Hydrocolloids are dimensionally unstable, they should be poured immediately. To avoid this, they should be stored at:

Once the hydrocolloid impression is removed from the mouth and exposed to air at room temperature, some shrinkage associated with syneresis and evaporation will occur. Conversely, if the impression is immersed in water, swelling caused by imbibition can occur. The storage medium, such as 2% potassium sulfate solution, or 100% relative humidity in a storage chamber, is suggested to reduce the dimensional change of agar impressions. 100% relative humidity is the best storage environment to preserve the normal water content of the impression.