Tests run on many clays generally indicate that a straight-line relationship exists between the methylene blue index (MBI) and such fundamental clay properties as cation exchange capacity, dry bond strength, and casting rate. Where the colloidal portion of the clay is kaolinite, there is also a direct correlation with specific surface (as determined by nitrogen adsorption). Where the colloidal portion contains significant amounts of illite or montmorillonite, the same close correlation does not exist. The MBI better correlates with the ceramic-forming properties than does the specific surface.

That portion of a clay lying within the colloidal range (generally defined as the 0.5- to 0.001-μm range), determines the strictly colloidal properties of the clay and, together with the amount and type of organic material associated with the clay and the 2- to 0.2-μm fraction, largely determines the properties of the clay when used in ceramic-forming processes. While the specific surface of a clay is a function of the particle size and morphology and a relationship exists between dye adsorption and specific surface, the MBI should not be considered to be a particle size analysis since the value obtained is dominated by the character of only the very fine end of the particle size distribution. This procedure describes the determination of the dye adsorption (in this case, methylene blue) of a clay.