strontium nitrate + meal dust = ammonia fumes?!

Wow, I just opened my ball mill of strontium nitrate and nearly gagged
on the ammonia!  I washed out the mill jug three times, media too,
before milling the S.N.  Anyway, the only other thing I've ball milled
is black powder.  The strontium came out warm to the touch and I
whisked it to the north forty in a hurry!  Am I missing something
basic here?  Dan