Several machines, such as jaw, gyratory or roll crushers use the physical effect of compression to crush hard and brittle rocks. Because of the tremendous expenditures of money and equipment necessary to investigate the comminution of large particles hardly any scientific investigations were carried out so far. Therefore, these crushers are still sized rather by empirical than by scientific approaches. In order to investigate the principles of comminution by compression of large rocks a test apparatus was built and rock: of different dimensions were tested. Since machines which use the effect of compression apply a defined displacement to a rock the main crushing properties were gained as a function of the applied throw and the particle size of the rock: likelihood of breakage, maximum crushing force, mass specific crushing work, crushed fragment size distribution. crushed fragment shape distribution. (C) 2004 Elsevier B.V. All rights reserved.