Quote:
Originally Posted by Maris
Undiluted "neat" rapid fixer (Ammonium thiosulphate based) fixes very slowly compared to a 1+4 or even a 1+9 dilution. Why? The reason is that fixer concentrate has very little water in it. The fixation products, silver tetrathionates etc, can't find water to dissolve in and tend to hang around in the film or paper. Ever noticed how heavy a bottle of rapid fixer concentrate is compared to its size. It's nearly all chemicals with just enough water to make it conveniently pourable.
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However if you place a print that's just been fixed in dilute fixer into concentrated fixer there's some diffusion into the emulsion and image bleaching is very rapid.
Ian