Copper and brass are both metals that can oxidize or react with oxygen in the air to form a layer of oxide on their surface. This process is called corrosion.
Copper oxide is a brown or black substance that forms on the surface of copper when it is exposed to oxygen and moisture. This can occur through a process called atmospheric corrosion, where the copper is exposed to the air and the humidity in the air causes the oxide to form. Copper oxide can also form when copper is in contact with certain types of acids or bases.
Brass is an alloy made up of copper and zinc. Like copper, brass can also oxidize when exposed to oxygen and moisture, forming a layer of oxide on its surface. The oxide layer that forms on brass is usually yellow or green in color.
Both copper and brass can be protected from corrosion by applying a coating, such as a paint or a clear finish, or by storing the metal in a dry, protected environment.
A lot of people actually like the color that comes from the oxidation. It will not affect the structural stability of the metal unlike steel when it rusts. Statues and roofing are often seen looking like this:
However, if you would like to keep the metal looking like it currently does you can apply a coating to protect it. We suggest using Everbright protective UV clear coat. You can find it here:
https://shop.everbritecoatings.com/index.php?main_page=product_info&cPath=6&products_id=93
We have also used SEM’s 1K HS Clear and it is also a good coating. It can be found here:
We personally prefer the Everbrite as it is made specifically for raw metals, but the SEM option is cheaper.
This link will show you how to use the product.
https://www.everbritecoatings.com/copper-cleaning-gel