Clay Colloids and the Potter

D. A. Holdridge
1950 Clay minerals  
The mode of formation and the particle size distribution of potters' clays, particularly china clays and ball clays, were discussed in this paper, emphasis being laid on the proportion of colloidal material present. From this information, it was found possible to estimate the proportion of colloid in pottery bodies. After a brief indication of the structure of the clay minerals involved, the colloidal properties of clay were considered in some detail, particular attention being paid to the
more » ... ng paid to the development of the Gouy-Freundlich diffuse double layer and the effect of exchangeable ions on this. The theory of deflocculation of clay slips is based on the diffuse double layer and the development of an adequate potential. The anions are considered to be located on the surface of the particles which have associated lyospheres the size of which is dependent on the cation present. The ~ potential being directly proportional to the charge on the particle, e, and the average thickness of the double layer, d, will therefore depend on the dimensions of the lyosphere. Kaolinite adsorbs OH--ions preferentially on its active points in the presence of low concentrations of anions, OH--ions therefore giving the highest charge and the greatest value for ~. If however, concentration of anions is high, anionic exchange can occur so that the presence of any quantity of SO4 z-, say, in the solution will result in reduced stability. Likewise cations are adsorbed by the negatively charged clay according to the Hofmeister series; the monovalent alkali ions being least likely to be adsorbed, give the greatest stability to the system. The water of hydration increases in the same order so that the ease with which a cation can approach the negatively charged particle is dependent on the water hull. Small alkali ions such as Na+, therefore tend to increase the thickness of the double layer and also ~ with consequent deflocculation. From a consideration of the relative efficiencies of various deflocculants, we conclude that for satisfactory deflocculation of clay slips : (1) The deflocculating electrolyte must be a salt of a monovalent cation having a high water of hydration; (2) The electrolyte must provide OH--ions, either directly by ionization, or indirectly by hydrolysis; (3) Elements high in the Hofmeister series appearing as exchangeable cations, either on the clay, or in the dispersion medium, must be removed as insoluble salts, e.g., Ca2+--> CaSiO3; (4) Anions low in the preferential absorption series must also be eliminated as insoluble salts, e.g., SO4Z----> BaSO4. 107
doi:10.1180/claymin.1950.000.4.03 fatcat:e2kma4hh25avneqy5imwcxqvhe