Inside a fourree

[Numerianus]

This seems to be an ancient forgery or ... or limes (of Gordianus III?). What is interesting: how it was produced?
It is brocken and  this reveal the interior structure which is completely surprising for
me.  The coin contains a very thin ``kernel"  of white metal (silver?) which resisted
corrosion.  The covering metal is  thicker (plomb, tin?). The central layer, apparently, is intended
to ensure the resistance to deformations!  I was thinking always that forgers
covers cheap bronze by a thin layer of expensive silver...  The problem is that the  adhesion
is not good and so one needs an intermediate layer of a third metal and this was indeed
noticed in a recent studies.  So the technology seems to be rather complicated.   
Could someone give a comment on this coin?

[Numerianus]

Larger pic:

[PeterD]

I read somewhere that fourees were made by soldering a thin layer of silver over a bronze flan. To do that you would need some "solder" made from a metal with a lower melting point than silver (lead?) and some form of anti-oxidant. Modern electrical solder contains just that - a thin wire of lead alloy with thin tubes of "flux" within.

Therefore a possible method of making a fourre would be to clean a bronze flan, dip it in some sort of flux, dip it into a bath of molten lead. When cool, dip it into the flux again, wrap in a thin layer of silver and re-heat. Then it would be ready for striking.

That could fit what you are seeing on your fouree.

[Rupert]

My hypothesis is a different one: your coin might not be a fourrée, but made of a low-quality silver alloy (billon). This is oxidized deep into the coin but not completely to the core, so after such an injury the core of non-oxidized billon may become visible.

Rupert

[Numerianus]

It seems that both hypothesis are not consistent  with the fact.
It is  natural to cover the  forgered coin (made from a homogeneous peace of cheaper metal)  by a thin layer of silver and we cannot exclude that this one was silver washed but the silver coating was weared out or corroded.
What I see in hand: this particular coin has a core, of a very different metal. It is impossible that such core was created
by a corrosion process.  My impression is that the original flan was bimetallic.
There is an article on a recently discovered process of silver coating but the method used for this coin  should be different.
http://dsc.discovery.com/news/briefs/20060417/forgery_arc.html

[moonmoth]

There can be odd layered effects within genuine coins quite like the one you describe and show.  I started a page on crystallisation and embrittlement, and this is starting to mutate into something about what's inside coins in general.  I have found layers in very thin siliquae and thicker denarii. 

You might expect the surface of a struck coin to have a different structure to the centre, given the stress it has been subjected to in the striking.  But some coins also show layers deep within.  The first photo below is a denarius of Q Pomponius Musa which has fractured along a rather lumpy layer line of weakness almost in the centre of the thickness of the coin. The second is the broken edge of an unattributed siliqua, broken by the digger, showing where the stress of breaking has separated the fabric of the coin along layered fault lines.

These are towards the bottom of my page here (hosted by Forvm), where there are also a few more photos:

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

So, given all this, I suggest it is possble that your coin has become corroded and this has exposed some existing layering which does not need to have been carefully crafted, but could just be the natural state of a struck coin.

Bill Welch

[Numerianus]

Hi, Bill,

Your theory  has a good ground but, probably, cannot  not applied to this particular coin.
Indeed, many coins have a layered structure, well visible at the edge.
However, I would classify it as  irregular.  It looks like ``pate" for bisquits. 
 By the way, is there a commonly accepted explanation for this effect?  It seems that a considerable mass
of the metal was forgered  and flattened when producing flans in the same way as for pastry...

As you wrote in your page, usually, the surface of coins is reinforced by the shock and is much more
resistant to cristallisation, oxidation and other chemical and physical processes.
I show below a nice example of a small bronze with a very nice surface but
claylike substence in its interior (at least, near the damaged part).
 This is quite typical and understandable. I have a fourree Voconius Vitellus which
resembles  a small  empty  silver box: the core  degraded and was dissolved.

On the other hand, the  core of this antoninianus seems to be crafted intentionally: it is very thin relatively to the overall
thickness of the coin and regular. Moreover, it is visible and intermediate level gluing the exterior thicker
layers of oxidated metal.   

 

[moonmoth]

Hi, Numerianus -

I haven't seen an explanation of the layering phenomenon as yet.  It doesn't seem to be in all coins by any means.  If flans were produced by pouring metal into a mould, there's no obvious reason why it should happen.  If strips were hammered and folded a few times, then cut into flans, then maybe, but why should flans be made like that?  It seems unlikely.  Perhaps it's another occasional effect of the stresses produced by hammering.

Coins having a stronger surface is common enough, though - that's a nice example you showed.  Here's another one.  This is a fouree core of Philip I, and you can see from the damaged edge that the interior has turned into a sponge!  The surface looks OK, though.

Bill

[Numerianus]

I started to think that the core of this antoninianus resembles
 an electrode and what we see is a result of galvanoplastic.

[moonmoth]

You suspect that the metal from the core has deposited on the surface through electrolysis?

[Numerianus]

It was a crazy idea which came when I was looking at photos. In hand the coin looks authentic.
Indeed, it may happen that the flan was composed  from several folds of silver of different quality
and the upper layers were more vulnerable. Was such a technology reported somewhere?