Ancient coins were made by hammering. A piece of metal about the right size for the coin would be heated up and placed on an anvil. A punch would be placed on top of it. Then the punch would be hit with a hammer, sometimes more than once, to impress onto the coin the designs contained in both the punch and the anvil.

This process sometimes left very distinctive marks on the coins. Coins made from silver of high purity might show flow marks, lines radiating out from the centre of the coin. Some coins of less high purity show a halo effect around the hollows in the dies.

Quirks and oddities like these make ancient coins far more interesting than modern ones.

A silver denarius of Antoninus Pius, 148 CE. It is 18x19mm across and weighs 3.4 grammes.

Detail from the denarius on the left.

This coin shows flow marks quite clearly. The enlargement on the right is the section in front of the forehead. You can see that the flow into the large hollow in the die that produced the shape of the head was a stronger effect than any flow into the smaller shapes of the letters. All the lines run in the same direction.

Where these flow lines are above the surface of the coin around them, they are caused by an old and damaged die. The damage will have been caused by a repeated flow of metal across the surface of the die and into the hollow at its centre each time a coin was struck. This sort of damage occurs most often, or most obviously, on the obverse of coins. This is the design that was carved into the anvil of the die, onto which the flan is laid to be struck with the upper part.

A silver denarius of Diva Faustina Senior, after 147 CE. It is 17x18mm across and weighs 3.3 grammes.

Detail from the denarius on the left.

This coin of Antoninus' wife Faustina has a delicate and pleasant portrait. It's on this page partly because it shows flow marks very clearly to the naked eye. The slight enlargement on the right demonstrates this. Flow marks are common on silver coins of this period, but this coin also has an interesting oddity – the hollows around the lettering on the reverse side. There is more about these on my depressions around lettering page.

Flow lines that are below the surface of the rest of the coin are specific to that particular coin. They can't have been applied by the die, because the die didn't reach that far. Instead, they are frozen evidence of the stresses which the coin was subjected to when it was struck. In coins that have them, these can sometimes be seen in the hollows around lettering, which tend to be more common or more obvious on the reverse sides of coins.

Details from the obverse of a denarius of Gordian III. Left, the lettering to the right of the bust. Right, a section from the top of the head.

On the right are some enlargements of parts of a denarius of Gordian III. The whole coin is shown further down the page. The first image is a section of the lettering on the obverse of the coin. The poor state of this lettering is pretty typical of the coins of the period once their dies have begun to wear. You can see some very clear flow marks in the hollows between the letters, and also around the top of the head, shown in the second cut. These flow marks are lower than the surrounding surface, so they are not on the die, but are stress marks which have been frozen into this particular coin.

The area above Gordian's head is part of another phenomenon that is common on coins of the period, which are made of impure silver. It looks like a halo, sometimes surrouding the whole of the head with a ghostly outline.

A silver antoninanus of Otacilia Severa, 244 CE. It is 21x25mm across and weighs 4.6 grammes. The reverse is Juno standing.

Detail from behind the head on the antoninianus on the left.

On this next coin, if you look at the area behind Otacilia's head you can see a light band surrounding her hair and a darker area around that. There is also the appearance of a dark mark between these two zones.

In the enlargement on the right, that mark or band runs from top to bottom, just left of centre. Here, you can also make out some stress marks on the coin's surface inside the line.

It seems likely that the band between the two zones marks a boundary between two different types of surface condition or surface structure. Perhaps it is simply the transition from one to the other..

A silver denarius of Gordian III, 240 CE. It is 20mm across and weighs 3.0 grammes. The reverse is Pietas standing.

Detail from the antoninianus on the left.

Here's the Gordian III coin which was enlarged earler on this page. This coin shows the halo effect much more clearly. You can see that it follows the contours of the face very closely, in an enlarged form. You can also see that it appears to be slightly lower than the surrounding metal, and has a slightly different surface texture. It looks from these coins as though the halos might be another flow phenomenon.

What causes these halos? Robert Kokotailo, an expert dealer who runs "Calgary Coin and Antique Gallery" on the Vcoins mall (see my useful links page) suggests that silver that has to flow into the head area gets more stressed than silver that doesn't have to flow as much. This gives it different surface properties and a different appearance. Purer silver flows more easily, so in less debased coins silver can flow across the whole of the coin and you don't see the same halo effect, but you do see flow damaging the die and producing widely spread flow lines.

Detail from the obverse of an antoninianus of Gordian III. Click on the picture to see the full coin.

Following on from this, one might add that silver which has been forced to bend and flow will have become hotter than the surrounding metal. Striking or bending metal will always make it hotter. This will make that small stressed area more fluid than the surrounding metal. This could easily be another reason why the halo area has a slightly different appearance and shows more evidence of having flowed.

Some of these halos appear quite similar to the depressions around lettering that I commented on at the top of the page. The similarities are the way they appear to be below the surrounding surface, and the flow marks in some of them. In fact I think the effect that causes the two phenomena is identical. On my depressions around lettering page the hollows around lettering are examined in more detail.

On the right is another Gordian III coin, an antoninianus this time. You can see on this one that the area around the head, where haloing and flowing might occur, seems to resist the formation of the slightly dirty patina that starts out past that zone. The patina even appears to have flaked away past the boundary between the two areas.

A billon antoninianus of Claudius II Gothicus, 269 CE. It is 19x21mm across and weighs 2.6 grammes. The reverse shows the emperor holding up a branch.

Detail from the antoninianus on the left.

Normally, you'd expect that area to be more dirty or encrusted, because it would be protected from natural rubbing and cleaning effects. This bears out the idea that the surface has different properties in this zone, even if it isn't obvious to the eye.

Finally, an example to show that it's not just silver that flows. Although this coin has some silver in the mix, it is mostly bronze. Like the debased silver coins, it shows flow marks in the region of the lettering. The enlargement is of the letter S to the right of Claudius' head on the obverse. These lines appear to be above the nearby surface, so must be caused by a damaged die.

Die damage caused by metal flow adds a slipstream to Diana's hunting dog on this Republican denarius.