This journal article describes methods conflicting opinions, ethical considerations and effects of cleaning coins. Differences between toning, tarnish, patination and active corrosion are discussed. Indirect cleaning refers to removal of foreign adherents (eg. dirt, grease, air pollutants, PVC residue, tobacco smoke, etc.) which contain no coinage metal. Direct cleaning alters the metal's surface. Chemical and physical cleaning methods include liquids (solvent, reactant, corrosive), abrasion, compressed gas (air, freon, steam), and electrolysis. Patinated surfaces are both protective and attractive and in great demand. Cleaning and toning are generally considered deceptive among numismatists and may actually damage the coin, reducing its value. Bibliographic references date from 1902-1984. This is a reference for coin collectors, dealers, investors and conservators, and also for surface enhancement of metal.