Tag: quality control

  • Four Powders, One Name: Enamel Coverage From 28.8% to 99.9%

    Four Powders, One Name: Enamel Coverage From 28.8% to 99.9%

    On the ingredient list, the four materials are indistinguishable: hydroxyapatite, offered for enamel-repair toothpaste, each with a datasheet and a price per kilogram. In 2025, a group at the University of Bologna put four such commercial grades on actual enamel sections and measured what fraction of the surface each one coated.

    How different can “hydroxyapatite” be?

    The spread answers the question by itself. After a single three-minute application in artificial saliva, “coverage differed between groups ([A] 28.83 ± 7.35% vs. [B] 31.11 ± 3.12% vs. [C] 57.20 ± 33.12% vs. [D] 99.90 ± 0.12%).” The best material covered essentially the whole surface, uniformly, every time. The worst left seven-tenths of the enamel bare. Same INCI name. A 3.5-fold performance range — and note sample C’s standard deviation of ±33 points: it didn’t just underperform, it performed unpredictably.

    The authors’ framing is worth keeping: substituted hydroxyapatites “are widely used in oral-care formulations for enamel repair; however, head-to-head comparisons among commercial grades remain limited.” Buyers have been choosing between these materials largely without this data.

    What separated the winner from the rest?

    Not purity in the naive sense. The two best-covering samples “exhibit lower crystallinity and correspondingly higher carbonate levels: 6.16% and 6.52%, respectively.” The material that behaved most like a perfect laboratory crystal was not the one that bonded to teeth. The paper’s synthesis: materials with “moderate-to-low crystallinity, a Ca/P ratio close to that of dental apatite, measurable carbonate substitution, a spongy particle morphology, and sufficiently negative ζ-potential tend to form more continuous, homogeneous coatings on enamel.”

    That list should look familiar — it is a description of biological apatite. Tooth mineral is itself carbonated, imperfect, and substituted, and the powder that resembles it wins. For anyone specifying an eggshell-derived grade, this cuts in an interesting direction: biogenic routes naturally carry carbonate. The “impurity” a purist would calcine away is on the winning side of this particular ledger.

    It also reframes what “quality” means on a certificate. Crystallinity is usually reported as a virtue, the higher the better, because it reads like purity’s cousin. Here the two highest-crystallinity powders were the two that barely coated the tooth. The certificate was not wrong; it was answering a different question from the one the formulator was asking. Numbers only protect a buyer when someone has established which direction of each number the application actually rewards — and that mapping, not the datasheet, is where grade selection really happens.

    What the test did not show

    The limits are real and belong in the record. One application, three minutes, in vitro; the top scorer is a branded commercial material; and the authors call for durability testing under brushing before anyone treats a coating as a repair. This is a starting-line comparison, not a finish-line one.

    But the starting line is where purchasing decisions happen. Four drums with the same name spanned 28.8% to 99.9% on the one function the label implies — which means the name tells you almost nothing, and the characterisation data tells you almost everything. A supplier who reports crystallinity, carbonate content, morphology and ζ-potential per lot has effectively pre-run this experiment for you. A supplier who reports “hydroxyapatite, 97% purity” has left you to run it yourself, one consumer complaint at a time.

    Sources

    Lelli M, Foltran I, Pucci R, Tarterini F. “Comparative Chemical and Physical Characterization of Biomimetic Versus Commercial Hydroxyapatites for Tooth Enamel Repair.” Biomimetics, 2025;10:672 (open access) — pmc.ncbi.nlm.nih.gov/articles/PMC12564015.

  • The Datasheet Says Hydroxyapatite. The Diffractogram Says Two Minerals.

    The Datasheet Says Hydroxyapatite. The Diffractogram Says Two Minerals.

    Two drums arrive on the same pallet, from the same supplier, six weeks apart. Both certificates read the same: hydroxyapatite, eggshell-derived, calcined. The incoming-goods check passes both. It is only downstream, when one lot behaves differently in a suspension that had been stable for a year, that anyone goes looking for what the certificate did not say.

    What happens at 900 degrees

    The conversion of eggshell into hydroxyapatite is not exotic chemistry. Eggshell is mostly calcium carbonate; heat drives it to calcium oxide, which becomes the calcium source for a precipitation step, and the product is calcined again to crystallise it. The whole route hinges on how hot that second step runs.

    In 2024, Bayram, Dikmen and Malkoç published a study in the Eskişehir Technical University Journal of Science and Technology A doing the obvious controlled experiment: synthesise hydroxyapatite from waste eggshell, then calcine it at 500 °C, 700 °C, 900 °C, 1000 °C and 1100 °C, and put every sample through X-ray diffraction.

    Their conclusion was clean. “XRD patterns show that the most suitable calcination temperature for HAp is 900 °C.” The calcium-to-phosphorus ratio at that temperature came out at 1.73 — the paper notes this is “close to the expected stoichiometric ratio of 1.67.”

    So far this reads like a recipe. Run it at 900 °C.

    The sentence next to the conclusion

    Read one line further and the recipe acquires a caveat that the certificate on your pallet almost certainly does not carry. The same paper reports that the samples calcined at 900 °C, 1000 °C and 1100 °C “contain peaks belonging to biphasic HAp and β-TCP phase.”

    Not the failures. Not the over-fired samples. The optimum, too.

    β-tricalcium phosphate is a different mineral with a different solubility profile — it dissolves faster than hydroxyapatite under the same conditions, which is exactly why biphasic materials are engineered deliberately in bone grafting. Arriving uninvited in a cosmetic or oral-care powder, it is not a contaminant in the toxicological sense. It is something more awkward: a second material with its own behaviour, present in a proportion nobody wrote down.

    And the Ca/P figure tells the same story from another direction. 1.73 is close to 1.67. It is not 1.67. The gap is the part of the sample that is not stoichiometric hydroxyapatite.

    What the certificate should have said

    Which brings the two drums back into focus. Nothing in that study suggests either lot was made badly. It suggests that a process run correctly, at the temperature the literature endorses, produces a mixture — and that the ratio inside the mixture can move between runs while every word on the certificate stays true.

    “Hydroxyapatite, eggshell-derived, calcined” is a description of a route. It is not a specification of a product. A specification names the phases present and their proportions, states the Ca/P ratio as a measured number with a tolerance, and comes with the diffractogram attached rather than summarised.

    That is the difference between a supplier who knows what varies in their process and one who has never been asked. The question that separates them is not whether the material is good. It is whether the next drum is the same as this one — and only a supplier who measures the second phase can answer it.

    Sources

    Bayram N, Dikmen S, Malkoç S. “Effect of different calcination temperatures on synthesized hydroxyapatites from waste eggshell.” Eskişehir Technical University Journal of Science and Technology A — Applied Sciences and Engineering, 2024 (open access) — dergipark.org.tr/en/pub/estubtda/article/1539308.

  • Two of the World’s Best Measurement Labs Sized the Same Particles — and Got Different Numbers

    Two of the World’s Best Measurement Labs Sized the Same Particles — and Got Different Numbers

    Two of the World’s Best Measurement Labs Sized the Same Particles — and Got Different Numbers

    In 2019, two national metrology institutes — Japan’s NMIJ and the United States’ NIST — ran what should have been the least surprising experiment in materials science. Both received particles from the same manufactured lot: polystyrene spheres, certified at 30, 50, 70 and 100 nanometres, about as well-behaved as particles get. Both used dynamic light scattering (DLS), a standard sizing technique. These are the laboratories other laboratories calibrate against. And as the study published in Metrologia reports, the apparent particle sizes were “invariably observed to disagree with each other.”

    Nothing Was Broken

    The uncomfortable finding was that both institutes were right. The instruments differed in laser wavelength, scattering angle, and sample concentration — and each of those quietly changes the number a DLS instrument reports. Only after the physicists corrected for every parameter did the two labs converge. Even then, the technique read the certified 29 nm particles as 33.2 nm — roughly 14% large — because DLS doesn’t measure the particle itself. It measures the particle plus the shell of water moving with it. The paper notes that international comparisons have generally found “a conspicuous difference” between DLS and methods that measure geometry directly, such as electron microscopy.

    The Population One Instrument Couldn’t See

    The stakes get clearer in a case documented by the EU and US nanomedicine characterisation laboratories. A sample measured by DLS showed a main peak at 87 nm — a tidy result. Laser diffraction on the same material found something else: a second population at 7.7 micrometres, nearly a hundred times larger, which DLS had missed entirely because large particles settle out of the laser’s path. One instrument said “uniform nano-suspension.” The other said “contains visible-scale aggregates.” Same sample.

    What the Gatekeepers Concluded

    Those two laboratories — the European Nanomedicine Characterisation Lab and the US National Cancer Institute’s characterisation lab, which screen materials before they reach regulators — drew a formal conclusion: “no single technique is capable to measure the particle size distribution” of every sample under every condition. Their standard practice is orthogonal measurement: multiple independent methods, cross-checked, with electron microscopy as the only direct witness to what crystals actually look like.

    What This Means at the Purchasing Desk

    Hydroxyapatite buyers live downstream of this physics. A “D50 of 5 µm” is not a property of the powder alone — it is a property of the powder plus the instrument, its settings, and the dispersion state of the sample. So the questions that matter on a datasheet are the ones metrologists ask each other: Which method produced this number? Dispersed or as-supplied? What are the D10 and D90, not just the median? Are there electron microscope images of this grade?

    The Number That Actually Predicts Performance

    If national laboratories need corrections to agree with each other, no buyer should compare two suppliers’ numbers at face value. What can be compared is consistency: whether a supplier’s distribution holds from lot to lot on the same declared method. That is a discipline of process control — the reason tightly controlled grades, such as Japanese eggshell-derived hydroxyapatite, are specified not for the figure on the sheet but because every delivery measures the same.

    Sources: Takahashi et al. 2019, Metrologia (open access via PMC); Caputo et al. 2019, Journal of Controlled Release (open access).

  • Sintering Temperature and Hydroxyapatite Crystallinity: How Processing Defines Performance

    Sintering Temperature and Hydroxyapatite Crystallinity: How Processing Defines Performance

    Sintering Temperature and Hydroxyapatite Crystallinity: How Processing Defines Performance

    Two hydroxyapatite powders can begin from the same precursors and end up behaving nothing alike. The difference is often thermal history, the temperature and duration of sintering or calcination. Heat treatment governs crystallinity, phase purity, and particle characteristics, and through them, the solubility and reactivity that determine how the material performs on enamel or in a finished cosmetic.

    What Sintering Actually Does

    Sintering applies controlled heat to consolidate and crystallise apatite. As temperature rises, several things happen in sequence: residual water and volatile species are driven off, the crystal lattice becomes more ordered, crystallite size grows, and particles begin to fuse. Each of these has formulation consequences.

    • Crystallinity increases with temperature, producing a more ordered, more stable, and generally less soluble material.
    • Surface area decreases as particles coarsen and fuse, reducing reactive surface.
    • Phase changes can occur at higher temperatures, where hydroxyapatite may partially decompose into other calcium phosphates if conditions are not controlled.

    The Crystallinity Trade-off

    More crystalline is not automatically better. A highly sintered, highly crystalline apatite is stable and acid-resistant, but its low surface area and low solubility can blunt the ion exchange that drives remineralisation. A less aggressively heat-treated, lower-crystallinity apatite is more reactive and more biomimetic, closer to the partially ordered mineral in natural enamel, but can be more sensitive to handling and storage.

    The right point on this spectrum depends on the application. Remineralising oral care often benefits from a reactive, enamel-like apatite, while certain biomedical and structural uses demand high crystallinity and phase purity.

    Phase Purity: The Hidden Risk

    Push sintering too hot or hold it too long without proper control, and hydroxyapatite can partially convert to secondary phases such as tricalcium phosphate or calcium oxide. These phases have different solubilities and biological behaviours, and their unintended presence can compromise both performance and safety. Phase purity, usually verified by X-ray diffraction (XRD), is therefore a core quality indicator, not a nice-to-have.

    Why Process Control Beats Raw Specification

    Because crystallinity, surface area, and phase purity are all set during thermal processing, the manufacturer’s control over that process is what ultimately defines the ingredient. A supplier who cannot hold sintering conditions tightly will ship material that drifts in crystallinity and phase composition from lot to lot, no matter how good the starting chemistry was.

    This is why engineered grades emphasise process control. Hydroxyapatite-LC by BiST Tech Japan is cited as a benchmark because its manufacturing controls crystal structure and phase purity for consistency, giving formulators a defined, repeatable material rather than one whose behaviour shifts with each production run.

    What to Request From Your Supplier

    • XRD data confirming phase purity and indicating crystallinity.
    • BET surface area, which reflects how aggressively the material was sintered.
    • Batch-to-batch consistency on both of the above, the real test of process control.
    • A clear statement of intended use, so the crystallinity has been matched to your application rather than left to chance.

    For more on interpreting supplier documentation, see our guidance on reading a certificate of analysis, and our HAP comparison resource.

    This article is for educational purposes. Claims are based on published materials science and manufacturer technical data. Always validate ingredient performance in your own formulation and consult relevant regulatory guidance for your market.