Fixed carbon and volatile matter

By Mohamad Sinno, Charcoal Expert · Reviewed by Budi Dharmawan, Head of QC, 4 September 2026

Fixed carbon is the fuel. Volatile matter is what smokes. They are read as a pair because they move against each other: material that was not fully carbonised keeps volatiles it should have lost, and every point of volatile matter is a point that is not fixed carbon.

The four figures are one measurement

A proximate analysis splits the sample four ways, and the four shares add to the whole:

fixed carbon = 100 − moisture − ash − volatile matter

That identity is the fastest audit available on any COA you are handed. Here it is on one of ours — Beckjorindo COA 000964, 24–25 June 2026, as-received:

Run it the other way and the fixed-carbon row is predicted rather than read: 100 − 2.63 − 1.34 − 17.82 = 78.21%, against the 78.21% the laboratory printed. When a COA's four rows do not close like that, something on it is worth asking about — a row on a different basis, or a figure that was not measured at all.

As received, or dry?

The same material reports two different fixed-carbon figures depending on whether the moisture is counted. As received includes it, the way the sample arrived. Dry basis removes it arithmetically and re-expresses the rest as shares of the dry mass — so every figure rises.

The size of the gap is worth seeing, so here it is on invented round numbers that are not ours and are not from any report: a charcoal measuring 70% fixed carbon with 5% moisture is 70 ÷ (100 − 5) × 100 = 73.7% on a dry basis. Same material, same measurement, two numbers nearly four points apart — which is roughly the margin sellers argue over. Every figure in that sentence is illustrative; none of ours appears in it, and none of ours has been converted to produce it.

None of our three certificates states which basis it used, and we will not convert one into the other to fill the gap — an arithmetic conversion is a figure no laboratory measured. So we publish what the reports print, as received, and say so. If a supplier's fixed carbon is quoted without a basis, that is the question to ask before comparing it with anything, including ours.

Which is why the measured envelope on our laboratory reports page quotes fixed carbon 77.03–80.84% as received on one stated basis, rather than merging figures from two into one wider-looking range.

Our lab reports are run on the finished briquette, which includes the tapioca binder — so part of the measured volatile matter comes from the binder rather than from the carbonised shell alone.

What the market calls good

• "≥80% fixed carbon" is a common premium claim, and one exporter's published specification pairs "min 75% fixed carbon" with "max 3% ash" (market convention, August 2026). Neither is a standard, and both are usually quoted without saying which basis they are on.
• "Smokeless" is commonly defined as volatile matter below 20% (market convention, August 2026). It describes a band, not a certification, and no body issues it.

Fixed carbon is not calorific value

This is the most common technical error in the trade's marketing. Fixed carbon is a proximate fraction — the share of mass left burning after volatiles go. Calorific value is an energy measurement, made in a bomb calorimeter, reported in kcal/kg. A high fixed carbon is consistent with a high calorific value; it is not a measurement of one.

We publish no calorific figure for our charcoal because none of our reports includes a calorimetric test. Converting our fixed carbon into a kcal/kg number would be arithmetic dressed as a measurement, and the first technical buyer to ask which laboratory produced it would find out. The reasoning, alongside the two other figures we do not publish, is on our laboratory reports page.