Bright old silver can cover a multitude of sins. They can be dipped or buffed or had the undivided attention of a Brillo Pad. Very, very occasionally you find silver that is genuinely untoned with original surfaces, but the vast majority of old coins have been dipped if there is no toning to them. It's a natural process, so the only way to prevent oxidation is for the coin to be stored with a protective layer or to remove the oxygen i.e. it doesn't happen very often.