Directional solidification is a common method for processing metals and for growing crystals.
Molten material is cooled, causing a solid to form at one end of the sample.
As the sample is cooled, the solidified region grows. The interface between solid and liquid material moves from one end of the sample to the other, hence the term directional solidification. Hotter zones are held at temperatures above the melting point of the material, while cooler zones are kept at temperatures below the melting point. The zones are controlled independently to establish a precise temperature profile down the length of the sample. Temperature sensors monitor the progress of solidification down the ampoule containing the sample material.