[Elfyn]

I've been cleaning the windows of a fairly old and interesting building these past few days and some of the window frames are, I think, made of copper. The outside glass has a hard, mineral(ish) coating over some of the exterior panes.
Is this from the copper in the frames? And, more importantly, how do I get it off??

[sunshine windows]

Winsol 500 is great for removing mineral deposits on glass

[Harry Robins]

For limestone deposits on windows an acid is often used to remove it ( brick and mortar cleaner etc).  The only trouble with this is that it will stain any metal on contact.  Be very careful with what you decide to use.