Why Is Cleaning So Bad?

depending with what you clean it with, it can leave scratches, spots and varied chemical reaction.

Plus a lot of collector prefer to see a natural coin than a little shiny thingy.

there's a lot of tread on this subject, try looking them up

I would add that its only considered bad right now. There was a time when cleaned coins were all the rage and pretty much anything 100+ years old has probably been cleaned at one point. Depending how you do it it can damage the coin which is obviously bad, but it wasnt always frowned upon. On the bright side if cleaning becomes the way to go its easy to clean uncleaned coins lol. But like mentioned its for numismatic coins only really. If the coins only value is its precious metal content it doesn't really matter if you clean it.

They are you coins though and if you really did want to clean them you can do what you want with your collection. Just know that if you do they will lose value right now and if you ever sell them be honest that you cleaned them and dont try and trick people into thinking its some pristine example

While there may MAY be RARE exceptions to the rule, cleaning coins is to be avoided at all costs.
Vermontensium is wrong in one respect: We like coins which are not grimey but HONEST (dirt, toning and all)!
To hold a coin which is 150 years old and which has NEVER be cleaned is a rare privilege.
(So many HAVE been cleaned.)
To own such a coin is, well, NIRVANA!

Before you give in to temptation, post a picture of the coin which you think could benefit from a cleaning.
Chances are, your replies will all be, "DO NOT DO IT!"

A coin is only Mint State condition (with original cart-wheel luster) once. If you clean it, the minute surface molecular alignment, that gives an MS coin its luster, can easily be, irrevocably, destroyed.

Therefore, it's best if the coin is left alone, in its present condition, as-is, for future generations.

Plus it kills the value.

In reality the main purpose of cleaning a coin, refinishing a 500 year old table, changing the radio to AM/FM on an old car, etc. is distruction of originality. Sort of like taking off the wrappings on an Egyptian Mummy and putting all nice clean ones on instead. Reshapening the Original Bowie Knife. The past is gone once anyone has changed an item from the past. You can not redo the past or not yet at least. So it's not just coins that shouldn't be tapered with, it is almost anyting from the past.
Of course some people take that to an extreame. For example I know a lady with a 10 year old car and so fare has never washed that either.

Hey hey! Dirt on a car is nature's way of protection, must like the antique dust I have in my home.
How DARE I take Pledge and a cloth to remove it?
Grime, dirt, dust, toning and what not are protecting the coin.
We protect these coins, grime, etc., and I, for one, do so with pride.

YES, Nevalite, cleaning removes something from the coin.
Metal and honest changes to the metal that only many years can provide.
Once gone, they are gone forever. With that, so is the value that we old-time collectors put on a given coin
which has never been cleaned.