A new concept for long-term solar thermal storage is based on the absorption properties of aqueous calcium chloride. Water, diluted and concentrated calcium chloride solutions are stored in a single tank. An immersed heat exchanger and stratification manifold are used to preserve long-term sorption storage, and to achieve thermal stratification. The feasibility of sensible heating the tank via large-scale natural convection without mixing salt solutions is demonstrated via measurements of velocity, CaCl2 mass fraction, and temperature in a 1500 l prototype tank. Experiments are conducted over a practical range of the relevant dimensionless parameters. For Rayleigh numbers from 3.4 × 108 to 5.6 × 1010 and buoyancy ratios from 0.8 to 46.2, measured Sherwood numbers are 11 ± 2 to 62 ± 9, and the tank is thermally stratified. Convective mixing between salt layers is inhibited by the presence of a steep density gradient at the interface between regions of differing mass fraction. The predicted storage time scales based on mixing via natural convection for the reported Sherwood numbers are 160–902 days.