How did the Romans mint coins?

I am not an expert on Roman mints and coins.

I do know that the Roman Empire contained many cities with their own governments and that many of those cities designed and issued their own coins.

And the imperial government also had a number of different mints in different cities.

You may have heard about the great emperor Aurelian https://en.wikipedia.org/wiki/Aurelian1 who reigned from 270 to 275 and helped to save the Roman Empire from the terrible Crises of the Third Century by defeating barbarian invaders and reconquering the breakaway Palmyrine https://en.wikipedia.org/wiki/Palmyrene_Empire2 and Gallic https://en.wikipedia.org/wiki/Gallic_Empire3 Empires.

According to the Augustan History:

There was also during the rule of Aurelian a revolt among the mint-workers, under the leadership of p271 Felicissimus, the supervisor of the privy-purse.142 This revolt he crushed with the utmost vigour and harshness, but still seven thousand of his soldiers were slain, as is shown by a letter addressed to Ulpius Crinitus,143 thrice consul, by whom he had formerly been adopted:

3 "From Aurelian Augustus to Ulpius his father. Just as though it were ordained for me by Fate that all the wars that I wage and all commotions only become more difficult, so also a revolt within the city has stirred up for me a most grievous struggle. For under the leadership of Felicissimus, the lowest of all my slaves, to whom I had committed the care of the privy-purse, the mint-workers have shown the spirit of rebellion. 4 They have indeed been crushed, but with the loss of seven thousand men, boatmen,144 bank-troops, camp-troops145 and Dacians. Hence it is clear that the immortal gods have granted me no victory without some hardship."

http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Historia_Augusta/Aurelian/3*.html4

142 This revolt is described also in Aur. Victor, Caes. 35.6; Epit. 35.2, and Eutropius, IX.14. According to these authors, the mint-workers, who, with the connivance of Felicissimus, had adulterated the metal appropriated for the coinage, fearing punishment, broke out into open war. It would appear that they had been keeping a part of the silver that was to have been used for the billon (i.e., adulterated) coins. Though the number of soldiers said to have fallen is, of course, greatly exaggerated, a battle seems to have been fought on the Caelian Hill, near the mint, which was on the Via Labicana. The date is uncertain; it may have been on the occasion of the German invasion of 270‑271 (see c. xxi.5) or in 274, just prior to the reform of the currency (see note to c. xxxv.3).

http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Historia_Augusta/Aurelian/3*.html4

So the mint in Rome was so large an establishment that the revolt of the mint workers had to be crushed by the army and was recorded in the few historical sources for that era.

This shows that Roman coin production was a large scale industry.

I am a sculptor and metal worker and for years I have seen beautiful ancient metal objects with small fine detail that to me seemed would have required magnification to make so I was also interested in that question and see that no one had a response to that. Nevertheless we do see these objects that would require using magnification if made today.

Then I had friend who wore rather thick glasses and who could focus almost on an object that was almost touching his eyeball and he said it was like looking though a reasonably powered loop. I wasn't sure I believed him until one time we were visiting a tech company that made computer disks. The made a small component that required a very high degree of polish and they were inspected under loops. It wasn't like a high power microscope, The inspection loops they were using were maybe 20x power. He could do the same inspections by holding the part right up to his eye. That made me realize that there were people who had the equivalent of 20x power close vision. He was near sighted of course and glasses allowed him to function normally, but without them he would have had a difficult life. Tt wasn't like he had only one or two feet of depth of vision. But without glasses he would have been useless as a soldier, or farmer. That made me think that in the ancient world perhaps it was people like him who did very fine work.

So I imagine someone like him, perhaps highly valued, crouched over those dies using tiny chisels to cut all that fine detail in those dies.

The Roman's used primarily two methods for minting coins, hot pressing and cold pressing. And just as it sounds cold pressing involves striking the coin while its cold while hot pressing strikes the coin while it has been heated up to make it more malleable.

Dies were used to impress an image on the front and on the back of the coin. Greeks typically relied on the weight of a hammer to pound the image onto the blank, while the Roman's typically used a hinged die set that allowed simultaneous striking on both sides. After the coin was pressed it would go to another craftsman final touch ups.

As to how long the die would last, that has been estimated at around 30k strikes from this source.

How many coins could a die strike before it broke, or wore out? This vexed question is a source of endless debate. Dies typically lasted three to four months in heavy use. A die might last five years or more in intermittent production. One estimate, based on a specific issue of Delphi (338-333 BCE), is that an obverse die was good for 23,000 to 47,000 strikes, while a reverse die could sustain 11,000 to 28,000 strikes (Howgego, 32)

Many academics extrapolate the amount of total coins per set to the amount of dies they have been able to locate.

Question:
Roman methods for minting coins

Skilled labor jobs in Roman times were performed by master and apprentice systems.

I found this.

How Ancient Coins were Made

Coin Production.
Flans or blank coins were produced by cutting pieces off of bars and then hammered into shape. The flan could be used as is or placed in an oven until soft. then placed into two sided dies, either bronze or Iron dies were used depending upon the coin. Then a hammer was used to smash the dies to imprint the coins. A team of workers could produce 20,000 strikes (coins) per day. During the Second century about 17 million Roman denarii were issued each year.

Die Production:
The mystery is how Romans produced the dies because the coin production process would require new dies everyday. How did they get the portraits so uniform if a new artist was individually cutting each die?

Perhaps they used a die production method described by 16th Century Medalist Benvenuto Cellini who was the first to employ a screw press to minting coins.

How Ancient Coins were Made

1. The central design is sunk in the dieblank, either by casting or hubbing.
2. The legend is added by letter punches which are individually struck, perhaps using afixture to align them.
3. The border is similarly added by punching dots around the legend.
4. The completed design is hand engraved for touchup where needed, then polished

Or perhaps as the source goes on dies were produced by separation of work with the Masters completing the portraits and their apprentices following up with the engraving and boarders.

Sources:

• How Ancient Coins were Made
• Screw Press
• Benvenuto Cellini