In reversible hydrocolloid the property by which the transformation from sol to gel and gel to sol occurs, is a function of the:
Concentration of fillers and plasticizer
Percentage composition by weight of water
Concentration of potassium sulphate
Temperature
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