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Bulk diffusion of point vacancies down a stress gradient. This is a volume diffusion through grains. Site vacancies flow from the faces that are in tension, where their concentration is high, to faces in compression, where their concentration is lower. Operates in conjunction with Coble creep. The two processes involve the non-random motion of point defects and can cause a change in shape of a crystal, and are thus deformation mechanisms. The vacancies movement through a crystal, is known as Nabarro-Herring Creep. If the vacancies diffuse around the grain margins then it is called Coble creep. That is, below a critical shearing stress for large dislocation movement mechanisms as Coble Creep take place.