Years ago, I used a sterling silver fork to retrieve items in my pickle pot when I couldn’t find my copper tongs. I discovered that if forgotten, the combo removed all kinds of stains on sterling silver parts after soldering. I recently did a diamond necklace in sterling silver, and overheated everything, leaving that copper blush behind. So I tried the sterling silver fork thing again and the copper vanished. For the life of me, despite repeated soldering steps on the same pieces, I couldn’t find any fire scale either.
Has anyone tried this and can they explain what is happening?