Hi Alan, it may also depend on the structure/composition of the silver halide crystals. Silver chloride is more readily dissolved than silver bromide, which is more readily dissolved than silver iodide. For example, sodium sulfite can fix a silver chloride emulsion, but might take very long to fix silver bromide, and might not be able to dissolve silver iodide or bromo-iodide. Apparently even sodium thiosulfate is quite inefficient at dissolving silver iodide (which may explain - at least in part - why sodium thiosulfate isn't recommended for fixing t-grain films etc.). Adsorbed dyes and other emulsion additives likely play a role as well.