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Need Help Troubleshooting A Glaze Defect


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Hi guys,

 

So I fired a couple turquoise-glazed tiles yesterday, laying them flat on shelves.

 

The results were marred by brownish patches. Closest resemblance : bronze spray paint, or ink from gold marker pens.

 

(Most of the glaze was unaffected.)

 

So basically, guys, I'm trying to ID the problem.

 

Best current guess: a closed side shutter (not the main vent!) led to oxide gas damage.

 

Schedule was 6 hours to 600 cent; 3 hours to 1220 cent, 15 min soak.

 

The shelves were spaced quite generously.

 

Copper carbonate based glaze.

 

 

Advice MUCH appreciated.

 

S.

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Does your recipe also have manganese in it? Copper and manganese can give a bronze/ gold look . Manganese in your clay?

Pretty difficult for folks to help without a recipe.

Electric..gas fired?

As Curt says above, could be reduction but would only happen in a gas kiln under quite heavy reduc.

SPecs falling from lid of kiln, underneath of shelf..

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Hi guys.

 

So this was an oxidation firing with no manganese.

 

Recipe:

Epk / China clay 20,

barium carbonate 40,

neph sy 40,

Copper carbonate 5.

 

Top temp 1220°c

 

Hope this info helps resolve the brown patches.

 

Thanks for your help,

 

S.

 

Ps

 

Perhaps the glaze melted too soon, and gas bubbles were trapped in it?

 

Or I should use distilled water to make new batch?

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I would personally expect anything with that much Barium in it to break and pool in wildly different colours. (Ie pink and turquoise in one glaze). Copper in that quantity can go metallic under its own power in oxidation.

The only thing I am surprised about is how much it's NOT doing that from your picture in the other post. Then again, I work reduction.

I don't know that it's actually a glaze defect. I think it's kind of supposed to happen. Glazes that unbalanced are that way because someone wanted a funky effect.

 

I hope that glaze is on decorative work only, and being mixed in full respirator, overalls and gloves.

Edit: I know Barium isn't likely to kill you unless you ingest about 50g on purpose, but precautions are still a good idea.

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Hi,

 

the barium carbonate - aka rat poison - is on a decorative tile. I was practically in a spacesuit when I mixed it.

 

Can you suggest a safer / stabler glaze reformulation? I'm going for an oceanic effect.

 

Ps. I also tried a commercial turquoise but this recipe lookd better, despite flaws.

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