Why Is Cleaning A Coin So Bad?

NO. Not to the concept, but to the terminology. Other disciplines have addressed these same issues and found terms and concepts that work.

I think the word you are groping for is Conservation. Which in every art context has a very specific meaning related to preservation of both the object and it's state to the best possible level.

That's why the art world differentiates between cleaning, conservation and restoration.

Conservation is stabilizing the item against further environmental damage. Conservation uses technologies that can be removed as future technologies improve.

Restoration is a permanent change to the item. It too may be the best we can do to preserve the object and it's state.

Say it better: "pinholecorrosion that could have been prevented by" careful chemical neutralization "and moving it into a better holder".

Had you done this when the corrosion first presented itself, you would have met the conservation statement above... stabilizing the item against further environmental damage.

Polishing too has a specific meaning - removing tarnish. With no other focus. And it's well known that repeated polishing removed detail. Check your Grannie's silver. Heck check my mom's silver which she doesn't polish but once a decade and you can still pick out the pieces she added for her 30th anniversary.

To answer the OP thread title:

Because cleaning causes permanent damage to the coin surfaces.

One more reply about cleaning coins. In fact there are those that don't care if a coin is cleaned or not. Same with old relics, paintings, etc. Some care, some don't. If you collect coins and they are now yours, you can do to them as you wish. If you never tell anyone you cleaned them, polished them, sandplasted them, acid dipped them, only you will know. Someone I know turned an old sewing machine table into a bench grinder stand. Every day people modify their cars to be what they like. In the past people have painted right over possible so called masterpeices.
The really only important thing about cleaning a coin, doing other things to paintings, old furnature, etc. is value. If you never plan on selling something, who would know what you did to it.
Regardless of what a problem was on a coin, and regardless of what you do to fix it, if you never tell anyone, only you would ever know.
The original question of what is wrong with cleaning a coin is mainly doing something to something that will bother someone in the future.
If your collecting coins as a hobby for yourself, no plans on selling any, giving any away, leaving them to someone, then as said, they are yours.
If, however, you ever want to sell any, think about what your doing with any type of cleaning.

All so true. Unfortunately so many just care about the present.

I believe scenario C is also cleaning. Once you get rid of the lustre between the letters in the legend you can't put it back. A circulation coin gets struck once. After that it's done, it's mint.

Simply touching the face of a proof coin "impairs" it.

Very interesting thread. I still consider myself a newbie after over a year of serious collecting. I don't do anything to my coin other than putting them in two by twos. @ baseball 21 you mentioned one time in the past cleaning was ok, do you know when this all changed? I'd really like an answer, I'm very curious when and why it changed? I hope someone will be able to she'd some light on the change.

Let me see if I can get past all the personal opinions and take this discussion to the nitty-gritty.

There's no "redo" button when you clean a coin.

Once you've removed the top surface of a coin (note that I don't describe what "surface" is), you've removed part of the coin's history.

The loss of a coin's history devalues the coin.

Those who try to squeeze in between the above two statements are fooling themselves or the rest of us by (paraphrasing now) trying to convince us that we can't really tell when a coin has been cleaned.

Those who understand the statements above are probably serious collectors.

Those who don't understand the statements above probably enjoy their coins as shiny, pretty objects. And who am I to say that they are wrong in doing so?