Application to products containing at least 98 % of elemental sulphur, expressed as a percentage by mass, of the product dried at 80 °C for 2 h. The difference between the ash and the residue corresponds to the "non-volatile bituminous matter" content at 200 °C. The principle of ashing consists in slow combustion in air of a test portion of the dried product. Heating in a furnace, controlled at 850 °C to 900 °C, and weighing. The determination of non-volatile matter is carried out by slow evaporation in a furnace, controlled at (200 10) °C, in a flow of nitrogen, of the volatile matter and the sulphur. Weighing of the residue after about 10 h.