Grain boundary sliding can be conveniently studied by loading helical springs wound of wire with a “bamboo” structure. Such tests on pure silver coils show that identically-stressed boundaries slide at widely different rates, and that the sliding rate in any one boundary is very anisotropic. A grain boundary precipitate of hard particles (SiO2 or A12O3) slows down the sliding, and renders all boundaries identical, and isotropic (as far as grain boundary sliding is concerned). This can be understood if the grain boundary particles are thought of as hard pegs, around which silver must diffuse if sliding is to continue.