Certain previously unobserved features of scale formed in an oil‐fired billet reheating furnace are described. It is shown that sulphur‐rich melts formed at the scale/metal interface penetrate the grain boundaries of the overlying scale: subsequently the sulphur is removed by an oxidation reaction in which the surrounding manganese‐containing oxide takes part, to form a complex manganese silicate. The thermodynamics of possible reactions are discussed briefly and hypotheses put forward for the transport mechanisms of sulphur from the furnace atmosphere to the scale/metal interface.