I was concerned about this and spoke with the guys at B&S a couple of months ago and they advised that THEIR ferric oxalate has a pretty long shelf life (their process is apparently proprietary/patented). I was told to expect in excess of 1 year. I have been using some that I have had for 6-8 months and it's still fine. Evidently the images start to look muddy with reduced contrast. I have yet to experience it with their product.