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Or Cone 6 Glaze Is Wierd


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#1 jennyd

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Posted 13 December 2010 - 07:16 PM

Our Orange Red - OR cone 6 glaze recipe is now turning out a very sad, plain brown. The recipe is:
Potash Feldspar - 46.7%
China Clay - 4%
Bentonite - 2%
Bone Ash - 15%
Lithium Carbonate - 4%
Talc - 16.9%
Fling - 11.4%

Red Iron Oxide 11.5%

Does anyone know if the make-up of any of the ingredients has changed or what might be the problem for us? The firing is taking almost 15 hours and we are allowing a natural cooldown. The cones (5, 6 & 7) were all totally bent. Someone told me that this may not actually be an overfire but is possibly that the firing took too long. May timing be the problem? In the last firing we had one piece that was sprayed with an old mix of the glaze and it turned out ok but the new mix did not.

This is the lovely OR we had in the past:
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And this is what we are getting now:
Posted Image

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#2 Pam S

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Posted 13 December 2010 - 08:01 PM

I can only sympathize. We're having the same problem with White Silk, a beautiful matte white with a soft waxy feel. It's now turning out slightly shiny with a bit of crackle. Pretty but not the White Silk we want. I'm chalking it up to several new batches of ingredients. After a year, we still can't reproduce the original. :o(

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#3 Ben

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Posted 14 December 2010 - 10:02 AM

What has changed?
How long do firings usually take (up and down)? Has that changed? (if your old batch of glaze worked in this firing you can ignore this variable)

Exactly which of each ingredient are you using and when did you get the new supply? For example, which potash feldspar are you using? Which china clay, which iron (yes, there is a difference in iron oxides)?


If you are using G200 feldspar you may want to check it. G200 has recently changed to a new product named G200HP with a different chemical analysis. More potassium, less sodium. It would be an easy fix if this is to blame.

Seems like Talc has changed recently too but I don't have any data on that.

In future to help avoid this you could write the date on the bag with a sharpie when you buy a new supply of a particular ingredient. That'll help some. Make fusion buttons of each new bag before using it in a glaze. Take digital pictures if you don't want to keep them around and compare each new batch. Make 1000 gram test batches of your glazes with the new ingredients to test them out.

#4 stonegarden

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Posted 14 December 2010 - 11:13 AM

The first result is beautiful- can you share some details for others to try this recipe? Like, the particular feldspar, and I'm guessing, the China clay is EPK, available at our local potters' chemical supply house? Last- I don't recognize the term "fling" in the context of glaze chemicals. Can you help me here?

Many thanks, and good luck with the formula.

#5 Ben

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Posted 14 December 2010 - 12:17 PM

"Fling" was probably a typo and should have been "Flint"

#6 jennyd

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Posted 15 December 2010 - 09:40 AM

The first result is beautiful- can you share some details for others to try this recipe? Like, the particular feldspar, and I'm guessing, the China clay is EPK, available at our local potters' chemical supply house? Last- I don't recognize the term "fling" in the context of glaze chemicals. Can you help me here?

Many thanks, and good luck with the formula.


sorry, that would be flint

#7 jennyd

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Posted 15 December 2010 - 09:56 AM

The first result is beautiful- can you share some details for others to try this recipe? Like, the particular feldspar, and I'm guessing, the China clay is EPK, available at our local potters' chemical supply house? Last- I don't recognize the term "fling" in the context of glaze chemicals. Can you help me here?

Many thanks, and good luck with the formula.


Yes, the color of the first piece is wonderful and we love the glaze and we are very sad that we seem to have lost it. The recipe comes from the ceramics handbook Glazes Cone 6 by Michael Bailey. We used a potash feldspar. If you test it, please let me know the results.

#8 Marcia Selsor

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Posted 15 December 2010 - 12:51 PM

I have used that glaze and had the same differences occur but with the same batch. Therefore, I think it could be the firing or the thickness.
I have been able to improve some of the brown pieces by refiring with a longer soak at ^6 (large witness cone down, 7 half down at the end of soak)) and also holding for a good hour at ^06.
Sometimes it improves and sometimes not. Not is when it has been on flat plates. I think maybe the plates need to be in plate setters to get the centers hotter.
Refired mugs all improved.
I have older G200 feldspar and talc.
Marcia

#9 stonegarden

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Posted 15 December 2010 - 02:18 PM

Thanks- if I'd had another cup of coffee, that might eventually have occurred to me, or not...I'll give the recipe a try after the holidays and see what happens here. We tend to fire at a hot cone 6, more like cone 7, range.




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