PottaFella Posted February 12, 2015 Report Share Posted February 12, 2015 Are Potash and Soda Feldspars interchangable? I'm looking at an Emmanuel Coper recipe for a pretty speckle semi-matt glaze and wondering if I can use my usual Soda feldspar and expect a decent result The recipe is... Feldspar (potash) 55 Lithium Carbonate 10 Whiting 10 Ball clay 5 Flint 20 Colourant x% (Cobalt oxide .25%) Book says 1200-1220 degrees C. i'd quite like to try and fire it to Cone six 1240 to tie in with my other stuff, do you think it will run? Link to comment Share on other sites More sharing options...
TJR Posted February 12, 2015 Report Share Posted February 12, 2015 No they are not interchangeable. Custer is a potash spar for high temperature work at cone 9/10. A soda feldspar like Nephelyn Syenite, if substituted directly for Custer will bring the glaze temperature down to Cone 6. TJR. If that is what you want, then test,test,test. Link to comment Share on other sites More sharing options...
Marcia Selsor Posted February 12, 2015 Report Share Posted February 12, 2015 Pot ash and soda ash can affect the glaze colorants differently as well. Marcia Link to comment Share on other sites More sharing options...
TJR Posted February 12, 2015 Report Share Posted February 12, 2015 Pot ash and soda ash can affect the glaze colorants differently as well. Marcia Marcia; Can you give an example-say, with cobalt carb, or copper carb? TJR. Link to comment Share on other sites More sharing options...
PeterH Posted February 12, 2015 Report Share Posted February 12, 2015 Marcia: Pot ash and soda ash can affect the glaze colorants differently as well. TJR: Can you give an example-say, with cobalt carb, or copper carb? From: http://digitalfire.com/4sight/material/soda_feldspar_1264.html Also, slightly different color responses will be encountered (i.e. cobalt tends toward purple copper toward blue with soda spar). Link to comment Share on other sites More sharing options...
Marcia Selsor Posted February 12, 2015 Report Share Posted February 12, 2015 cobalt really is influence by Magnesium as in Dolomite for turning lavender. Barium or Strontium with Copper will go more turquoise with certain combinations and slight variations in quantity.Iron will be yellow and bright blue with cobalt. Copper needs a soda ash to go red in reduction imo. So it isn't just soda or pot ash but they do influence color too. You can check out Hopper's Ceramic Spectrum for more in depth inf. Marcia Link to comment Share on other sites More sharing options...
neilestrick Posted February 12, 2015 Report Share Posted February 12, 2015 No they are not interchangeable. Custer is a potash spar for high temperature work at cone 9/10. A soda feldspar like Nephelyn Syenite, if substituted directly for Custer will bring the glaze temperature down to Cone 6. TJR. If that is what you want, then test,test,test. Changing the feldspar from potash to soda won't necessarily bring it all the way from cone 10 down to cone 6, but it will definitely bring it down. How far depends on the rest of glaze formula. But it is a good first step to lowering the melt. Potash spars can be used at cone 6, but they do melt differently than soda spars, and give a different surface look as well. Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted February 13, 2015 Report Share Posted February 13, 2015 Doesn't soda spar have a slight tendency to craze more than potash, all else being equal? Link to comment Share on other sites More sharing options...
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