I've cooked maybe a dozen times with my flat-bottom wok (carbon steel, uncoated).  A layer of ‘tar’ has developed in the bottom, which I've been unable to clean out. The ‘tar’ doesn't seem to affect flavor, but recently when I cooked tofu for the first time I saw unsightly black flakes on the tofu.  What can I do short of scraping? Can I loosen the ‘tar’ by boiling water in the wok?  Or should I just live with it?  Next day: I woke up late and found that the elves had taken off most of the ‘tar’ with oil, kosher salt and paper towels. Now for the boiling water. 
It is not absorbing water from the atmosphere, rather, moisture is migrating from within the bread.  Basically, as soon as a loaf of bread is removed from the oven, moisture migration begins to happen.  It doesn't take long (hours-days) for that moisture to impact the crustiness of bread.  Moisture migration accelerates staling (which turns out to be more complicated than simply drying out, but that was not your question).  The good thing is that heat can effectively and temporarily delay or limit moisture migration.  Just reheat your bread in the oven or toaster, and it should regain much of its crunch.   