Green stain - another newbie question

[Tudor]

I have an old fedora that has a green stain in the felt right near the sweatband, back center. Over this past year I have seen on eBay two other hats that try to describe a green stain near where the brim meets the crown. What is this and what causes it? Does anyone have a suggestion on how to safely get it out. I have read much on hat cleaning here, I just do not recall any prose on this specific issue. I have learned so much from all of you. I hope that you do not tire from all the newbie questions.

 

[Art Fawcett]

Tudor, at the back of the leather (sweatband) you will see that it is sewn. At the top ot the leather is a reed that gives the leather shape and inside that leather covering is a "ferrule" usually made of brass or alloys containing copper as an alloy. What you are seeing is the copper in the ferrule oxidizing, thus the green. Naptha usually takes this out but I caution , please use gloves when working with it.