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A one millimeter side bulk material cube offers 6 mm2 of exchange surface. In cutting it out following 3 axis, the exchange surface doubles at every cutting operation. After 16 operations, the exchange surface area is 6.6.104 times greater. For a same volume, the "reactivity" of a material, therefore, depends significantly on its granulometry. Moreover, the visible color of certain materials is changed depending on the size of the particles.