Glaze material · Fluxes, feldspars & frits
Gerstley borate
Also sold as: GB
The classic boron flux: boron is both flux and viscosity agent, and Gerstley also suspends and hardens the raw glaze.
In published recipes
Used in 1,926 of 10,859 glaze recipes (18%)
Typical amount when present
14.5% of the dry batch — half of recipes fall between 9.9% and 25%.
Fired surface of those recipes
Glossy 65% · Semi-glossy 15% · Matte 10% · Satin matte 9%
Most often mixed with
Materials that appear in the same recipes most often.
Chemistry
- Raw formula
- 0.177Na2O·0.823CaO·0.886B2O3·0.658SiO2 (+3.049 LOI)
- Molecular weight
- 213.3
- Fired formula
- same as raw
- Equivalent weight
- 213.3
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
Named after the operator of a Californian colemanite mine; mining ceased in 1999 and stocks are finite. Substitutes (Gillespie borate, calcium borate frits) supply the boron but not the thixotropic bucket behaviour.
Handling and safety
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 Gerstley borate do potters use in a glaze?
- Across 1,926 published glaze recipes containing it, the typical amount is 14.5% of the dry batch, with half of recipes between 9.9% and 25%. 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 Gerstley borate used at?
- It appears in recipes across the range; the most common cones among published recipes are cone 6, cone 10, cone 5. Where a material sits depends on the rest of the recipe more than on the material itself.
- Is Gerstley borate 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 |
|---|---|---|
| Gillespie borate | 215 | 13.5% |
| Colemanite | 293 | 12.9% |
| Ferro frit 3134 | 1,907 | 20% |
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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