Now, I read somewhere recently that if you dissolve copper sulphate and salt (sodium chloride)together, you get a really nasty liquid that's used to etch metal. I've not heard what crystals you get when it evaporates, though. Of course, it's far too dangerous to do this at the museum - they wouldn't like that at all. So I've done it in my kitchen instead. :)
On the right is the aluminium after 20 seconds of dunking. Good, eh? Any guesses as to which crystals will end up growing in that horrible gloop first? Sodium sulphate? Copper chloride? Just some weird green gloopy stuff?