Why does everyone recommend misting a loaf with water before recrisping an already-baked loaf of bread?
The initial bit of mist is to compensate for the drying-out effects of reheating. The finished loaf had exactly the desired ratio of moisture (right?-). If you then put that loaf back in a hot oven, it would lose a little more moisture and become dryer than you intended. Adding a bit of moisture in the form of a mist compensates for this, so the net result is the same moisture content in the loaf coming out of the oven the second time as it had the first time.
(Arguably you might ideally like this "compensation" moisture to start out inside the loaf [of course not all in one spot:-]. That's not straightforward though, so misting the outside of the crust [sometimes more on the botton and sides than on the top] is typical.)
Going a little further, most re-crisping operations are actually doing two things at once: making the crust crispy again, and either un-staling or thawing the bread.
In either case, most (not all) of the mist recommended for "re-crisping" is actually helping either an un-staling or a thawing operation that's going on at the same time as making the crust crispy again.
My guess (no evidence at all here) as to why "misting" doesn't automatically lead to a "soggy crust" is all the added moisture moves because of the heat. Some of it goes off into the air in the oven; some of it goes into the crumb; but not much of it remains in the crust.
That's the best I can do at a "theoretical" explanation. But a lot of bread-baking is theory-be-damned do what works. If you can construct a reasonable theoretical explanation after the fact, that's nice. If you can use theoretical reasoning as a guide to what to try next, that's even better. But sometimes it's necessary to just "hold your rational nose" and do what works:-)
Thank you Chuck for such a complete answer! My main puzzlement derived from the fact that to make the bread crisp, we need to remove water from the crust; therefore, it made no sense to make our job harder by making it damp in the first place.
I think what you're suggesting is that some of the water migrates inward and only the thin outer layer gets dried out. To me, it's hard to imagine that after a quick spray and insertion into a hot oven that a significant amount of water manages to penetrate into the crumb. I actually haven't ever tried to recrisp bread without spraying it, so I don't know what effect that has on bread.
and time is needed to re-crisp?
There's a pretty good discussion at this old thread. The very last posting on the thread contains some specific times and temperatures.