A proof coin is a coin struck using a special, high-quality minting process, and made especially for collectors. Modern proof coins often have mirror-like fields and frosted devices, although proof coins struck prior to the mid twentieth century are often distinguished only by their high quality surfaces. In all cases, the term proof always refers to a type of coin, or the way it was struck, and not to a coin's grade.
The difference in the two finishes that you see on a proof coin is known as cameo contrast. This is where the devices on the coin exhibit a frosted finish and the field exhibits a mirrored surface. This is achieved by polishing the planchets before they are struck to remove any surface imperfections. Additionally, the coin dies are also specially prepared for use in striking proof coins.
Special Coin Dies
The coin dies used to strike proof coins are specifically manufactured to bring out the minute details of the design. Since the raised area of a coin are recessed into the die the field is the highest surface of the die. This allows the mint technicians to impart a frosted surface into the recessed areas of the die and then polish the surface of the field. When the specially prepared planchet is struck with the proof die, the devices will be frosted and the field will exhibit mirror like qualities.
Why Don't All U.S. Proof Coins Exhibit Cameo Contrast?
Before 1971 the United States Mint used a different process than what is used today. A coin die that is going to be use to produce proof coins was "pickled" in an acidic solution. This etched the entire surface of the die with a frosted effect. The die was then taken to a polishing machine which only polished the highest surface of the die which would produce the field of the coin.
The frosted finish on the coin was very delicate and only the first few hundred proof coins produced with these dies exhibited the frosted cameo contrast effect. As additional coins were produced, the intense pressure of the coin striking process wore a way the delicate frosted effect of the die. This resulted in coins that were mirror like across the field and the devices. These are known as "Brilliant Proofs."
Production of Modern Proof Coins
Thought United States Mint now employs a special process that allows the mint technician to selectively frost specific areas of the coin. The coin die that will be used to produce proof coins his first polished to a brilliant mirrored surface. The die is then loaded into a machine that is computer controlled and selectively uses a laser to frost specific areas of the die. They also have the ability to control the density and depth of the frosting process to yield different finishes on the same coin.
Edited by: James Bucki