I found my first non-riveted button today. It is 11/16” diameter. Front appears plain w/ a nice green color (patina?). The back has writing. I want to clean it without damaging it. It probably isn’t valuable, but to me it is. Rub toothpaste into it dry, soak in white distilled vinegar, other? What is recommended to clean without damage? It appears to say: “WARR” and possibly “WARRANTY”. Also an “OK”. Any help appreciated