Rub it with sandpaper and wipe the oxidized material out. Want to learn more about buttons: accessible BCI Cotton fabric is a kind of natural yarn fabric. It is environmental and comfortable. We have 100%cotton, CVC and TC using the BCI cotton yarn. The fabric we do is solid and printed. You can specify the color or design you want, we will do a lab dip or strike off for your approval before bulk, to make sure the final goods are the ones you want. This cotton fabric is used to do all kinds of garments for children and does sports or garments for spring and summer. We are a member of BCI, so we can issue transaction certificates for buyers. So it can sell to any market in the world. BCI Cotton Fabric,Polyester Cotton Rayon Terry,Cotton French,BCI Cotton Shaoxing Concord Textile Co., Ltd. , https://www.ctknites.com
Or buy something like hydrochloric acid, bubbles, and then take it out with a cloth coat just fine.
Copper button blackening method