Opening that can of powder will certainly reduce its collector value. However, IF it has not deteriorated, ie., it looks and smells OK and does not have any red dust or lumps in it, it is probably useable. You might find some data for that powder in Phil Sharpe's COMPLETE GUIDE TO HANDLOADING, Belding & Mull Reloading Handbook, or the Ideal Handbook from that era..

I have fired GI .45 ball ammo that was loaded in 1918, and it shot OK.