A proof coin is a coin struck using a special, high-quality minting process to produce coins, especially for collectors. Modern proof coins often have mirror-like fields and frosted devices. Proof coins struck before 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 produced. Proof it is not a coin grade. However, coin collectors and numismatists do grade proof coins.
The difference in the two finishes that you see on a proof coin is known as cameo contrast. Proof coins can exhibit a difference in the finish when the devices on the coin have a frosted finish and the fields exhibit a mirrored surface. Mint workers achieve the cameo contrast by polishing the planchets before they are struck to remove any surface imperfections.
They polish the planchets by tumbling them in a rotating barrel with small stainless steel balls. The friction from the steel balls burnishes the surface of the planchet and smooths out any imperfections. After the planchets are polished, they are then washed in a special solution to remove any foreign contaminants on the surface.
Special Coin Dies
The coin dies used to strike proof coins are specifically manufactured to bring out the most minute details of the design. Since the raised area of a coin is recessed into the coin die, the field is the highest surface of the die. This process 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.
Early in the history of the United States Mint, mint workers made proof coin dies from dies originally intended for producing business strike coins. The process would begin by submerging the face of the die in a solution of acid. The acid would remove microscopic pieces of metal from the surface of the die. This process gave a frosted effect on the entire surface of the coin die. Mint workers would then polish the die since the highest points on the die would be the fields. The polishing would impart a mirror-like finish on the field while the devices would have a frosted finish.
The modern minting process requires that proof coin dies are created separately. Only coin dies with the best possible detail are selected. A mechanical process that uses horse hairbrushes polished the dies to a mirrorlike surface. Mint technicians then load the coin dies into a machine that uses a computer and a laser to impart the frosted finish on the surface of the coin die.
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 used to produce proof coins was "pickled" in an acidic solution. This process etched the entire surface of the die with a frosted effect. The die maker would then take the coin die to a polishing machine, which only polished the highest surface of the die. This would produce the mirrored-like field on 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 mint workers made additional coins from the same set of dies, the intense pressure of the coin striking process wore away the delicate frosted effect of the die. This friction resulted in coins that were mirror-like across the field and the devices. These coins are known as "brilliant proofs."
Production of Modern Proof Coins
The United States Mint now employs a special process that allows the mint technician to frost specific areas of the coin selectively. The coin die that will be used to produce proof coins is first polished to a brilliant mirrored surface. The technician then loads the coin die into a machine that is computer controlled and selectively uses a laser to frost specific areas of the die. He or she also can control the density and depth of the frosting process to yield different finishes on the same coin.
Modern proof coins are sold in sets, and they come with a Certificate of Authenticity (COA) issued by the United States Mint.