I would be very cautious with any photo that had cracking, crazing, or otherwise allowed water to get to the paper substrate. I emphasize again, go slow, be careful, and if the gelatin surface appears to be getting gummy or sticky or otherwise absorbing the water, stop and let it dry completely.
This is NOT the same as soaking cards from an album, and there may well be an age or "brittleness" of the gelatin surface beyond which one would not want to attempt this on an amateur level. I do not think I have attempted it on any 100+ year old photos, and do not think I would personally be comfortable doing so on a photo that had surface damage (loss or cracking of the emulsion surface).
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