Topic: How did this blemish occur, please?

[Fen]

This vase is a lovely blue colour and six and a half inches high. I only purchased it from a junk shop very cheaply so I'm not complaining and it certainly still looks lovely to me but... there's a strange blemish in the glass. I have photographed it to show a brown, oil like stain. It is inside the glass, would it have been there when it was made, did it somehow develop later, will the patches grow so that the discoloured patches increase in size? I would appreciate it if anyone could enlighten me.

[Frank]

Probably just contamination, I doubt it will change.