• Materials for Ceramics

Custer Feldspar (pre-2000)

Alternate Names: Custer Spar, Custer Feldspar, pre 2000 Custer spar

Description: Potash Feldspar

Notes

Starting in 2024, this material became unavailable, and their website went down. Little information was given about what happened. It was later learned that the quality of their ore body was deteriorating over time (more quartz, less potash feldspar). The declining K₂O and broader financial, ownership, equipment and mica-mining issues led to their shutdown.

G200, Mahavir and soda feldspars (also nepheline syenite) can be substituted in most cases; they are lower in iron and should produce as good or better results in most glazes or bodies. If the percentage is high or your feldspar has a significantly different chemistry, calculations can be done to compensate (see video demo link below).

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

Since the 2000s, we have received independent reports of a significant reduction in the potash content and increase in SiO₂ (compared to that stated on the official data sheet). Traditionally, Pacer, the manufacturer, had reported KNaO as around 13%. However, independent testing done (e.g. by Ron Roy and others) suggested that around the year 2000 the K₂O content dropped to about 7.5 (with no accompanying change in the Na₂O). Pacer disputed this. The chemical analyses they provided with individual shipments of material continued to report chemistries similar to those stated on their datasheet. 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 became somewhat of a scapegoat, people blaming it when online recipes they try do not work!

Since 2021, their data sheet for the P325 grade claimed 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 have expected it to melt better than in the recent past. However, what actually happened was a decrease in the K₂O, and it did not melt as well.

As with any feldspar, production users had to be vigilant to do sieve analysis testing to spot any iron-bearing particles in the plus 100 mesh range. A melt fluidity test was also practical to gauge its behaviour as a flux.

Related Information

Custer Feldspar update

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The chemistry for Custer Feldspar has been updated in the materials at Digitalfire and Insight-live. Until now, the Pacer-published values have been used, but at Ron Roy's request, this has been changed (see the link to his article below). The average of assays done from 2000 to 2012 has been used for the chemistry. The material name has been changed to "Custer Feldspar (post-2000)". However, Insight-live will still find it in recipes when needing its chemistry, since "Custer Feldspar" has been included as an alternate name. The old material's name has been changed to "Custer Feldspar (pre-2000)".

In this image, you can see the material chemistries on the right and each of them being used in a one-material recipe on the left. To verify they are calculating correctly, the calculation of each has been set to formula plus analysis.

If you have defined your own material for this (typically done to set a cost), everything should still work correctly (using your chemistry if you provided it).

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 is the champion melter of American/Canadian feldspars?

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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 (we consider a feldspar a material that sources KNaO). Nepheline Syenite, this is A270, is 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. 3-5%).

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

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Our melt fluidity tester is being used to compare cone 8 melt fluidities 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. As a demonstration of that, note how well they flow with only 10% frit added (top), even though fired three cones lower.

Custer vs. G200 feldspar at cone 10:

These are fluxes; why aren't they melting more?

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This melt-flow test compares two feldspars at cone 10, with no additives. Feldspars are among the most common flux sources in glazes, yet by themselves they flow surprisingly little. How is this possible?

Feldspar supplies potassium and sodium (K₂O, Na₂O), but these come packaged with substantial alumina and silica (Al₂O₃ and SiO₂). The result is a viscous melt: These specimens have softened and become glossy, but barely moved. In a glaze, feldspar interacts with other materials to create a different oxide balance, one that can melt more readily and flow much farther. Its usefulness as a flux source depends on the chemistry of the whole recipe—not simply on how far it runs by itself.

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 was consistent between these two shipments.

Don’t overthink Custer Feldspar replacement:

It’s usually straightforward

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In a clay body: In a clay body: Compare the original body with one containing the proposed replacement using SHAB test bars fired together. Adjust the replacement percentage to approach the original absorption and firing shrinkage (and fired color, deformation and glaze fit). Nepheline syenite or a suitable soda feldspar may achieve the required maturity at a lower percentage.

In a glaze: Start by matching KNaO—the combined molar amounts of K₂O and Na₂O. This gives more freedom to use available feldspars without requiring an exact match of potassium and sodium separately. Confirm the result by firing tests. The following are practical starting points, not fixed percentage rules:

1. About 20% feldspar or less: Try a one-for-one substitution with a comparable feldspar recommended by your supplier. Compare fired tests before changing a production batch.
2. About 20–40% feldspar: A chemistry adjustment becomes more worthwhile. Use the replacement feldspar to match KNaO, then adjust the other ingredients to bring Al₂O₃, SiO₂ and the remaining oxides into line.
3. More than about 40% feldspar: Check glaze fit carefully. If the glaze crazes, this may be an opportunity to fix both issues (reduce KNaO and increase suitable lower-expansion fluxes, such as CaO or MgO). Test for changes in melting, surface and color.
4. Special-effect or reactive glazes: Identify what produces the effect and preserve the relevant chemistry, materials and firing conditions. Matching total KNaO is a useful first step; if the effect changes, investigate the K₂O:Na₂O ratio as well as other differences.
5. Glazes with too little clay to suspend or adhere well: Consider fixing this issue at the same time, supplying some KNaO from a low-alumina frit such as Frit 3110. This can make room for more clay while maintaining the Al₂O₃ target. Rebalance the other oxides and test both slurry behavior and fired results.

Custer Feldspar vs Nepheline Syenite at cone 8 oxidation

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Although Nepheline Syenite and Custer Feldspar are used as effective body maturing agents and fluxes in glazes past cone 6, curiously, neither of them melt well by themselves. Thus, both of these come 6 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.

Craze city: Pure Feldspar and Nepheline Syenite

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These pure material powders were applied to bisque as slurries, suspended by gelling with Epsom salts, and fired at cone 10R. On the left is Custer feldspar on a Grolleg porcelain (P700); on the right is Covia nepheline syenite on a whiteware body (P600).

Feldspars and nepheline syenite supply large amounts of K₂O and Na₂O, oxides having very high thermal-expansion contributions in glaze chemistry. When these materials are melted essentially by themselves, the resulting glasses have very high thermal expansion relative to typical ceramic bodies - thus the spectacular crazing seen here. Glaze recipes containing high percentages of them can likewise be prone to crazing unless enough lower-expansion oxides, especially SiO₂, Al₂O₃ and MgO, are present.

Replace Custer Feldspar in a cone 10R glaze:

Two minutes of glaze chemistry

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My G2571A and G1947U cone 10R glaze recipes are both in use in Ecuador. One user has been importing Custer feldspar, but now needs to substitute a local one. That material is much higher in SiO₂ and much lower in Al₂O₃ and KNaO (so a pound-for-pound substitution is likely not going to work). Here is how I did the chemistry. It turned out to be a matter of increasing the new feldspar to match the KNaO (which also matched the Al₂O₃) and then reducing the silica to match the SiO₂. The amount by which I had to change the feldspar is a testament to how different the chemistry of these materials can be from country to country.

If you do this calculation, please note the following: In August 2026 the name for Custer Feldspar was changed to "Custer Feldspar (post-2000)" (both here and in the Insight-live reference materials). There is also one for "Custer Feldspar (pre-2000)".

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