If there is one sure method of lowering the value of your rare coins, it is to clean them. Cleaning your coins will generally reduce their value by 80% to 90%, because coin collectors value the original, pristine surfaces of their coins. Even a tarnished, grimy-looking coin is preferred over one that has been improperly cleaned. Being able to tell if a coin you want to buy has been cleaned is an important skill to have. Although reputable grading services such as NGC and PCGS claim they don't slab cleaned coins, mistakes do happen, so you should learn to tell the difference between a coin with original surfaces and one that has suffered the indignity of being damaged by "cleaning."