Mike is obviously quite right here, you mention that they were brass or bronze, are these the original metal without any specific surface finish applied at the time of their manufacture? if not then cleaning as mentioned in previous posts shouldn't harm them. If however a specific surface finish was applied when manufactured, you may destroy it. That being said, if they have obviously been cleaned with a silver polish at some stage (The one you now wish to remove) then the damage may well have been done many years ago. got any photo's to give us a better idea.? I have cleaned some British medals both bronze and silver in a similar way, as these are "As they come" metal, they have in time once again regained a natural "Patina" but without the crud and no visible signs of damage, so as you say, once removed, who would know that it was there in the first place. Just a thought regards Alex