I would break it up into small chunks but not totally powder it, as you will block off most of it from the air, which may impair its ability to grab moisture from the air, but I can't back that up with data, just a hunch, I just smash it up a bit. Doesn't seem to damage the microwave none. One thing I'm wondering is if one could use this for the full process of drying mushrooms, simply adding fresh shrooms and the anhydrous epsom salt to an airproof container, dry them partially, re-nuke the epsom salts, and dry them further?