Is Cleaning Coins Ever Acceptable?

I would try to conserve your coins with Virdi-care. Use with a Q-tip sparingly. Be sure to rinse with distiller water and pat dry with a very soft cotton towel. Just a suggestion and I don't know if Virdi-care is available in the UK.

If all its got on it is "tarnish" - we coin collectors prefer to call it "toning" or "patina" - then leave it alone. Cleaning it will only make it look worse. There are circumstances where cleaning a coin becomes necessary; dark toning is not one of them. If a coin is "too dark to see", then turn on some more lights, or go outside on a sunny day.

Coins dug up out of the ground, whether ancient or more modern, usually need cleaning. How harsh depends on what the coin is made of.

Coins with active corrosion spots need to have the corrosion neutralized to prevent further damage. "Bronze disease", "aluminium cancer" and "zinc rot" are all good examples of dangerously contagious corrosion that must be cleaned off before it turns your whole coin into a powdery mess.

Coins with some foreign matter stuck to the surface, such as paint, glue, varnish, wax, old stickytape residue, or the green slime that accumulates on coins stored in cheap plastic coin albums, can have this foreign matter washed off with whatever solvent is effective - water, methylated spirits, acetone or xylene can all be effective (depending on the nature of the adhered matter) and will not harm either the raw metal surface nor the patina/toning/tarnish on the coin.

Nobody can stop an old soldier from polishing up their medals, so we've given up trying. War medals can be polished until they glow in the dark, without affecting their value.

Hi Demarco Bishopp.

Digging up old coins from under the ground by metal detectors in Australia would be very uncommon.

But looking for 2kg Gold Nuggets under the ground by metal detectors is far more common in comparison to finding coins.

Bronze and copper can't really be cleaned without changing the appearance and removing any original colour or lustre. Whereas silver will retone in time if cleaned carefully, copper and brass will always show it's been cleaned.

That said, if you're talking about low value and want to see detail better, then soaking in olive oil (can take months!) provides a reasonably minimal damage way to get rid of crud from coins.

If the toning is uniform and the detail good however, I'd be inclined to leave them alone.

As already stated cleaning is never a good thing. Remember that in most instances, what you see on the coin that appears as dirt is actually the results of a chemical reaction with the invironment. This means that some of the metal of a coin has reacted with something. Removing that STUFF would also remove part of the coins original metal.
If it was just dirt, distilled water or possibly some Acetone would help but otherwise, best leave alone.
And remember that you should never use soap and water on a coin. ALL soaps are manufactured differently and what is in them could do more damage than good. And also with water there are so many possible contaminates, that too could do some damage. Usually the best thing to do is put old coins in something to protect them from any further corrosion, tarnishing, staining, toning, etc.
So basically it is not a good idea to clean coins.

@kg5: Quite often, gold nuggets are acid washed to clean them. It helps to brighten them up, and to get closer to a nett gold weight.

I have seen one or two gold coins that have suffered from burial in a rusty tin can. A complex ferro gold salt can be formed over 100 years or more of burial, especially so if there is a 10% copper alloy in the gold composition. I have seen a result that looks similar to a verdigris scar.

Okay, it's pretty easy to ignore advice about cleaning when you're dead set on shining something up - I've done it myself. But listen to this! You can have some fun cleaning if you dig coins or are experimenting with worn/dateless items.

When I started out, I polished a few coppers in vinegar for kicks. None of them turned out well, they got a horrible salmon colour. Different acids and additives continually failed to produce a nice, shiny surface on any of them, and even if it did work, a shiny surface will never match a worn coin. I found a few cleaned and worn silver Canadian quarters for below spot - they were worn to silhouettes and yet they were bright white. But I can totally get behind chemical use on valueless coins. If a date's worn off, put it in something caustic (okay, fine, know what you're soaking it in first) and see if it becomes revealed. I have a horrible and holed large American cent (how else could I afford one of those :P), and I soaked it in 25% acetic acid (basically 5x vinegar, available at the sort of shops that encourage you to bring your own containers to refill and make your own dish soap) to try and reveal the worn-down date. With a good jeweler's loupe and lots of photoreference, it was revealed to be an 1828! I was pretty happy at that.

If you really want to clean something (I have this urge still, you just have to know what to indulge it on), find some cheap worn coins and see what you can reveal. Maybe you'll find a date rare enough to offset the value drop from cleaning!