A method of comminuting the size of particles of frangible blast media from each particle's respective initial size to a size smaller than a desired maximum size includes directing a flow of entrained particles toward a gap, at a first location splitting the flow into at least a first and second flow, the particles are entrained in the first flow, the first flow travels through the gap and substantially no particles are entrained in the second flow, and rejoining the second flow with the first flow at a second location downstream of and proximal to the gap.