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Substitute Minspar 200 with Custer feldspar


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Recipe is

M200 46.10

Silica 19.7

Gerstley 13.5

cal carb 8.20

dolomite 5.9

zinc oxide 3.9

can I substitute Custer Feld for M200 1:1 or do I need to adjust ?

oxidization cone 6 

Edited by Min
Title edit by Min for clarity of topic
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  • Gin changed the title to Substitute M200 with Custer feldspar

I assume you mean G200 feldspar. I’m basing this answer off of that assumption, but if I’m wrong, let me know.

The analyses of Custer and G200 are similar enough that it’ll be pretty close just doing a 1:1 substitution. The sodium and potassium are close enough that it shouldn’t matter, but there’s differences in silica and calcium that may have a minor effect. 

An exact adjustment (or as close as I think it’s possible to get would be):

51 Custer

17 silica

13 gerstley borate

9 whiting (calcium carb)

10 dolomite

4 zinc oxide

I rounded the batch numbers for easy measuring. The silica:alumina ratio is as close to identical as I can get it, the flux ratio is identical, and the Stull chart position are identical. If you were to add some epk to this recipe, it probably wouldn’t craze as badly. 


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  • Min changed the title to Substitute Minspar 200 with Custer feldspar

Feldspars are classified as either a soda spar or a potassium spar based on which oxide they contain the most of. (Although we treat Nepheline Syenite as a feldspar it is composed of an alkali feldspar plus nepheline.) What we call a soda spar (such as Minspar 200), or potassium spar (such as Custer), will have both sodium and potassium but a greater amount of one or the other hence the categorization as a soda spar or potassium / pot/ potash spar. 

If you have Nepheline Syenite and can't get Minspar you could try a test using Neph Sy subbed for it but the forumula needs to be balanced so other materials need to be tweaked. If you can't get either Minspar or Nepheline Syenite the simplest way to balance it with Custer would be to use glaze calc to drop the Custer enough so the Potassium is balanced then add Ferro Frit 3110 to supply the missing sodium. If you have some of the old Kona F4 feldspar just do a direct 1:1 swap of that for the Minspar.

Recipe below where I used Nepheline Syenite to supply the sodium and potassium. Rebalanced to 100. I noticed the original recipe didn't total 100, is there something missing, maybe some kaolin or ball clay? I added some epk to the recipe below to bring the alumina up to match original formula.


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  • 1 year later...
4 hours ago, Sara Trangsrud said:

I have run out of custer spar but have a lot of minspar can I use minspar in place of custer? I fire to cone 9.

As noted above, potash spar (Custer) and soda spar (Minspar) have opposite balances of potassium and sodium. As Min notes, one can spike up the Custer with some nepheline syenite to boost the sodium content to equal the Minspar or use the neph sye to get all the required sodium (together with some other adjustments), but there is no good potassium source to spike the Minspar to equal the Custer.

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