I remember a post that you made a while back where you asserted that the reason that fiber displays more of a dry down effect than RC paper is because FB shrinks as it dries, concentrating and darkening the tones. I looked for that thread and couldn't find it so I'm referring back to it in a new topic.
Last night I was printing some snow pictures on FB with very delicate highlight tones. The lightest of which were absolutely indistinguishable from paper White while in solution but merely lifting the print and allowing it to drain slightly revealed a perceptable tone in these areas. Blotting the print surface increases the effect further. Of course this darkening continued as the prints completely dried but I was curious, since this effect can not be explained by shrinkage, what would you, as our resident drydown expert, say is the cause of the perceived darkening effect of simply lifting a wet print out of the fixer or water bath?