Describes the preparation of the calibration graph and the dissolution of a test portion in concentrated sulphuric acid and heating on a hot-plate until the hydrofluoric acid has evaporated. Treatment of the residue with concentrated hydrochloric acid. Aspiration of the solution into an acetylene/dinitrogen monoxide flame and spectrophotometric measurement of the absorption of the 422.7-nm line emitted by a calcium hollow cathode lamp.