of the most easily influenced glaze colorants, by the other materials in the glaze.  mix a little boron or alkali material into your glaze and wallah the green is now in leaning more toward blue color range.



#14 NancyAmores

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Posted 07 May 2014 - 08:54 PM

Whoops, I apologize about the mermaid.Nobody has called me honey in a long time. Darling.

Black stain can be purchased at your local clay supplier. Many artists use it to enhance the texture. Apply as a wash with water and then sponge it.

Copper carb. tend to move around at high temperatures in a gas kiln. It is a volatile oxide, and people like its unpredictability. May be stable at lower temperatures.

I use it in stoneware/porcelain firings.

TJR.

This might be a good time to bring this up, TJR. It was only last week, after seeing your avatar pic for more than a year that I realized your platter had fishes on it...as it appears out of the corner of my eye to be the head of a velociraptor. Surely I can't be the only one who thought this...hello...anyone? OK I'll just see myself out now :0



#15 Isculpt

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Posted 08 May 2014 - 11:02 PM

I'm confused! 

 

After Marcia's warning about the dangers of manganese, I spent a couple of hours researching the subject and found the same concerns mentioned elsewhere, including old posts here on CAD. Perkolator's similar concern sent me back to my research.  I was ready to forget the whole manganese idea until I visited the Amaco website where I found the product that had been recommended to me: Amaco Ceramic decorating Wash, Manganese, WA-44.  It carried the AP seal, which is detailed as :

The new AP (Approved Product) Seal, with or without Performance Certification, identifies art materials that are safe and that are certified in a toxicological evaluation by a medical expert to contain no materials in sufficient quantities to be toxic or injurious to humans, including children, or to cause acute or chronic health problems. This seal is currently replacing the previous non-toxic seals: CP (Certified Product), AP (Approved Product), and HL Health Label (Non-Toxic) over a 10-year phase-in period. AP glazes are intended for use in grades Pre-K and up.

 

So.....Did I mention that I'm confused?

 

Jayne



#16 Chilly

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Posted 09 May 2014 - 01:55 AM

 

This might be a good time to bring this up, TJR. It was only last week, after seeing your avatar pic for more than a year that I realized your platter had fishes on it...as it appears out of the corner of my eye to be the head of a velociraptor. Surely I can't be the only one who thought this...hello...anyone? OK I'll just see myself out now :0

 

 

Fishes?  Oh-oh, I thought it was a three-eyed-monster ;)


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#17 Babs

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Posted 09 May 2014 - 02:39 AM

TJR forgive them they know not what they see! :)






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