The physical change of agar from sol to gel is induced by lowering the temperature. The gel liquefies to a sol when it is heated to a temperature known as the liquefaction temperature (70°C to 100°C). When the sol is cooled, it becomes a gel at a point known as the gelation temperature (between 37°C and 50°C). Thus, it is called a reversible hydrocolloid. The gelation temperature is critical for impression making. Phillips dental materials 12th edition page no 168,170