The role of magnesium chloride in zinc oxide eugenol impression paste:

Chemical constituents and setting reaction The chemical constituents and setting reaction of impression paste is essentially similar to the zinc oxide eugenol cement. The gum rosin is an important constituent of both pastes and acts as a binder, giving the paste body and reducing the risk of separation of the other components. It also has the property of being thermoplastic so that the impression can be softened in hot water. This aids removal from the plaster model which is subsequently poured. The magnesium chloride and other metallic salts of acetic and hydrochloric acid act as accelerators, so that the material will set in a clinically acceptable time. As with zinc oxide eugenol cements, water catalyses the setting reaction of impression paste.