Coking coals are characterized by their ability to swell as they devolatilize during heating. However this capability is reduced by oxidation. Caking tests, such as the Free Swelling Index or Crucible Swelling Number, quantify the swelling, which is a function of rank, vitrinite content (type), and oxidation.

The ASTM Standard D5263-93 – Test Method for determining the Relative Degree of Oxidation in Bituminous Coal by Alkali Extration, is a commonly used test for coal Oxidation. However, this method is not without its problems. Among many coals, suspended clays cloud the liquid and produce a false positive oxidation result.

In our labs, we use an alkaline organic dye solution which stains oxidized coal grains green-yellow. The uptake of the stain is highly selective and is restricted to oxidized grains, as shown below.