Impression compound base:
Low thermal conductivity
Crystalline structure
Formation of cross linkage with heating
High fusion temperature
Impression compound 
The thermal conductivity of these materials is very low, indicating the need to allow an extended time to achieve thorough cooling and heating of the compound. The material should be uniformly soft at the time it is placed in the tray and thoroughly cooled in the tray before the impression is withdrawn from the mouth. Cold water can be sprayed on the tray while it is in the mouth until the compound is thoroughly hardened prior to removal of the impression tray.
Failure to attain a complete hardening of the material before withdrawing the impression can result in a significant distortion of the impression by relaxation.
Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 178