This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
The date and denomination of a coin worn smooth can be determined in the following manner: Take an ordinary coal shovel, or a piece of sheet metal, and place it in a hot fire. Allow it to become red hot, then remove, and place the coin on the hot surface of the metal. Any figures or letters can be readily seen when heated in this manner. This test seldom fails even when the inscriptions have been worn so smooth that they are invisible to the naked eye.