Paraffin wax has a very high heat capacity, meaning it is able to absorb and retain a great amount of heat. A paraffin wax spa takes advantage of paraffin's heat-retaining capacity in a simple process called heat transfer. In technical terms, heat is transferred to the core of the affected area as paraffin goes through a phase change. As it melts, the paraffin becomes a liquid and is able to retain more heat. Then, when a hand, foot, or other area is dipped into the heated bath, the phase of the paraffin that surrounds the dipped area quickly changes into a solid. The heat that is given off is called the heat of fusion.