I thought it may have to do with the plastic used in the ink vial, but that makes sense, too. Whatever made up that precipitate is a much darker blue than the actual ink. It's almost like there was a reaction of something with something, like when iron rusts or copper oxidizes and turns green over time.
My particular sample had been in that vial for at least a year. Once I abandoned passive water soaks, and actually used an abrasive (pipe cleaner), it cleaned up with the merest touch. I could have used a paper towel or even my finger instead -- in fact some rubbed off on my finger when I accidentally touched it. The vial was left perfectly clear, too, with no staining. So I'm not going to worry.
A lot of us have used the ink now, and I don't think anyone has had cleaning issues with an actual pen, and to me that's what matters.
But it was a cool thing to see! Fun! And it's a good reminder not to leave a pen inked for years.
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