Glaze material · Fluxes, feldspars & frits
Custer feldspar
The popular potash (K) feldspar: flux, alumina and silica together, in roughly a 1:1:6 ratio.
In published recipes
Used in 2,210 of 10,859 glaze recipes (20%)
Typical amount when present
37.5% of the dry batch — half of recipes fall between 25% and 47%.
Fired surface of those recipes
Glossy 61% · Semi-glossy 17% · Matte 14% · Satin matte 9%
Most often mixed with
Materials that appear in the same recipes most often.
Chemistry
- Raw formula
- K2O·Al2O3·6SiO2
- Molecular weight
- 556
- Fired formula
- K2O·Al2O3·6SiO2
- Equivalent weight
- 556
- idealized potash feldspar
From Linda Arbuckle's materials table. Mineral formulas are idealized; buy an analysis for the bag you have. Equivalent weight is the weight that supplies one molecule of the fired oxide — the number glaze calculation uses.
What potters should know
Feldspars flux clays and glazes at high temperatures; most commercial spars also carry traces of iron and magnesium. Custer is the K-fluxed workhorse; where a sodium spar is wanted, Minspar 200 stands in for the discontinued Kona F-4.
Handling and safety
Crystalline silica: the primary respiratory hazard in a ceramic studio (silicosis with chronic exposure). Wet processes, ventilation and a fitted respirator when dry.
Dry ceramic materials generate respirable silica dust. Use a NIOSH-approved N95 or P100 respirator and adequate ventilation when mixing dry, and wet-clean surfaces.
This information is provided for planning purposes and is derived from publicly available sources including OSHA regulations, NIOSH guidance, and manufacturer safety data sheets. It does not constitute professional safety advice. Consult your material suppliers' SDS documents and applicable local regulations for authoritative safety guidance.
Questions
- How much Custer feldspar do potters use in a glaze?
- Across 2,210 published glaze recipes containing it, the typical amount is 37.5% of the dry batch, with half of recipes between 25% and 47%. Colorants and additives sit far lower than fluxes and glass formers; treat these as a range to test around, not a target.
- What cone is Custer feldspar used at?
- It appears in recipes across the range; the most common cones among published recipes are cone 10, cone 6, cone 9. Where a material sits depends on the rest of the recipe more than on the material itself.
- Is Custer feldspar safe to use?
- It carries no specific hazard flag beyond the general rule for all dry ceramic materials: avoid breathing the dust and read the supplier's SDS.
Related materials
| Glaze material | recipes | typical |
|---|---|---|
| Potash feldspar | 1,446 | 36% |
| G-200 feldspar | 320 | 34% |
| Nepheline syenite | 3,138 | 30% |
Sources. Usage figures are computed by Claybench over 10,859 glaze recipes compiled from publicly shared community recipe collections (snapshot 2026-02-27) — counts only; no recipe is reproduced, and the figures describe that collection, not every recipe in existence. Material roles and notes are paraphrased from Linda Arbuckle's Ceramic Materials handout unless noted. Hazard notes summarise supplier SDS guidance; read the SDS for the product you buy.
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