How much Bone ash do potters use in a glaze?

Across 681 published glaze recipes containing it, the typical amount is 7% of the dry batch, with half of recipes between 2% and 10.6%. 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 Bone ash 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 Bone ash 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.

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Glaze material · Fluxes, feldspars & frits

Bone ash

Also sold as: calcium phosphate, tricalcium phosphate

Calcium phosphate — a secondary flux whose phosphorus promotes red-brown iron colours and milky, mottled glazes.

fluxContributes: CaO · P₂O₅

In published recipes

Used in 681 of 10,859 glaze recipes (6.3%)

Typical amount when present

7% of the dry batch — half of recipes fall between 2% and 10.6%.

Fired surface of those recipes

Glossy 51% · Semi-glossy 22% · Matte 15% · Satin 12%

Cones where it appears

Share of recipes containing this material that fire to each cone.

Most often mixed with

Materials that appear in the same recipes most often.

Silica72%Red iron oxide54%Whiting50%Talc45%EPK38%Dolomite32%Bentonite32%Custer feldspar27%

Chemistry

Raw formula: Ca₃(PO₄)₂
Molecular weight: 310
Fired formula: CaO
Equivalent weight: 103
: phosphorus burns out in firing

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

Encourages breaking over edges; used in saturated iron reds. Boiling during firing can cause crawling and blistering, and around 20% it makes low-fire foam glazes.

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 Bone ash do potters use in a glaze?: Across 681 published glaze recipes containing it, the typical amount is 7% of the dry batch, with half of recipes between 2% and 10.6%. 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 Bone ash 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 Bone ash 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
Whiting	5,462	12.5%

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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