From Vihtavuori Reloading Guide
Although modern smokeless powders are basically free from
deterioration under proper storage conditions, safe practices
require a recognition of the signs of deterioration and its
possible effects.
Powder deterioration can be checked by opening the cap on
the container and smelling the contents.
Powder undergoing deterioration has an irritating acidic
odor. (Don’t confuse this with common solvent odors such as
alcohol, ether and acetone).
Check to make certain that powder is not exposed to extreme
heat as this may cause deterioration. Such exposure produces
an acidity which accelerates further reaction and has been
known, because of the heat generated by the reaction, to
cause spontaneous combustion.
Never salvage powder from old cartridges and do not attempt
to blend salvaged powder with new powder. Don’t accumulate
old powder stocks. The best way to dispose of deteriorated
smokeless powder is to bum it out in the open at an isolated
location in small shallow piles (not over 1” deep). The quantity
burned in any one pile should never exceed one pound. Use an
ignition train of slow burning combustible material so that the
person may retreat to a safe distance before powder is ignited