A Certificate of Analysis (COA) is the contract-adjacent snapshot of what a supplier reports for a specific batch or lot of carbon additive. Buyers use it to decide whether material can enter a qualified supplier list, a particular furnace campaign, or a tight chemistry window. This guide explains common lines on a recarburizer COA in plain language.
Fixed carbon (FC)
Fixed carbon indicates how much of the material is available as elemental carbon for dissolution—relative to other mass that behaves differently in the melt. Higher FC often means less additive mass for the same carbon contribution, assuming comparable absorption and losses. Always check whether FC is reported on a dry basis or as received.
Buyer check: compare FC across quotes on a consistent basis (dry vs as-received) and reconcile with invoice weight terms.
Ash
Ash is the non-combustible residue tied to minerals in the feedstock. Ash influences slag volume, inclusion risk, and how much “extra chemistry” arrives with carbon. Two materials can have similar FC but diverge sharply on ash depending on source.
Buyer check: ask whether ash is typical for the grade family you specified and whether any cargo treatment changed fines content.
Volatile matter (VM)
Volatile matter is material driven off under the standard test conditions used in the COA. It indicates how “calcined” or “graphitized” the product is relative to expectations. Unexpectedly high VM can signal process drift or moisture/organics that affect feeding behavior.
Buyer check: compare VM to prior accepted lots, not only to a brochure range.
Sulfur (S)
Sulfur is one of the most audited lines because it affects steel grades, ductile iron nodularity, and downstream inclusion budgets. It is not “bad” in the abstract—it must fit your limit.
Buyer check: confirm whether sulfur is total or tied to a specific method note on the COA, and whether the sample is representative of bulk cargo.
Nitrogen (N)
Nitrogen can matter in steels and iron where pinhole or porosity sensitivity appears at the edge of process control. Not every COA includes nitrogen for every grade; when it matters, request the latest N data and align on sampling.
Buyer check: for nitrogen-sensitive applications, request latest N data or third-party test report before ordering.
Moisture
Moisture influences weight, dusting, and sometimes feeding consistency. A shipment can meet chemistry on a dry basis yet surprise handlers if moisture diverges from practice.
Buyer check: confirm storage expectations and whether weights are dry-adjusted commercially.
Putting it together: a practical review order
- Identify the basis (dry vs as-received) for each line.
- Compare FC + sulfur + ash as your first triangle.
- Add VM if you suspect process or packaging issues.
- Add nitrogen if your plant tracks gas-related defects.
- Close with moisture + size distribution (often on the same document or an annex).
When documentation needs to go further
If your plant ties release to a narrow window, ask whether additional testing or split sampling is supported. Third-party inspection can be arranged when both parties want an independent chain of custody.