Short answer: a Certificate of Analysis (COA) reports what a laboratory measured on one sample of one lot. Read it in five steps:
- Check the basis of every line: dry, air-dried or as-received.
- Read fixed carbon, ash and sulfur together.
- Check volatile matter and moisture.
- Look for nitrogen if your castings are sensitive to gas porosity.
- Compare particle size with the way you feed the furnace.
Then confirm that the test methods are named and that the sample represents your cargo.
What each line on a recarburizer COA means
| COA line | What it measures | Basis to check | What to ask the supplier |
|---|---|---|---|
| Fixed carbon (FC) | Carbon left after moisture, ash and volatile matter are removed. In proximate analysis it is usually calculated by difference, not measured directly | Dry (d) is most common. Never compare a dry FC with an as-received FC | Is FC calculated or measured, and on which basis? |
| Ash (A) | Mineral residue left after the sample is burned | Dry | Is ash composition (SiO₂, Al₂O₃, Fe₂O₃) available if slag volume matters to you? |
| Volatile matter (V, VM) | Mass lost when the sample is heated without air under standard conditions | Dry | How does this lot compare with lots you accepted before? An unusually high VM can mean under-calcination |
| Total sulfur (S, St) | All sulfur in the sample | Dry | Does it meet your maximum? Which method was used? |
| Nitrogen (N) | Nitrogen content, often reported in ppm | Ask | N is not on every COA. Request it if your plant tracks pinholes or gas porosity |
| Moisture (M, Mad, Mt) | Water content | Air-dried (ad) or total / as-received (ar) | Which moisture is reported? Is invoice weight wet or dry? |
| Particle size | Share of material inside the agreed size range, by sieving | Not applicable | Which sieve type was used, and how much is below and above the range? |
The arithmetic check. On a dry basis, fixed carbon + ash + volatile matter = 100%. If the three dry-basis numbers on a COA do not add up to about 100%, ask why before you accept the lot.
A worked example: a real laboratory test report
In March 2023, an independent laboratory tested two samples of carbon additive that Panson submitted, labelled “93%” and “95%”. Buyers can request the full report. These are the reported results:
| Test item | Basis | Sample “93%“ | Sample “95%“ | Method cited in the report |
|---|---|---|---|---|
| Moisture (air-dried) | ad | 0.04% | 0.03% | GB/T 24527-2009 |
| Ash | d | 5.14% | 3.51% | YB/T 192-2015 |
| Volatile matter | d | 0.60% | 0.92% | YB/T 5189-2007 |
| Fixed carbon | d | 94.26% | 95.57% | YB/T 192-2015 |
| Total sulfur | d | 0.18% | 0.16% | GB/T 24526-2009 |
| Size 1–5 mm | — | 86.2% | 91.5% | YB/T 192-2015, round-hole sieve |
How to read it:
- Fixed carbon was calculated. 100 − 5.14 − 0.60 = 94.26, and 100 − 3.51 − 0.92 = 95.57. Both match the report, so the basis is consistent.
- Both samples measured above their labels. Treat a grade name as a floor, not as the typical value. Put the minimum FC you need in your purchase order.
- Sulfur was 0.16–0.18%. That is low for an anthracite-based carbon. Graphitized petroleum coke is typically lower still, so choose by your own sulfur limit.
- Size is a share, not a guarantee. 86.2% and 91.5% fell within 1–5 mm on a round-hole sieve. Ask how the remainder splits between fines and oversize, and use the same sieve type when you test.
- The report covers only the samples tested. The samples were submitted by the supplier, so the report says nothing about how they were drawn or whether they represent later production. For every shipment, ask for a lot-specific COA.
Test methods. The report cites Chinese standards (YB/T and GB/T). If your laboratory uses ASTM methods, such as ASTM D3174 for ash, D3175 for volatile matter and D4239 for sulfur, differences in test conditions can shift results slightly. Compare like with like, and write the method you expect into the contract.
Red flags on a COA
- The basis (d, ad, ar) is not stated.
- Dry-basis FC, ash and VM do not add up to about 100%.
- No laboratory name, test method or report date.
- No lot number, or the same COA appears for different shipments.
- Size is given only as a range, without the percentage inside it.
- You asked for nitrogen and it is missing.
Put these in your RFQ, and the COAs you receive will be comparable
- Product family and grade (for example CAC 94%, GPC, CPC low-sulfur)
- Minimum fixed carbon, with basis
- Maximum ash, volatile matter, sulfur and moisture
- Maximum nitrogen, if your process tracks it
- Size range, percentage inside the range, and sieve type
- Test standards (ASTM, ISO, GB or YB) and who samples
- Quantity, packaging and destination port
You can send these fields through the request-a-quote form. Your current COA is the quickest starting point.
Frequently asked questions
Is fixed carbon measured directly?
Usually not. Proximate analysis measures moisture, ash and volatile matter, and fixed carbon is the remainder. That is why FC depends on the basis: the same material shows a lower FC as-received than on a dry basis.
What is the difference between dry, air-dried and as-received?
Dry (d) values are recalculated as if all moisture were removed. Air-dried (ad) values refer to the sample after it reaches equilibrium with laboratory air. As-received (ar) values refer to the material as delivered. Commercial weights are often as-received, while chemistry is usually reported dry.
Why does my laboratory get a different sulfur number?
The common causes are sampling (one bag is not the whole lot), the method and the basis. Agree on the method and keep a retained sample from each lot so disputes can be settled.
Does a third-party report prove every shipment meets the specification?
No. A report like the example above covers only the samples tested. Ongoing proof comes from a lot COA for each shipment and, where needed, inspection at loading.
Which matters more for ductile iron, sulfur or nitrogen?
Both can matter. Sulfur consumes magnesium and affects nodularity. Nitrogen is linked to pinholes when the process is near its limits. See Why low sulfur and low nitrogen matter in GPC.