What Makes A Proof A Proof?

To quote RedBook:

How a Proof Coin is Made
Selected dies are inspected for perfection and are highly polished and cleaned. They are again wiped clean or polished after every 15 to 25 impressions and are replaced frequently to avoid imperfections from worn dies. Coinage blanks are polished and cleaned to assure high quality in striking. They are then hand fed into the coinage press one at a time, each blank receiving two or more blows from the dies to bring up sharp, high relief details. The coinage operation is done at slow speed with extra pressure. Finished Proofs are individually inspected and are handled by gloves or tongs.

Eaglefoot,
for older proofs that is certainly the case but modern proofs are minted in a manner that it would be rare to not have a DCAM

Proof coins will typically have a very flat (not rounded) edge which is squared off...almost sharp. The devices will be also squared off to the field. This is all due to the striking pressure of the dies against the planchet. On later 20th century coins and onward, all Proofs should have deeply mirrored fields and no weak details (although I'm sure someone might be able to cite examples to the contrary). Also, expect the edge to have a mirror finish. 19th century Proofs (an area I somewhat specialize in) are a whole different ballgame with quality going all over the place depending on series, number minted, and length of the series.

Thanks for the response Prethen and GBchaosmaster. If I run across any penny that is deeply mirrored, no weak details, and I can see the details of my ugly face in it haha, I'll definitely hold on to it.

But I haven't yet, and instead have a lot set aside that have been heavily circulated but still wonder if they were once proofs. So I still would have to ask, is there a way to tell a proof from a non-proof if it is heavily circulated? Would it even matter at that point? Are there even year and mintages that the proofs and non-proofs only differ in look when they were first produced but heavily circulated look like one another? I think I'm probably definitely missing some important piece of info here. Sorry for such a newb question...

It depends on the series, if the coin is worn. If too few business strikes were made and they made Proofs off of business strike dies or vice versa, then all bets are off if it's too worn. I know this as a fact as I'm dealing with just this very situation now. Several 3CN coins can be dastardly difficult to impossible to tell once they get below AU. I'm sure other series can be the same way.

I think the answer is that there will be times when you will not be able to identify the proof coin, if it has been heavily circulated.

I suggest buying a RedBook (A Guide Book of United States coins by R. S. Yeoman). It has a wealth of useful information, and will identify year/mintmark combinations that HAVE to be proof.

Usually, a proof coin in circulation will have the sharp features discussed earlier in this thread and/or still have somewhat mirror-like surfaces, and can be identified in that way; but I'm sure there will be exceptions. By the time the proof coin gets to that point, unless it is a rarity in proof, it realistically won't matter that it is technically a proof coin.

"Proof coins are the masterpieces of our craftsmanship. Clean, good quality steel is used to make proof dies. The die impression is grit-blasted with very fine zirconium sand to give the surface a frosted finish - the common term for this is sand-blasting. The surrounding background is then highly polished.

"Hand polishing of the sections of the die face is performed under a microscope and takes two to three hours. To achieve a mirror-like finish, different grades of diamond lapping paste are applied to the surface and polished with wooden sticks (usually soft birch wood). This is followed by polishing with a soft felt pad attached to a dental drill.

"The entire face is covered with clear tape and then, using a fine scalpel, the tape is cut away leaving the design exposed. This area is sand blasted again and given an even frosted appearance and a clean sharp profile around the impression.

"After polishing, the die is chrome-plated or titanium nitride coated to resist damage from minute foreign particles and to extend the die life. This in turn reduces the number of times the die is refurbished.

"Proof coins are manually and individually coined in a specially designed and air conditioned environment. Laboratory coats, hair ties, gloves, sticky mats on the floor, lint free material to wipe dies, and fans continually blowing air across the die in the coining presses are some of the measures taken to minimise the risk of blemishes on the coins.

"On each manually operated press, using forces of up to 360 tonnes, we can produce about 500 proof coins per day.

"Within the coining press a collar is used to restrain the blank. The size and shape of the collar determines the shape of the finished product.

"Operators, with gloves and/or tongs, place the blank onto the coining press and strike it up to 4 times. The number of strikes is dependent on the size and the depth of relief. The die is wiped with alcohol, to remove any dust, before the next coin is struck.

"The operator inspects each coin and then Quality Control randomly examines coins within every batch. If one fault is found the entire batch is thoroughly examined.

"Finally, proof coins made from bronze, cupro nickel, aluminium bronze and sterling silver are sprayed with a nitro-cellulose lacquer to protect against oxidation. The coins are inspected once more.

"Proof coins are usually packed into capsules and high quality display packaging; along with information about the coins."


(Taken from the Royal Australian Mint website)