Anyone seen this type of damage on bronze coins before?

[Hydatius]

I have a small AE 4 of Valentinian III. It is in absolutely perfect condition and shows little wear. While it was sitting in its 2 x 2 flip the edge sort of exploded, filling the flip with tiny granules of bronze. All along the edge the nice dark bronze had come off exposing a light, greenish granular interior. The 'perfect' bronze exterior looks just like a coating around the outside. I can only assume that this is some kind of corrosion from the inside. Has anyone seen anything like this before with bronze coins? I would have assumed that the whole coin would corrode, not just the inside.

Richard

[areich]

In Germany in the 70s or so, coins were often 'zaponiert' (varnished).
That sometimes produced this 'perfect bronze' look, though under magnification was not
so perfect anymore. Could this be the case with your coin, combined with bronze disease?

Andreas

[Hydatius]

There's no obvious sign of anything applied to the surface, which is matte not shiny.

[moonmoth]

Have a look at this coin, a dupondius of Marcus Aurelius.  The exterior looks lovely, but there is no metal left inside.  I think the exterior is just the original patina, and the rest has gone to either bronze disease or some similar corrosion (the colour looks wrong for bd).  Is yours like this?

Bill

[Hydatius]

That's it exactly. I could never get a decent photo like that, I'm afraid. It has that same whitish-green crumbly stuff on the inside. You start with a great coin and end up with one that's valueless.

Richard

[moonmoth]

At least you know you are not alone, though I can't recall seeing this kind of corrosion written about, so it is probably uncommon.

Well, I wouldn't say my coin is valueless, though the monetary value is certainly much reduced by the damage.  It's still perfectly good for die studies, and its remaining good looks would fetch some sort of a price. 

It is quite brittle.  Those edges come apart with not very much effort.  But that is also true of many crystallised silver coins, and they retain most of their value even when you can see the traces of crystallisation on their surfaces.  (Within reason.) 

My coin has stayed stable since I stopped messing with it, but it sounds as though yours might be actively disintegrating, which is not so good for value!

Could yours be getting some moisture from the air?  If so you might think about keeping your coins in a dryer environment, such as a box with silica gel sachets.

Bill

[Arminius]

These coins remind me to Roman bronze coins from modern agricultural areas of western Europe, especially over-fertilized land situated on or near former Roman camps.

These coins are often a 100% mixture of compact base metal oxides without any metallic rests.

Humid climate, frost, 60-80 years treatment of fertilizers, liquid manure and acid rain oxidizes all base metals in upper soils.

[aj]

I have a number of recently acquired uncleaned Romans in this exact condition.

[Hydatius]

Well, I wouldn't say my coin is valueless, though the monetary value is certainly much reduced by the damage.  It's still perfectly good for die studies, and its remaining good looks would fetch some sort of a price. 

Could yours be getting some moisture from the air?  If so you might think about keeping your coins in a dryer environment, such as a box with silica gel sachets.

I meant in monetary terms. It certainly has value for me!  I like the idea of the silica gel. Thanks for all the help!

Richard

[moonmoth]

These coins remind me to Roman bronze coins from modern agricultural areas of western Europe, especially over-fertilized land situated on or near former Roman camps.

These coins are often a 100% mixture of compact base metal oxides without any metallic rests.

Humid climate, frost, 60-80 years treatment of fertilizers, liquid manure and acid rain oxidizes all base metals in upper soils.

I have watched coins dug up on TV (Time Team) that disintegrated completely as they were removed from the soil, where the conditions were as you describe, though I can't  vouch for the exact state of fertilisation.  But I haven't seen other examples that have solid, stable patinas with no metal underneath. That suggests that the conditions surrounding them must have changed drastically at some time: first, to allow the thick patina to form, then to disintegrate the interior.

I have a number of recently acquired uncleaned Romans in this exact condition.

Any chance of a photo?

Bill