The short answer is no, neither should coins be ‘dipped’ (a practice of less scrupulous dealers of yesteryear).
Neither would i recommend any form of abrasive or bristle brush. And I would certainly not recommend lemon juice, which is effectively a citric acid.
If you simply need to remove surface dirt, then Washing up liquid, warm water and a soft cloth only. You can also use an ultrasonic bath, but again be careful as you will devalue your coin if you get it wrong.
Soaking in olive oil is okay as it is not destructive, but loosens up stubborn dirt as engine oil keeps it clean in a car engine. A practice used by many metal detectorists. But do not attempt this on coins with a high value.
Once a coin has been incorrectly cleaned with abrasive, it would put scratches on the plain field of the coin. This will devalue the coin and serious collectors or investors will not consider buying the coin.
Whilst you cannot reverse scratches and wear, there is a trick for putting a form of toning back.
Boil an egg and place it with the coin in a sealed Tupperware type box. Leave it in for half an hour and the gas – hydrogen sulphide – a toxic gas will start to tone the coin. The longer you leave it the deeper the tone will get, so you need to keep an eye it or it will go too far.
Once you have a light tone, remove the coin and wipe it gently with a soft cloth to remove any excess.
But once again, I cannot emphasise enough, you should not attempt any form of cleaning or restoration unless you know what you are doing.
