Carboy Cleaning Question

[scarfaceshim]

So I found an old 5 gallon glass water cooler jug at a garage sale and thought it would make an awesome carboy. I bought it for 10 bucks and after noticed that around the top on the inside it looks rainbow like, like you would see on an oil slick. Does anyone know of anyway I should try cleaning it or if it just how the glass aged to become like this, keep in mind its about 50 years old.

 

[jaytizzle]

Soak in hot water with oxyclean and see what you get.