Pure acetone (can pick it up at any hardware store) will not hurt or diminish the value of coins. The acetone merely removes the layer of dirt and grunge from it, so go ahead and soak your "dirty" coins, then rinse thoroughly and pat dry, or take them out of the acetone and just let them air dry as the acetone will evaporate quickly. Don't try to reuse acetone as the debris from previous coins, with simple add to the coins surface.