A salt certificate of analysis should show sodium chloride, moisture, insoluble matter, calcium, magnesium and sulphate, plus any limits your grade needs. A bentonite COA should show moisture, particle size, swelling and the performance tests for its grade, such as viscosity for drilling or green strength for foundry.
Every figure should name its test method and the lot it covers. Without that, the numbers are hard to check. Your contract should list the tests you need, so the COA becomes a check against agreed limits rather than a sales document.
Tests for salt
- Sodium chloride content, with the basis stated as dry or as received.
- Moisture, measured by drying at a stated temperature.
- Water-insoluble matter, mostly sand and clay.
- Calcium and magnesium, often reported as ions or as their sulphate or chloride salts.
- Sulphate, which matters to chlor-alkali and soda ash plants.
- Iron and heavy metals such as lead, arsenic, cadmium and mercury for edible grades.
- Iodine content and the anti-caking agent for iodised or free-flowing edible salt.
- Particle size distribution, which matters for de-icing, water softener and food grades.
Tests for bentonite
- Moisture, which affects both freight weight and performance.
- Swelling index in mL per 2 g, usually by ASTM D5890.
- Viscometer readings, yield point and filtrate loss for drilling grades under API Specification 13A.
- Residue on a 75 micron sieve, or a full particle size distribution.
- Green compression strength and methylene blue value for foundry grades.
- Water absorption, clumping strength and dust for cat litter grades.
- pH, and sometimes a chemical analysis of the main oxides by XRF.
Signs of a weak COA
Some certificates look complete but tell you little. Watch for these signs:
- The test methods are missing, so you cannot repeat the test.
- The certificate shows typical values rather than results for the lot shipped.
- The lot number does not match the bags or the commercial invoice.
- The figures are identical across many lots, which is unlikely for natural materials.
- The certificate is unsigned, undated or issued before the goods were packed.
Make it useful
List the required tests, methods and limits in your contract, and ask for the COA before the cargo sails. Then have samples drawn at loading and tested independently. Reading a lab certificate explains what to look for, and checking a COA is genuine covers how to test a certificate against the batch. A certificate from the producer's own laboratory is a starting point, not proof. For disputes and claims, buyers usually rely on an independent laboratory, often one accredited by NABL, India's national accreditation board for laboratories, or by a body in the buyer's country.
Next step
We check COAs against the contract, draw samples at loading and have them tested by an independent laboratory. To set the right test list for your grade, book a free sourcing review.