The keys are probably a 19th‑century brass alloy—mostly copper, maybe 20–30 % zinc, with a small amount of tin or lead for softness and machinability. That mix gives it a nice shine but also makes it prone to tarnish. Use a mild, non‑acidic cleaner—like a water‑based polish or a very light vinegar solution—applied with a soft cloth. Avoid harsh solvents or abrasive pads, and dry the surface immediately to keep the alloy from oxidizing. A light coat of mineral oil or a commercial brass polish will protect it without altering its character. Keep it out of damp places and rotate the keys a bit if you can; that keeps the metal from settling and preserves its “soul.”