The procedure includes examining a small quantity of the sample under the microscope and, if grains of different colours or diameter are present, selecting a few grains of each, placing the selected grains between two glass microscope slides and, whilst applying pressure, slowly moving one slide over the other with a reciprocating motion for 10 s, examining the glass surfaces for scratches. The object of the method is to assess whether the sample has a minimum hardness of 6 on Mohs' scale. Annex A gives information on the international standards series for non-metallic blast-cleaning abrasives ISO 11126 and ISO 11127.