Specifies a method for determining the mass distribution of the particle sizes of diameter 50 m and more. The procedure based on mechanical sieving of the crude sample, dried at 100 °C, through woven wire sieves in an atmosphere of relative humidity not more than 50 %. Weighing of each of the separate fractions. Verification, by means of a lens or microscope, of the degree of obstruction of the sieve meshes (maximum permitted obstruction: 1 %). Has also been approved by the IUPAC.