Colour Variations in Bituminous Coal with Thickness of Sections

Abstract

THE colour of bituminous coal has been variously attributed to large polynuclear condensed aromatics^1–3, free radicals^4–6, charge-transfer complexes^2,7 and free π-electrons⁸. The chemical-physical causes of colour remain matters of conjecture and will not be treated here; the optical aspects and simulation of colour in coal will be discussed. Thick sections of coal vitrain are black, or opaque in the visible region of the spectrum. For thin sections, however, the transmitted colour progressively changes from brown to red to yellow with decrease in thickness^9,10. The yellow colour is characteristic of the thinnest sections that can be successfully made by the normal procedure. The recent success at this laboratory with the grinding of a thin section to form a wedge suggests that, at thicknesses represented by regions near the tip of the wedge, white light is transmitted; that is, the vitrain is clear and colourless¹¹. Similar variations in colour with thickness are observed for carbonaceous derivatives such as some heavy oils, asphalts and pitches from coal and petroleum. This communication presents an explanation for these colour changes, and the production of identical colour-changes with thickness is demonstrated with pure compounds that are lightly coloured or colourless.