ISO 1063:1974

Solutions of the substance to be tested in differing concentrations are mixed with hard water solutions of differing known calcium hardness. After leaving the solutions to stand in specified conditions their appearance, i.e. clearness, opalescence, cloudiness or precipitation is observed. The method is applicable to surface active agents soluble in water at 20 °C. It can be extended to those which are soluble at approximately 50 °C.