When I Lay Out Several Calcium Phosphate Samples Side By Side, Their P₂O₅ Values Do Not Differ Dramatically.

Calcium Phosphate: Beyond P₂O₅ Content - What Really Matters for Bone China Bodies & Opaque Glazes

For bone china bodies, calcium phosphate-based raw materials account for a substantial proportion of the formulation. When used in opaque glazes or soft matte glazes, they also shape fired whiteness, hiding power and surface gloss.

Below I break down the observed variations, along with points that remain inconclusive without further testing.

Most glaze ingredients are dosed at only a few percentage points in standard glaze formulas. Bone china works differently. Industry conventional formulations load bone ash or calcium phosphate raw materials at 40%–50% within the body recipe.

 

At such high addition rates, minor variations in the raw material get amplified across the whole ceramic body.

The fired microstructure of bone china generally consists of β‑tricalcium phosphate, anorthite and glassy phases. These crystalline and glassy phases directly determine the final whiteness, translucency, mechanical strength and firing stability.

 

That is why, when selecting calcium phosphate for bone china, I avoid focusing solely on the P₂O₅ figure.

The calcium phosphate grade we actively promote and supply delivers the following test results:

CaO: 48.84%

P₂O₅: 45.94%

Na₂O: 2.04%

BaO: 2.11%

LOI: 0.22%

For comparison, a competing sample shows:

CaO: 52.01%

P₂O₅: 46.61%

Na₂O: 0.48%

BaO: trace / negligible

LOI: 0.07%

At first glance, both samples carry roughly 46% P₂O₅ and appear comparable. But the full oxide sheet reveals obvious gaps in Na₂O and BaO content.

I do not draw oversimplified conclusions such as "higher sodium is better" or "barium content delivers fixed benefits". The final fired performance depends on the complete glaze or body recipe, loading percentage and firing schedule. One fact is certain, though:

Switching between these grades is not merely changing one numerical value. You are introducing a whole different chemical assembly into your body or glaze system.

Our flagship grade has low LOI, indicating limited mass loss under standard test conditions. Even so, XRF elemental data alone cannot confirm its crystalline phase. Chemical analysis answers "what oxides are present". XRD testing is required to identify the mineral phase.

Some years ago, I visited an overseas bone china factory and collected a lump‑form calcium phosphate sample they were using. Unlike common fine powder grades, this material was supplied in solid chunks. The customer referred to it as washed calcium phosphate, an imported raw material.

Later, I tested it against multiple domestic calcium phosphate powders and realised: products sharing the same name "calcium phosphate" can be manufactured via completely different source routes and processing workflows.

Test data for this washed calcium phosphate lump:

CaO: 53.81%

P₂O₅: 41.87%

MgO: 1.14%

Na₂O: 0.70%

BaO: 0.27%

LOI: 1.14%

SiO₂: 0.44%

Compared with the powdered grades above, it has a distinct Ca/P ratio, plus higher MgO and LOI. These differences make these questions critical:

Is the calcium phosphate synthetically precipitated, or derived from animal bone feedstock?

Has it undergone washing, calcination and milling?

If calcined, what is the degree of thermal treatment?

These details are often more valuable than the generic label "calcium phosphate" printed on the packaging.

Important note: XRF oxide data alone is not enough to classify this washed calcium phosphate as a specific crystalline phase, nor can we confirm it is bone ash. XRD phase identification is mandatory for that conclusion.

Particle size test results for our promoted grade: D50: ~19.62 μm D90: ~162.9 μm D97: ~235.6 μm D[4,3]: ~52.83 μm

While the median particle size sits around 20 μm, the distribution carries a long coarse tail extending to 160–230 μm.

Describing this material only by mesh size is insufficient. The particle test was run with 1‑minute sonication and no dispersant added. We cannot confirm whether this coarse tail consists of genuinely oversized hard particles or soft agglomerates from the powder.

To reliably correlate this fraction with glaze pinholes, body fineness or slip flow behaviour, parallel testing under consistent dispersion conditions is recommended.

Many of our customers use calcium phosphate not only for bone china bodies but also inside opaque glazes and soft matte glazes. In practical trials, this raw material modulates glaze whiteness, hiding power and surface lustre. Some formulators use it as part of their whitening strategy.

Still, "higher whiteness after addition" is not a universal rule. Glaze performance is governed by the base glaze formulation, addition rate, particle size, glaze application thickness and firing cycle. The same calcium phosphate can produce varied opacity, gloss shifts and defect risks across different glaze systems.

When selecting a grade targeted for opacity or soft matte effects, I recommend structured side‑by‑side trials:

This method delivers actionable data that can be applied directly back to your production formula, rather than a subjective visual judgement of which sample looks whiter.

Calcium phosphate carries a simple product name, yet its application in bone china and ceramic glazes should never be judged solely by the P₂O₅ value. Full oxide composition, raw material origin, processing route, crystalline phase, particle size profile and your specific formulation all shape the final result. Changing calcium phosphate means adjusting the complete chemical input into your body or glaze, not just tuning one single number.

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