You could lose a lot of detail if the raised parts slope down to the base. For exampe, a coin (to use your example) such as a 'dime' (I have one in my hand) has a figure of a head on it. The head has topographical features that your method might not reveal to a scanner. Only the highest parts would touch the glass bottom through the liquid. Therefore, in the case of the dime, you would see the guy's ears and perhaps part of his hair or jaw-line, but the 'lower' features would be missing.

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All the Woes of a World by Jonathan Icknield aka The Bryster

And in my final hours - I would cling rather to the tattooed hand of kindness - than the unblemished hand of hate...