Cleaning Coins

please dont clean coins

warm wishes.

Here is a success but one in which I would NOT recommend!

Some years ago, I found in a junk box what was barely recognisable as a coin. I asked the dealer his opinion. He said that there was a vague possibility that it may be silver, and may be ancient, do I want to buy it? I said "OK" but how do I clean it for identification? I decided to buy it and take the gamble. It cost me the princely sum of $7 at the time.

The dealer said if I wanted to risk the complete loss of the item, he had a weak solution of hydrochloric and sulphuric acid mixture, and to place the item in it for a few minutes. After two minutes, it looked as if it may possibly be an ancient silver tetradrachm. He weighed it at about 13.5 grammes. He asked if I wanted to continue, I said "yes".

He diluted the acid solution down to about 20 to 1 to what it was previously, to slow down the reaction. This way, it would be possible to stop when it had been far enough. After about half an hour, his experience said to take it out of the acid.

He passivated it with a quick wash in a mild alkaline solution, rinsed with water and dried it.

The coin turned out to be a magnificent find; I still have it.

What is the coin? It is a silver tetradrachm of Ptolemy X11 in about nearly EF condition, and is worth perhaps $250 today.

Thanks to all that have posted, it helps to ask if one doesn't know.

Most of the times cleaning coins is not recommended but sometimes its a matter of preference or a must. But what one means by cleaning, it can be something innocent like the first four steps described below or the more heavy duty solutions that require to know what you are doing or else better don't try it at home!
First I would rub it with a velvet cloth piece that would remove fingerprints, easy dirt, etc... If still ugly, brush it with a toothbrush using soap and hot water, that would remove the more persistent dirt. If still not happy, rub it with your fingers using baking soda powder + few drops of water until it gets muddy. Soda will not damage the coin, only the dirt. You can also soak coins in olive oil for one or more days, I have tried this with excellent results with bronze/copper coins. Now for more heavy duty solutions and for those who know what they are doing, chemicals sometimes combined with a tool of some kind that can help remove that ugly spot that have become one with the coin. You can always practice with cheap coins and possibly master the art of cleaning coins after some effort studying and practicing, and money buying cleaners,tools,etc... Unless someone is a dealer, with few exceptions, I don't think he would bother too much with cleaning, especially with chemicals and tools, its boring and risky.

Baking soda is a very mild abrasive...

More pleasant to the eye or not, those slight scratches are going to reduce the value.

Yes, a velvet cloth will still leave hairlines as would any other type of physical wiping of the coin. Soaps can also cause problems with a coin and should be avoided. You may consider baking soda to be a very mild abrasive but rubbing it on a coin is not much different than using 100 grit sandpaper. Regarding the 1920 Aussie Penny, that spot can be removed with wet chemical methods that will not harm the coin at all.

Remember that an ugly coin may not always be ugly if it can be treated with proper conservation but a cleaned harshly hairlined coin is damaged for life.