[Limitless]

So I have had some Potassium Nitrate lying around in a close drawer (dark) but the ziploc bag was partially open. I am now wondering if the potassium nitrate is expired or not.
I do know that potassium nitrate has an unlimited shelf life if contained properly, but this potassium nitrate is around 1-2 years old.  I tried to do a little test with sugar and it did burn, however, I am still unsure if it is usable for more experiments. 
Is it expired? Is it still usable?

Thank you in advance. 

[rucik5]

Hi,
I must say I don't know much about it's general stability in air but I don't think anything bad should happen to it. It's not very hygroscopic - according to 'wikipedia' it takes almost 2 months for it to absorb 0.03% of water in 80% (standard?) humidity. Another forum, however, claims that it absorbs moisture quite easily. If it's not lumpy, sticky then I think it should be fine... You could do a small test - take a portion, weight it accurately [perhaps grind it] and either dry it in vacuum dessicator or in the oven at 120-150°C? See if the weight changes a lot and if it turns nice and crispy. It melts at 334°C and decomposes above 400°C.