I'm not talking about sheet foil, Like a solid chunk of copper. https://smile.amazon.com/Copper-Sheet-Metal-Scraps-Lead-Free/dp/B07RNNY34R/ref=sr_1_2?_encoding=UTF8&c=ts&dchild=1&keywords=Copper+Sheets&qid=1613076665&s=industrial&sr=1-2&ts_id=11260304011
This or something perhaps a bit thinner would work too.

Sounds like your iron is too hot or you are working one area too much.