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Really? I saw they were different on Digitalfire; I guess I didn't investigate more than that.  

Liambesaw, I thought the higher potassium was a deal breaker, requiring more heat to make it viscous enough...?

Thanks, you two!

 

Edited by Rippity
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13 hours ago, Rippity said:

Really? I saw they were different on Digitalfire; I guess I didn't investigate more than that.  

Liambesaw, I thought the higher potassium was a deal breaker, requiring more heat to make it viscous enough...?

Thanks, you two!

 

Potash feldspar does have a higher melting point usually, so at cone 6 there's usually some boron to aid melting.  I'm not sure about the viscosity, I haven't noticed any issues with that.

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19 hours ago, Rippity said:

I would like to make underglaze pencils from THIS website

Even if you used a soda spar in place of the potash spar it probably wouldn't make any difference for this usage. In glazes it can make a difference but for what it's being used for I don't think it would matter, perhaps very slight increase in melt but highly doubt you would notice it.

Re potash feldspars, as mines come and go potters have to adjust their recipes to what is available. Buckingham, Kingman etc are long gone and Custer became the go to (in North America). After about the year 2000 the potassium content decreased by approx 2.5% in Custer feldspar.  Reason I bring this up is because the article you linked to is from 2020 but the original recipe could well predate the newer low potassium content Custer feldspar. We don't know which potash feldspar Hopper was using when he wrote the original recipe. These days in North America Custer and more recently Mahavir feldspar (mined in India) are the go to potash feldspars. 

Ron Roy article on Custer feldspar changes here.

 

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