Defined particle bed arrangements are an appropriate and useful tool for studying the substantial comminution behaviour of a particle collective. In this study, a totally open particle bed experienced a single, slow compressive load between two parallel plates. Material flow is possible for this kind of particle bed arrangement. The mass of feed and the load intensity were varied. It could be shown that the breakage and compaction of the particles mainly took place in the part of the material. which stayed in the stressing zone until the end of the compression. This crushed mass can be used as the reference mass for the evaluation of energy consumption and comminution results. The particle size distributions of the comminution products are self-preserving. Furthermore. these distributions can be constructed as a superposition of several statistical particle assemblies, each of which represents a logarithmic-normal distribution. Based on these results, an unconfined particle bed arrangement was proved to be an appropriate test model for fundamental research on comminution. (C) 2002 Elsevier Science Ltd. All rights reserved.