Old stuff, depends on the ppm and how much the particles settled/agglomerated over time, and how well it was kept. You should probably at least strain it through a coffee filter or two before trying to use it if it has been left sitting for a long time tho. Depending on the original ppm, you might get some black sediment at the bottom, which can just be decanted/strained out. That shows that it does change over time. The question is how long before it becomes useless, and the answer will vary depending on how it was stored.
