Specifies the procedure for the preparation of crude samples and dried samples and of natural and synthetic materials having a molar ratio sodium fluoride/aluminium trifluoride between 3 and 1,7 approximately. The crude sample is intended for the determination of geometric characteristics, for certain physical and physico-chemical tests and for moisture determination. The dried sample serves especially for chemical tests. The principle of the procedure consists in grinding followed by sieving of the sample until the whole passes through a sieve of mesh aperture 0,125 mm. Thorough mixing and drying at approximately 110 °C.