It is either the anhydrous salt (doubtful) or the decahydrate, which has zero drying capability as is. The salt loses .10H2O (the decahydrate) at only 100C, so heat it to 150C in the oven to speed the process up, and bake it for........I dunno, 30-45 minutes should be good, if not overkill. But better safe than sorry, right? Right.
Don't leave the post dried salt out in the open or it will re-absorb moisture from the air. Also, do not pluverize the chunks into dust or even small granules, or you will have nasty cloudy solvents when you go to dry them, and it will take years to settle. (not literally, but a long time!)
Leave them in the small rock-salt sized chunks they come in, and just bake them.
PrimoPyro
Vivent Longtemps La Ruche! How's my posting? Call 1-800-EAT-SHIT