Feldspar changes

Somone just asked me about a spar substitution. My experience is that for many applications, spar substitutions within the categories of soda spars or pot (potassium) spars have some leeway and will often endure substitutions fairly easily. Unless they don’t. The “pot” or “soda” spar refers to the dominant flux in the feldspar. I suggest testing, and I’d test the easiest fix first – substitute a spar you have, and see if it changes things in an unacceptable way. If it does, try the things suggested below. At home I’m still slogging through my big bucket of old majolica glaze. At workshops, I’ve had people use various substitutions, and other than some alternate soda spars deflocculating the raw glaze, it’s been fine. If you need to review floc/deflocc, see the  Ceramic Arts Daily video demo on this.

feldspar Idealized formula KNaO• Al2O3• 6SiO2

In the idealized (simplified) feldspar formula above, KNaO refers to any combination of sodium or potassium. K2O is potassium oxide – 2 potassium for every oxygen. Ditto Na2O. So KNaO maintains the 2 units of flux for every oxygen, but is shorthand for “any combination of” because they often operate interchangably and it’s easier to calculate if you overlook the distinction because if often doesn’t matter. An actual feldspar, say F4, will have traces of other oxides in it, and not be quite that simple. These small inclusions are often overlooked in calculation of glazes because they often are insignificant. Since sodium and potassium have different atomic weights, the weights of soda spar and pot spar differ in idealized form. If you plug in two K instead of KNaO, the weight is 556 units vs. 524 for sodium. So, you might get a weight difference if you jump pot vs. soda categories. It would be conservative to stay within K or Na family of the spar you’re substituting. Wikipedia on feldspar. Image from Wikipedia.

Info below thanks to NM Clay in 2010.  If you go to Brant’s blog post below, you can download the datasheets for the spars.

Soda Feldspar – Kona F4 There is a supply change in the Soda Feldspar Kona F4. It is not available from the mine any longer due to a fire and mine closure. We will have Minspar 200 available. They are both very close to the F4 in composition. A chemical analysis of the materials is below.

 Minspar 200 Feldspar  Kona F-4 Feldspar   Sodium Feldspar
CaO 1.50 CaO 1.70
~~~ MgO 0.05
K2O 4.10 K2O 4.80
Na2O 6.50 Na2O 6.90
Al2O3 18.50 Al2O3 19.60
SiO2 68.60 SiO2 66.80
Fe2O3 0.06 Fe2O3 0.04
Volatiles % LOI 0.30 Volatiles %    LOI 0.20

Minspar 200 was Formerly NC-4 Feldspar.

G200 update From Jeff Zamek Article from Ceramics Technical (November 1, 2009)

“G-200 felspar, mined by Imerys North America Ceramics, has been discontinued. While there are still stocks in potters’ studios and ceramics supply bins it will eventually join Albany Slip, Gerstley borate, Kingman felspar, Oxford felspar, and other materials that have gone out of production. G-200’s demise is based on economic considerations that potters do not control. Originally, G-200, as potters know it, was blended at the processing plant by using 70% Minspar 200, a sodium based felspar, and 30% HP G-200, a potassium based felspar. The blended felspars then became G-200 and were shipped throughout the United States.

The increasing costs of shipping Minspar 200 from Spruce Pine, NC and G-200 HP, mined in Siloam, GA, to the processing plant in Monticello, GA, a trip of over 200 miles, made this situation uneconomical. Potters are now faced with either using Custer feldspar in place of or finding a new potassium based felspar.”

Thank you Mr. Zamek I couldn’t have said it better or at all!

Laguna Clay will be blending G-200 so it will be available. G-200HP is the new product and according to Jon Pacini from Laguna the major difference is slightly more fluxing power, similar to (the also defunct) Kingman Feldspar of old.

Posted by Brant from New Mexico Clay Inc on his blog.



By arbuck

Potter, Professor Emerita, Ceramics at University of FL, Gainesville.


  1. Recent (since perhaps the year 2000) changes in Custer Feldspar and the increasing amount of silica have left many low silica glazes difficult or impossible to mix with Custer Feldspar. Using Nepheline Syenite as a replacement for part of the Feldspar may help with some of the problems, but G200 HP Feldspar is a better solution. It is a much better Potash Feldspar than Custer feldspar as it is higher in Potash, lower in soda and calcia and lower in silica. You get more potash gram for gram. Thanks for posting this Linda!

    1. Completely new to glazes, and my first hurdle is feldspar. I have a recipe for Custer feldspar, but have access only to Potash feldspar. Recipe calls for 20% Custer. Would the substitute remain the same or do the proportions need change? I have yet to make my own glazes, and any help will be great!

      1. Custer is a potassium-fluxed (potash) spar, so your Potash feldspar is some brand of potassium-fluxed spar and should work as a sub. Best wishes on your glaze mixing.

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