A process of exchanging one ion for another ion on a charged, solid substrate, such as a natural clay, zeolite or resin. Cation exchange on clay minerals occurs in muds and during mud testing. In muds, various positive ions (for example, Ca⁺²) on clay surfaces can be replaced by other positive ions (for example, Na⁺) that occur in higher concentration or have a higher affinity for the substrate. In water muds, cation exchange is an ongoing and dynamic process. In mud testing, the methylene blue dye is a cationic dye that goes onto clays in the mud sample being tested to show the extent of cation-exchange capacity. Ion exchange is the basis for clay peptization, which can improve a poorly performing clay.