Abstract: Process and apparatus for growing crystals using float zone techniques. A rod (34) of crystalline materials is disposed in a cylindrical container (36), with a space being left between the rod and container walls. The space is filled with an encapsulant (72), selected to have a slightly lower melting point than the crystalline material. The rod is secured to a container end cap (38) at one end and to a shaft (60) at its other end. A piston (64) slides over the rod and provides pressure to prevent loss of volatile components upon melting of the rod. Prior to melting the rod the container is first heated to melt the encapsulant, with any off-gas from this step being vented to a cavity behind the piston. The piston moves slightly forward owing to volume change upon melting of the encapsulant, and the vent passageway (74) is closed. The container is then moved longitudinally through a heated zone (32) to progressively melt sections of the rod as in conventional float zone processes.