Freezing and meltingMelting of sideledgeSideledge in aluminium electrolysis cellsAluminium electrolysis cells was treated in a pragmatic model including coupled mass and heat transferHeat transfer. During slow freezing (“equilibrium freezing”, less than 0.3 mm ledge growth per hour), cryolite with AlF3 and CaF2 in solid solution will be formed, and the rate of freezing is much slower than what can be calculated by common formulas where mass transferMass transfer is not considered. Rapid freezing produces a sideledgeSideledge with composition close to the bath composition, which can be explained by formation of dendrites close to the ledge and subsequent freezing of bath between dendrites. It can be recommended that ledge formation in cells with stable operation is estimated using the liquidus temperature at the ledge surface, i.e., taking mass transferMass transfer into account. Ledge formation during rapid freezing and meltingMelting (e.g., during power cycling) should be treated by the standard equations using the liquidus temperature for bulk bath. If the bath temperature falls below the bulk liquidus temperature, formation of cryolite particlesCryolite particles will take place because carbonCarbon particles act as crystallization boundaries. Freezing in the bulk of the bath should be accounted for in dynamic cell models.