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Custer Feldspar

Alternate Names: Custer Spar

Description: Potash Feldspar

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% -
LOI 0.30%n/a
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 had a page on their website named "Clarification of Custer Feldspar Chemical Composition Data" in which they claimed their analysis was always 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!

Since 2021 their data sheet for the P325 grade is claiming SiO2 - 67.6, Al2O3 - 17.2, Fe2O3 - 0.1, K2O - 10, Na2O - 4.5, CaO - 0.3, LOI - 0.3. This represents an increase in the Na2O content, so one might expect it to melt better than in the recent past.

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. A melt fluidity test is also practical to gauge its behavior as a flux.

Related Information

Original container bag of Custer Feldspar

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The back of the bag is blank. The warning label on the front mentions feldspar, crystalline silica and mica.

Custer Feldspar 2021 data sheet

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It is worth noting that ceramics is not among the recommended applications for this material. The Pacer Minerals website does not have information that indicates there are multiple grades having different chemistries.

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

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Feldspars melting

Feldspars are employed in glaze recipes as melters. So comparing their melt fluidities should be helpful in deciding if one can substitute for another (of course, if possible a soda predominant feldspar should be substituted for another soda spar). Feldspars don't melt alone at cone 6 (2200F) so we mixed each with 15% Ferro Frit 3195. Nepheline Syenite is obviously the champion melter here. Other similar ones can be spotted easily. In the end, degree of melt is a valid consideration in determining if one feldspar is a viable substitute for another in a recipe. Even if the feldspar you want to substitute does not melt as much a little frit can be added to the recipe to make up for the difference (e.g. even just 1 or 2%).

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

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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.

Comparing the melt fluidity of two shipments of Custer Feldspar

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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.

Custer Feldspar vs Nepheline Syenite at cone 8 oxidation

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Feldspar and nepheline melting

Although Nepheline Syenite and Custer Feldspar are used as effective fluxes in glazes at cone 8, curiously, neither of them melt well by themselves. Thus, both of these melt fluidity tests add 20% Ferro Frit 3134 to get them flowing. This is a 2021 shipment of the feldspar and a 2022 shipment of the nepheline.


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
In ceramics, feldspars are used in glazes and clay bodies. They vitrify stonewares and porcelains. They supply KNaO flux to glazes to help them melt.
Materials Potash Feldspar
Hazards Feldspar
Feldspars are abundant and varied in nature. They contain small amounts of quartz (while nepheline syenite does not).
Typecodes Feldspar
The most common source of fluxes for high and medium temperature glazes and bodies.


Sieve Analysis 35-325 Wet 0.3% (0.5% max)
Sieve Analysis 35-325 Wet 100
Sieve Analysis 35-325 Wet 4% (5% max)
Density (Specific Gravity) 2.6
pH for dry powder 8.0
By Tony Hansen
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