Even though the coins many look more shiny & pretty after cleaning, all routine cleaning methods do some damage to the surface of the coin. Even gently applying polish with a soft cloth will do slight damage to the coin's surface, as will dipping it into a cleaning solution. Cleaning can be damaging for a couple of reasons. First, a small amount of the coin's metal is stripped away with the tarnish, making the coin weigh slightly less and lose value. (Of course, this can also happen to coins that are in circulation, just from routine handling.) Second, the surface of the coin is altered from its original mint state, which can be seen in a microscope. This makes the coin less desirable to some collectors, who avoid purchasing coins for their collection which have ever been cleaned. So, what is it that can be seen in a microscope...? Flow lines!
when a coin is struck, the metal flows outward from the center, resulting in microscopic lines that add to the luster of a coin. [source]
When a coin (or bullion round) is minted, tremendous pressure is applied to stamp the design into the metal. This causes the solid metal to "flow" a bit into an outward direction, radially, across the surface of the coin. This creates "flow lines" that cause a pretty diffraction of light when the coin is tilted. Since the pattern appears as radial lines akin to a wheel of a wagon or cart, this is called the "cartwheel effect" and can be seen in the video, below.
• VIDEO BY ME •
So, should a coin ever be cleaned? My source for this article suggests:
About the only time I can think of that it might be appropriate to consider cleaning a coin would be if you dug up a rare U.S. 1804 silver dollar with the help of a metal detector! Then I think, because of the coin's extremely high value and rarity, it might be worth sending it off to someone like the Numismatic Conservation Service to have it professionally cleaned. [1]
SOURCES
1 The Spruce Crafts: How to Tell if a Coin Has Been Cleaned
