I've searched and searched to learn more about mint marks, but all I found was history on how and why the mint marks came to be assigned, but not on how they were applied. I know that there are some errors from the middle of the century involving repunched mint marks (such as D over S, or D over D, or D over horizontal D, etc) and I was wondering if mint marks were, back then, punched after the coins have been struck, and if so, were they punched with a machine? Or were they hand punched? Are mint marks still punched that way these days? Or are the mint marks now engraved onto the dies themselves?
That would take an eternity to strike each coin with the mint marks! They were hand punched into the Die. It would be incused as all the other numerals, letters and images on the die.
In the US, up until 1989, they were hand punched into the working dies, allowing for the variation and mistakes that brought us the mint mark varieties you mention. Starting in 1990, they were moved to the master die (the die that makes the working hubs that are used to make the working dies), eliminating the possibility for those varieties.
Not all over punched mint marks, like O over CC on a variety of 1900 dollar, were errors. Dies are expensive, and if there was a good one with a now defunct mint mark on it, mint personnel simply over punched it.
The US Mint die shop is in Philadelphia. Dies would be manufactured there and then sent out to the various mint. Manufacturing dies was difficult and expensive, so when dies were returned, sometimes in the early days the dates were repunched too. Dies were sent unhardened in case they were stolen in transit. Poorly hardening dies at the mints caused problems in some years.
Huh. I would've thought hardening a die would've been a fairly minor challenge in the counterfeiting process.
Thank you! That explains all the variation in mint mark positions relative to the date on older coins, whereas coins made in the last 30 years or so have the mint mark consistently placed!
You might think so, but there is correspondence I've seen posted on the boards that indicates otherwise even for the mint itself.
Not really. There are records from San Francisco in 1878 about problems they were having with one employee not following instructions for properly hardening dies, resulting in dies that didn't last very long. I'm trying to remember the entire story, but there was an experienced employee who was out sick at the time, so someone else couldn't be fired or they'd be short-handed. I'm messing this up. Anyway, drama, and this was at a facility that was equipped for and instructed in properly hardening the dies for use. A counterfeiter isn't going to be as careful or skilled, or would have the intelligence and wisdom of the average criminal, and would not get much use of the dies before they mushed. If a smart, capable counterfeiter got his hands on dies in transit, he'd use them as master dies to make hubs, not coins, and end up with an more endless supply of working dies.
The first Philadelphia Mint had a terrible time with harding dies. If they stuck a red hot die in the water and sizzelled, they figured they were in good shape. If it "POPPED!" they knew they had just wasted hours of engraving work for nothing or very little. The quality of the steel they used was far from consistent.
Also hardening dies is not as easy as it sounds. You have to heat it to the right temperature, keep it there for the right amount of time and then quench cool it at the right rate. This was not easy to do when there are no sensors to let you know what the actual temperature is. It had to be judged by eye (by the color of the metal) and experience. Get it right you have a good die. Get it wrong and the have a die that fails quickly. Get it too far wrong you have a worthless broken chunk of tool steel.
On top of that, you had to have the chemical composition of the furnace just right. Too much carbon and it would be too brittle, too little and it would be too soft. There have been incredible advances in metallurgy in the past 300 years!
But that's all done by the time the die is in transit. Hardening just changes the structure of the metal, not its chemical composition, right? It doesn't involve a net addition or removal of carbon.
The intention is that the chemical composition should be the same - which is why the amount of carbon in the atmosphere of the furnace is so important. If there is less carbon, there will be carbon offgassing which reduces the carbon content of the steel. The carbon is initially added to the alloy when forging the metal - but any annealing or hardening process must maintain that balance when the metal is heated to super high temperatures.
