180F for 6 hours is waaaay too much for fumarates drying to a state where they can be powdered. Often 30min at 130F is more than enough.
I suspect that since your source is a.confusa, you are dealing with a substance whose salt is quite hygroscopic even as fumarate salt. Maybe, for instance you have NMT-fumarate and that is very hygroscopic by nature? (that is just guesswork).
Did you wash your fumarates with acetone or isopropanol? that might take away residual lipids and aid solidifying. Also, do you plan on turning them onto freebase? If yes, then you do not have to worry about powdery fumarates, but be warned that your freebases might be very gooey.
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