Desiccation
Following drying or ignition operations, the residues and their containers [crucibles, evaporating dishes, or filter] must be cooled to room temperature before weighing on the analytical balance. If such cooling were allowed to take place in the open air, moisture would be picked up from the air by the residue and container. The amount of moisture pickup would depend upon the time of exposure and relative humidity. To overcome this difficulty, residues and their containers are cooled in desiccators where the relative humidity is kept near zero percent by means of a desiccant such as anhydrous calcium chloride.