Trace concentrations of metal ion impurities, such as Fe²⁺, Fe³⁺ and Mg²⁺, which are commonly encountered in water, can retard significantly the precipitation of CaCO₃. In view of the paucity of viable techniques for overcoming scale deposition difficulties, it is important to examine options for CaCO₃ scale control by trace amounts of inhibiting metal ions. The literature cites Zn ²⁺ ions as being particularly effective in suppressing CaCO₃ scale deposition. This paper reviews phenomena induced by the presence of ion impurities. Literature data relating to the retardation of CaCO₃ nucleation and crystal growth processes by adsorption of impurities on crystal growth sites are analyzed and correlated. The effect of the presence of Zn²⁺ ions on CaCO₃ scale deposition from hard waters having a Langelier saturation index of 1.0 to 1.5 was measured in two experimental heating systems. A marked scale suppression effect was obtained with as little as 1 to 1.5 PPM Zn²⁺ ion, added to the water either by dosing a Zn²⁺ solution or by the release of Zn²⁺ ions through contact with a redox Zn-Cu alloy medium such as a granular bed of KDF-55®. Includes references, tables, figures.