One of the oldest and most intriguing problems in the handling of materials is how a collection of solid grains packs together. While granular packing is normally determined by how grains are poured or shaken, we find that a systematic and controllable increase in packing is induced by simply raising and lowering the temperature, e.g., without the input of mechanical energy. The results demonstrate that thermal processing provides a largely unexplored mechanism of grain dynamics, as well as an important practical consideration in the handling and storage of granular materials.

7 pages, 1 figure