How to Verify a Chrome Oxide Green Batch: The QC Checklist

A supplier's certificate of analysis is a promise. Your incoming inspection is the proof. This checklist walks through the six checks that separate a usable batch of Cr₂O₃ from a formulation disaster.

Check 1 — Cr₂O₃ content (ISO 4621-1986)

Ask for ≥ 98.0 % as the contractual floor, not the marketing number. A 97.5 % batch is 0.5 % of something else — usually alumina or silica, and usually enough to dull tint strength.

Check 2 — Hexavalent chromium

Standard grades: ≤ 10 ppm. Micronized grades: not detected. If your market is Europe, this single number decides whether the shipment clears REACH-driven customer audits. Request the raw test report, not just the pass/fail line.

Check 3 — Colour: ΔE against your reference

The most reliable incoming check: tint 1 part pigment with 10 parts TiO₂, draw down against your retained standard, and measure ΔE. A ΔE ≤ 1.0 against your own standard is realistic; above 1.5, call the supplier before you release the batch.

Check 4 — Particle size (D50)

D50 tells you how the batch will disperse. Standard grades run D50 1.0–2.0 μm; micronized ≤ 0.5 μm. Laser diffraction takes minutes and predicts grind time, gloss and tint development.

Check 5 — Water-soluble salts and pH

Soluble salts ≤ 0.70 % and slurry pH 6–8 protect your resin system from flocculation. A pH outside band is the classic silent cause of "the same grade that suddenly behaves differently".

Check 6 — Screenings and foreign matter

Screenings on 0.045 mm ≤ 0.30 % catches coarse oversize that leaves streaks in films and scratches in polished finishes. One sieve, one minute.

The verdict

You do not need a full analytical lab to police your pigment supply — you need a reference standard, a drawdown kit, a sieve and a laser-diffraction run. Ask your supplier for the raw QC protocol; the good ones publish it.

Why QC fails: the four failure modes

Quality control of chrome oxide green fails in predictable ways, and almost none of them involve the pigment being bad. The four classic failure modes are worth naming, because each demands a different fix.

The most expensive of the four is the broken reference chain, because it is invisible: every decision made against a faded master is internally consistent and consistently wrong. We have audited customer labs where the “approved standard” predated the current product line — the pigment had changed, the standard had yellowed, and the two had drifted in opposite directions for years without anyone noticing.

Sampling done properly

Chromium oxide green is a dense powder (relative density 5.1) that settles and segregates in transit. A sample scooped from one corner of a bag is a photograph of one corner of a bag, not of the lot. The professional procedure is the ISO 15528 technique: sample from at least three levels of several containers, combine, reduce by quartering or a riffle splitter, and only then prepare the test specimens. Where the lot is shipped in supersacks, use a thief sampler through the top and side ports. The sampling discipline matters doubly for colour and D50 — the two properties most sensitive to particle segregation.

Reading the certificate like an auditor

A certificate of analysis is either a claim or an instrument record, and the difference is visible in seconds. The professional reads it in this order: (1) the lot number — does it match the bag? (2) the test methods — are ISO or DIN numbers named, or only “conforms”? (3) the units and limits — are the numbers inside the specification window, or just present? (4) the signatory and date — a certificate without a dated signature is decoration. And (5) the absence list — what the certificate does not mention. The properties a supplier leaves off the CoA are the ones they are not proud of. A chromium oxide certificate without a Cr(VI) line is not incomplete by accident; it is silent by decision.

Building the goods-inwards routine

The inspection plan becomes valuable only when it becomes routine. The professional set-up is three tiers: every lot — identity check, Cr₂O₃ content, colour ΔE, Cr(VI); every fifth lot — full chemistry including salts, pH, oil absorption; type approval — heat stability, lightfastness, weathering, full toxicological documentation. Tier one catches the failures that stop production; tier two catches the failures that reach the customer weeks later; tier three protects the formulation itself. The tiers map onto the supplier's own release testing, so a buyer who runs tier one is verifying the supplier's claim, not re-doing their work.

The joint testing protocol: what to agree with the supplier

The most productive hour a buyer can spend with a pigment supplier is not negotiating price — it is agreeing on a joint testing protocol: the same methods, the same reference materials, the same rounding rules, and a named referee laboratory for disputes. Most goods-inwards disputes are not about bad material; they are about two laboratories measuring different things and calling them the same name. A joint protocol fixes five items: (1) the test methods with their ISO/DIN numbers; (2) the reference standard and who holds the master; (3) the sampling procedure; (4) the rounding and reporting conventions (does 99.04 % round to 99.0 or 99.1?); (5) the dispute path — which third-party laboratory arbitrates, and who pays. Once the protocol exists, disagreements become rare and cheap, because both sides measure the same quantity in the same way. Suppliers who resist a joint protocol are usually not hiding anything — but the discipline of the protocol will reveal their process control anyway.

An annotated certificate walk-through

Consider a typical DELIKECOLORS certificate line by line. Lot number and production date: tie the bag to the record — this is the traceability anchor. Cr₂O₃ 99.2 % (ISO 12485): the purity claim with its method; a purity without a method is a rumour. Colour: L* 38.5, a* −14.2, b* 8.5; ΔE 0.4 vs master: the colour position and the consistency claim — the second number is the one that protects production. Cr(VI) ≤ 5 ppm: the regulatory line; its absence would be the loudest line on the certificate. D50 1.2 μm (laser diffraction): the process-fit line; a window, not a point. Water-soluble salts ≤ 0.5 %: the defect-prevention line. Sieve residue 0.05 % on 45 μm: the coarse-particle insurance. Every line answers a question; a certificate with twenty lines answers twenty questions before they are asked — which is precisely what a certificate is for.

Where QC data meets the supplier's process

A buyer who charts incoming QC data over a year learns something no certificate can show: the supplier's process signature. Plot ΔE against lot number and the drift, if any, appears as a slow trend — a sign of equipment wear, a changing ore source, or a deliberate reformulation. Plot D50 and the same story emerges. Suppliers with tight control show a flat, noisy cloud around the target; suppliers under pressure show structure in the noise. The chart is also the buyer's leverage: it turns a subjective argument (“the colour looks off”) into an objective one (“your ΔE has moved from 0.3 to 0.9 over the last six lots”). We publish our release data because we want buyers to chart it — the flat cloud is our best sales material. The buyers who keep those charts are the ones who never argue about colour again.