Layers in denarii

[moonmoth]

For a while now I have been looking at the insides of coins, what can be seen of them.  I have found several examples of struck silver coins which show a distinct layering effect; the surface layer has different properties from the interior, and there is sometimes evidence of further layering such as a discontinuity across the middle of the interior.  This is an interesting specimen in this regard.

At first glance it looks like a fourree, because a patch has broken off in just the same manner as the silver skin of a fourree.  But on this coin, there is no evidence of a base metal core.  The exposed flat surface is dirty, but has no green discolouration as a base metal core might have, and at the edge, only solid silver can be seen; examination with a lens shows this to be so even deep inside the edge cracks which occur during striking.  I actually probed at a suspicious area on the edge with a knife, exposing only a silvery gleam.

So this would seem to be an unusually good demonstration of the layered nature of a denarius! 

I started looking for this kind of thing when looking at coins which have crystallised or show other metallurgical deterioration, shown on my page:

https://www.forumancientcoins.com/moonmoth/crystal_coins.html

The photos show the coin as a whole, a close-up of the area round the flat exposed surface, and a close-up of a section of the edge.

[Joe Sermarini]

It seems that the compression of striking can create layers.  When the outer layer flakes off it is called a lamination defect. 

[moonmoth]

There's not just the compression to consider, but also the way the surface metal is forced to flow at speed into the hollows of the die, which must cause it to heat up considerably, taking its energy from the kinetic energy of the hammer.  Flowing, heating and cooling are good candidates for what gives the surface layer different metallurgical properties from the underlying metal. 

Do you have other examples that demonstrate such an effect? 

[Heliodromus]

I don't know about the layering, but the photography is great!

Ben

[moonmoth]

Thanks!

https://www.forumancientcoins.com/moonmoth/coin_photos.html

[moonmoth]

Does anyone know of any articles on this effect?  I can locate a few photos, but little more.

[basemetal]

Here's an interesting example.
I would likely never have known, but I dropped the coin.
Ping!
It looks as if moisture got into an already existing crack on the edge and over time
percolated through. Maybe.
Bruce
Basemetal

[moonmoth]

Yes, interesting!  It looks as though it has suffered a good deal of corrosion on the outside, too.

On the surface that is left behind, can you see whether there are tiny lumps standing up, or is it pitted?  And does the piece that has come away look like the incuse in its fine detail, or does it look as though some material has been lost in between?

The picture here is what I mean by "lumps standing up."

Bill

[Numerianus]

I recall our old discussion on layers
https://www.forumancientcoins.com/board/index.php?topic=29388.0
could the layering be explained than the fabric (even in the second
century) was "improved"  by a higher quality silver applied on the surface?

[basemetal]

The effect on the denarius I showed is more like the splitting of a shale type rock.  And there is no correlation on the exposed surface of the reverse inscription with the exception of the "lump" or raised area at what corresponds to the top of the reverse.
Either the flaw /fracture was there and water got into the space, or freezing thawing/heating cooling widened a very small area at the rim. The  exposed surface underneath looks as it it would "polish" to a silver color if buffed or cleaned.  My two cents.
Bruce

[moonmoth]

I recall our old discussion on layers
https://www.forumancientcoins.com/board/index.php?topic=29388.0
could the layering be explained than the fabric (even in the second
century) was "improved"  by a higher quality silver applied on the surface?

That's an interesting idea, but I am doubtful. There's not much evidence of poorer quality silver inside my own specimens.  The silver in layered coins like the one I started this thread with looks just as good deep inside the edge cracks as it does on the surface.  I have also seen layering effects in silver coins from Corinth c. 330 BCE, Persis c. 50-1 BCE, the Roman Republic 66 BCE (the Pomponius Musa coin I just posted) and a Constantius II siliqua of 356 CE, among others.  That's quite widespread in space and time, and suggests a physical side-effect of striking rather than an applied layer.

Bill

[Rugser]

I think that the phenomenon is initiated when in to make the alloy they have wanted to impoverish adding copper granules.
Because the point of fusion of the two metals is different the amalgam results scarce.
Now we see how these copper granules have been being uncovered for the oxidization
I have two examples of these denarii that have suffered the natural electrolysis in the ground and the copper it is completely disappeared.

[Joe Sermarini]

I think that the phenomenon is initiated when in to make the alloy they have wanted to impoverish adding copper granules.
Because the point of fusion of the two metals is different the amalgam results scarce.
Now we see how these copper granules have been being uncovered for the oxidization
I have two examples of these denarii that have suffered the natural electrolysis in the ground and the copper it is completely disappeared.

Although interesting, I think that is something different from a lamination defect.   

[Rugser]

Hi Joe
I think not defect of lamination I think  defect of alloy (silver-copper=scarce amalgam)

Best regards
ser