Marks on copper, known as tarnishing (or patina), is a familiar sight to all of us. Copper tarnishes naturally when copper reacts with environmental factors, including moisture and even the air itself. As a result, copper will inevitably tarnish unless it is kept in a vacuum!
The good news is that tarnishing can be quickly and easily removed, and your copper products will once again look as good as new.
To remove tarnishing: simply apply citric or acetic acid (such as lemon juice or white vinegar) and salt (acts as the catalyst for the acid) to the mug and scrub gently using a soft cloth. Alternatively, you can use a professional copper polish. Popular choices include Bar Keeper's Friend and Wright's Copper Cream. Simply apply polish with a soft cloth and scrub gently.