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Custer Feldspar

Alternate Names: Custer Spar

Oxide Analysis Formula
CaO 0.30% 0.03
K2O 10.00% 0.66
Na2O 3.00% 0.30
Al2O3 17.00% 1.04
SiO2 68.50% 7.13
Fe2O3 0.10% 0.00
Oxide Weight 618.54
Formula Weight 620.40


This is one of the main feldspars used in the ceramic industry in North America. It is used in industries such as abrasives, sanitary ware, floor and wall tile, dinnerware, pottery, and electrical porcelain. It is a ceramic grade, high potash feldspar and is available in crude, 200, 325 mesh and chip form.

Since 2012 we have been getting independent reports of a reduction in the potash content (below that stated on the data sheet). Traditionally Pacer, the manufacturer, has reported it as around 10% (as is shown in the chemistry given here, and from their online data sheet). However Ron Roy claims that, from his independent analysis of specimens over time, around the year 2000 the K2O content dropped to about 7.5 (with no accompanying change in the Na2O). Pacer disputes this. The chemical analyses they provide with individual shipments of material continued to report K2O, Na2O and Fe2O3 levels at quite similar to those stated on their data sheet. Additionally, they have a page on their website named "Clarification of Custer Feldspar Chemical Composition Data" in which they claim their analysis is, and has been, accurate. Whether justified or not, Custer Feldspar has become somewhat of a scapegoat, people actually blaming it when online recipes they try do not work!

As with any feldspar, production users should be vigilant to do sieve analysis testing to spot any iron bearing particles in the plus 100 mesh range.

Related Information

Comparing the melt fluidity of two shipments of Custer Feldspar

A GLFL test for melt-flow to compare Custer Feldspar from Feb/2012 (right) with Mar/2011 (fired at cone 6). Custer Feldspar does not melt like this by itself at cone 10. It was mixed 80:20 Feldspar:Ferro Frit 3134. This test demonstrates that the material has been very consistent between these two shipments.

Feldspars, the primary high temperature flux, melt less than you think.

A cone 8 comparative flow tests of Custer, G-200 and i-minerals high soda and high potassium feldspars. Notice how little the pure materials are moving (bottom), even though they are fired to cone 11. In addition, the sodium feldspars move better than the potassium ones. But feldspars do their real fluxing work when they can interact with other materials. Notice how well they flow with only 10% frit added (top), even though they are being fired three cones lower.

Which common American/Canadian feldspars can substitute for each other?

After comparing the chemistries of an original feldspar and a tentative substitute, these melt flow tests are an excellent way to confirm physical similarity also. These were done at cone 6 (2200F). Each feldspar is mixed with 15% Ferro Frit 3195. Some things to note: Nepheline Syenite is the champion melter. Mahavir is very similar to G200. Kingman and Custer are very similar. Our Minspar substitute is very similar to Minspar itself.


Clarification of Custer Feldspar Chemical Composition from Pacer
Custer Feldspar SDS
Custer Data Sheet
Product Reference Guide
Materials MC 200K Feldspar
Materials Mahavir Potash Feldspar
Materials Fortispar
Materials Eureka Feldspar
Materials Kingman Feldspar
Materials Chesterfield Feldspar
Materials G-200 Feldspar
Materials Feldspar
Materials Potash Feldspar
Typecodes Feldspar
The most common source of fluxes for high and medium temperature glazes and bodies.


Sieve Analysis 35-325 Wet0.3% (0.5% max)
Sieve Analysis 35-325 Wet100
Sieve Analysis 35-325 Wet4% (5% max)
Density (Specific Gravity)2.6
pH for dry powder8.0

By Tony Hansen

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