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Primus-P Feldspar

potash feldspar

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#1 MargaretK



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Posted 15 June 2014 - 06:37 PM

I used to use a potash feldspar from Mexico called Primus. Now it is no longer available and I've switched back to Custer, but it changed my main glaze and I'm having big problems with it (blistering). Does anyone know if it might still be available somewhere in the US? Or, which feldspar it would be best for me to use now?

#2 bciskepottery


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Posted 15 June 2014 - 08:47 PM

This might be helpful.


#3 Dick White

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Posted 15 June 2014 - 09:39 PM

Sadly, Custer is not what it used to be. A few years ago, people began noticing the very problems you are having - blistering, pin holes, lack of melt -  with glazes containing significant amounts of Custer.  It was finally traced to an unannounced and as yet unacknowledged (by Pacer Corp, the producer of Custer) change in the properties of Custer. Compared to 5 or 10 years ago, the current product has about a quarter less potassium (which belies it being called a potash spar...), an eighth less alumina, and about 6% more silica than before. That makes for an unfluxed, non-melting, pin holing glaze. To restore your glaze, you need to reformulate the recipe using more (much more) Custer and adjusting the silica downward. This is easy enough to figure out if you are facile with glaze calculation software but tricky to stumble around with line blends and other traditional glaze testing methods. If you have glaze calculation software, I can give you the new material analysis, or if you aren't comfortable with glaze calculations, post your glaze recipe and I'll recalculate it for you.

#4 MargaretK



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Posted 16 June 2014 - 09:31 AM


I don't have the glaze calculation software, but would rather not publish this recipe online. But thanks for the tips. If you google Margaret Kunzler you can get my information on Siskiyou Arts Council webpage. Another question I have about possible changes in this glaze is the barium. Is there a big difference between Chinese barium and German Barium? Another possible problem is that I added a little chlorine bleach to the bucket because of growth of green slime. (Could this affect the glaze?) I'm going to run a bunch of tests. I will also try G-200 feldspar. Strangely enough, this matte glaze has become more melted with Custer. I'm thinking of removing some of the flux.


Thanks again,  Margaret

#5 Wyndham


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Posted 16 June 2014 - 01:12 PM

There are several online glaze calculation programs that will work very well for what you need, just Google.


#6 JBaymore



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Posted 16 June 2014 - 03:54 PM

You can get a fully functional demo of Insight software at Digitalfire.com.





John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art


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