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Directional solidification advances a freezing region preferentially along an imposed temperature gradient. Pulling material through fixed heat exchangers can make the mushy layer stationary in the apparatus frame even while solid and liquid pass through it.

The eutectic temperature is the temperature of simultaneous liquid–two-solid phase coexistence in an eutectic system. In directional freezing of a solution away from the eutectic composition, a mushy layer can end at this temperature, where its remaining liquid solidifies.

The liquid fraction is the volume fraction occupied by liquid in a solid–liquid mixture. For a saturated rigid mushy layer, it is the porosity available to Darcy flux.