Don't store in a humidor. You want as dry as possible.
You can dry epsom salt in the oven and it will lose its water of crystallization and drop the salt in the bottom of your jar and they will keep it dry and absorb moisture from the air.
Here is a c/p from the merck index
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Magnesium Sulfate: Epsom or bitter salt. MgSO4 • 7H2O; mol. wt. 246.50. Anhydrous salt 48.84%, H2O 51.16% (45-52%, U.S.P.), Mg 9.87%, MgO 16.36%, SO3 32.48%, SO4 38.97%, H2O4 39.79%.
Colorless or white, somewhat efflorescent crystals or white powder; bitter, saline, cooling taste. On exposure to dry air at ordinary temp. it loses about 1 H2O (7%); at 70-80° loses 4 H2O (about 28%); at 100° loses 5 H2O (36.5%); at 120° loses 6 H2O, rapidly reabsorbing water when exposed to moist air; loses the last mol. of water at about 250°. d. 1.67. One gram dissolves in 0.8 mil. water, 0.2 ml. boil. water; very slowly soluble in 1.1 ml. glycerol; slightly soluble in alcohol. Its aq. soln. is neutral. pH 6-7.
Keep well closed.
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I would assume that those temps are in C.
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