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pink underglaze turned brown


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#1 Val

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Posted 16 January 2013 - 11:34 AM

I am using Amaco velvet underglaze at cone 6 and on my test tiles the clear brush on glaze turned the pinks to brown while the dip glaze did not. Does anyone know what glaze ingredient to avoid in the future? I know about zinc turning greens to brown, but can't find much info on pink

#2 Chris Campbell

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Posted 17 January 2013 - 01:14 PM

Clear glazes can mess with all colors, not just green ... zinc is an ingredient I always avoid but I have seen color change with one of my old gerstley borate recipes too. The clear glazes I have had the least problems with are the Amaco zinc free clears ... but, that said, I have not tried them all.

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#3 timbo_heff

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Posted 17 January 2013 - 01:54 PM

Chris is correct: Amaco recommends their zinc-free clear over underglazes.

#4 Benzine

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Posted 17 January 2013 - 07:26 PM

I have never had an issue with Amaco clear. The students have a tough time believing, that it will be clear, as the low fire clear I use, looks like Pepto Bismol. I always say, "If it doesn't come out clear, I'll buy you a pop..........Or you did something terribly wrong".

I did discover that the clear, when applied over a black iron oxide stain, will turn it green.
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#5 ayjay

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Posted 18 January 2013 - 06:09 AM

I have some Amaco Velvet underglazes, they are about five years old now and the firing range recommended is up to 05/04.

I tested them up to cone 8 and some of the colours burnt out to a nasty brown/grey, surprisingly the reds and yellows were less affected.

Maybe a newer incarnation of the product is OK to be fired hotter. What temperature is reccomended for what you have?

#6 Chris Campbell

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Posted 18 January 2013 - 01:20 PM

If you look in the newest Amaco catalogs ( and perhaps online too ) you will see they now show the Velvet underglaze color variations at both Cone 5 and Cone 10; glazed and unglazed. These images are good but they cannot replicate exactly what it will look like on your clay so I am posting a couple images of test tiles for color.

I make a bunch of flat tiles and carve the lines into them when hardened. You can have as many rows for information as you need. Place a hole in the corner so you can easily join them together and hang them on a wall for easy reference.

Fire the highest cone temp first, then the cooler one. Put glaze across half of the color so you can see the difference.
I also use these same test tiles for my glazes and my majolica colors. You can use them for technique references as well.

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Chris Campbell
Contemporary Fine Colored Porcelain
www.ccpottery.com

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