There is a need to establish a caking test that gives a measure of the caking propensity of powders due to the presence of moisture in a relatively short time. A suitable test has been developed for materials that cake by the partial dissolution and recrystallisation of material to form solid bridges between particles. Boric acid (H3BO3) has been used as a candidate material due to its modest tendency to cake. The developed test involves addition of a specific amount of water to a powder, and then subjecting the contained powder to temperature cycling. The amount of water for addition is determined by independent measurement of the equilibrium moisture sorption isotherm. A caking index can be obtained by comparing the mass of cake formed to the mass of original sample. Tests in which the temperature of the sample was held constant yielded cakes that were comparable in size and strength to the cakes formed during temperature cycling. This interesting anomaly has been explained by the enhanced solubility of submicron surface features. The resulting supersaturated solution promotes crystal growth on surfaces of larger radii of curvature. This phenomenon can be termed Ostwald caking after the analogous process of "ripening" during crystallisation processes. Evidence to support this proposed caking mechanism has been gathered by scanning electron microscopy (SEM), by inducing caking in boric acid by condensation of water from the vapour phase at constant temperature, and by visualizing the disappearance of submicron surface features after exposure to humid air, using atomic force microscopy (AFM). (C) 2004 Elsevier B.V. All rights reserved.