Gerstley Borate
A standard material in Glazecalc's library: its oxide analysis, unity contribution and hazards.
What it is
- Kind
- Boron sources
- Status
- Hard to get since 2023
- Formula
- B₂O₃ 26.8%, CaO 19.4%, SiO₂ 14.8%, Na₂O 3.95%, MgO 3.54%, Al₂O₃ 0.98%, Fe₂O₃ 0.425%, K₂O 0.399%, P₂O₅ 0.053%, TiO₂ 0.05%, MnO 0.024%, LOI 29.58%
- Also called
- GB, Gerstley Borate Original, Gerstley Borate Rasorite
- Made by
- Laguna Clay Company (milled from U.S. Borax Gerstley mine ore; mine closed 2000)
- Sold in
- US, AU
- Similar materials
- Gillespie Borate , Calcium Borate Frit (Potterycrafts P2954)
- Source
- Laguna Clay Gerstley Borate analysis (posted by Laguna June 2011), as reproduced by New Mexico Clay (2011-06)
Natural sodium-calcium borate ore (colemanite and ulexite with a hectorite-like clay), long the main low- and mid-fire boron flux in US recipes. The mine closed in 2000; remaining stock is limited and costly, and Gillespie Borate is the recommended pound-for-pound replacement.
Oxide analysis and unity formula
Glazecalc has the analysis of Gerstley Borate as weight percent of the raw material. The unity contribution is the moles of each oxide in the amount of Gerstley Borate that holds 1.0 mole of flux, which is how Glazecalc adds it into a recipe's unity formula.
| Oxide | Weight % | Unity contribution |
|---|---|---|
| B₂O₃ | 26.8 | 0.767 |
| CaO | 19.4 | 0.689 |
| SiO₂ | 14.8 | 0.491 |
| Na₂O | 3.95 | 0.127 |
| MgO | 3.54 | 0.175 |
| Al₂O₃ | 0.98 | 0.019 |
| Fe₂O₃ | 0.425 | 0.005 |
| K₂O | 0.399 | 0.008 |
| P₂O₅ | 0.053 | 0.001 |
| TiO₂ | 0.05 | 0.001 |
| MnO | 0.024 | 0.001 |
- Equivalent weight
- 199.17
- Fired weight
- 140.25
- Loss on ignition
- 29.58%
The equivalent weight is the grams of Gerstley Borate that hold 1.0 mole of flux. The fired weight is what is left of that after firing.
Hazards and safety
Hazards: Borate: do not ingest; high-dose animal studies of inorganic borates show reproductive/developmental effects (U.S. Borax Gerstley Borate MSDS, Apr 2013). Avoid breathing dust. Supplier carries a California Prop 65 crystalline-silica warning.
Use it in a recipe
Add Gerstley Borate to a recipe in the Glazecalc calculator to see what it does to a glaze's unity formula.