Glaze material · Clays
EPK
Also sold as: Edgar Plastic Kaolin, EP kaolin
The default kaolin in North American glazes: alumina and silica in a form that also keeps the glaze suspended and helps the raw coat handle.
In published recipes
Used in 4,397 of 10,859 glaze recipes (40%)
Typical amount when present
11.7% of the dry batch — half of recipes fall between 7% and 20%.
Fired surface of those recipes
Glossy 58% · Matte 16% · Semi-glossy 16% · Satin matte 11%
Most often mixed with
Materials that appear in the same recipes most often.
Chemistry
- Raw formula
- Al2O3·2SiO2·2H2O
- Molecular weight
- 258
- Fired formula
- Al2O3·2SiO2
- Equivalent weight
- 258
- ideal kaolinite formula
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
Clay in a glaze raises the melting point, reduces flow and, in excess, matts the surface. Where a raw glaze cracks as it dries, potters swap part of the clay for calcined clay. Kaolin is the purest clay form; other clays also bring iron and trace minerals.
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 EPK do potters use in a glaze?
- Across 4,397 published glaze recipes containing it, the typical amount is 11.7% of the dry batch, with half of recipes between 7% and 20%. 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 EPK used at?
- It appears in recipes across the range; the most common cones among published recipes are cone 6, cone 10, cone 9. Where a material sits depends on the rest of the recipe more than on the material itself.
- Is EPK 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 |
|---|---|---|
| Kaolin | 2,677 | 11% |
| Grolleg kaolin | 255 | 12.1% |
| Calcined kaolin | 285 | 12% |
| Ball clay | 1,683 | 11.4% |
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.
Claybench tracks your glaze materials, not just recipes
Inventory, hazard flags, and per-piece material cost — the same reference behind this page, wired into your studio.