In order to enlarge the surface of particles which absorb liquids at leasto a limited extent, the particles are exposed to a liquid or to a humid atmosphere containing this liquid at least until said liquid has penetrated into the area of the particles close to their surface, and the liquid-containing particles are then irradiated with micro-waves until the penetrated liquid evaporates. The surface of the particles is torn down to the depth of penetration of the liquid and thus enlarged, or the particles break up, creating small broken pieces with an irregular and enlarged surface.