Denarius with bubbles

[Optimo Principi]

Currently for sale on eBay from a seller not on the NFSL.

He states condition of the coin may be from a fire?? Look like casting bubbles to me. Does fire cause this appearance also?

[Joe Sermarini]

Yes, that looks like fire damage. In hand, the flan cracks would probably be either clearly genuine cracks from striking or cast. From the photos, I think more likely fire damaged than cast.

[Rupert]

I think so to. If THESE were casting bubbles, this would be an extremely low-quality cast. And the reliefs are too sharp for a low-quality cast IMHO.

Rupert

[Joe Sermarini]

By the way, the bubbles result from tiny pockets of air in the flan expanding with heat. Sometimes a bubble can or will have already been pushed flat, but that does l not return it to a normal appearance and sometimes the thin shell of the top of the bubble will break leaving a pit.

[Lee S]

I have also noticed after experimenting heating (bronze) slugs that what I presume must be lead bubbles out to the surface .... Given that much silver ore is found alongside lead ore, although I've not tried heating silver coins it's not impossible that they have a high lead content which would melt and bubble when heated...

[djmacdo]

Studies of cross sections of early third century Roman silver denarii show isolated patches of lead not fully incorporated into the alloy--such could bubble out in a fire as you suggest.

[areich]

I have also noticed after experimenting heating (bronze) slugs that what I presume must be lead bubbles out to the surface .... Given that much silver ore is found alongside lead ore, although I've not tried heating silver coins it's not impossible that they have a high lead content which would melt and bubble when heated...

That was discussed here years ago as a way of 'restoring silvering'. I think they were Byzantine scyphates.

[Joe Sermarini]

I have examined bubbles on fire damaged silver denarii, similar to the bubbles on the coin above. The bubbles were silver, not lead, and were hollow, created by expanding heated air. Certainly lead would melt and perhaps bubble out, but I didn't notice that on the fire damaged coins I examined.

[djmacdo]

I wonder whether lead or even copper elements might have vaporized and left voids, which then lead to silver bubbles?  So much I do not know about how alloys behave!

[Howard Cole]

Lead vaporizes at 1750 degrees C. (3182 degrees F).  I don't think a fire in a building or other structure reaches that temperature.  Copper is even higher.

[jmuona]

A coin of this type would have had much less than 80% silver bullion. This being the case, there would have been copper-rich cells as the metals do not mix properly with such oncentrations. Such copper pockets loose the metal due to corrosion quite easily. These empty pockets might be the cause of explosive pubbles in fire - sort of like hat Joe has observed.
Jyrki Muona

[Joe Sermarini]

I suspect, however, that the empty pockets were simply just very tiny little bubbles of air in the flan, as it was cast before striking. I am no expert on metal corrosion, but I think it unlikely that many copper pockets corroded away in the short period while the coin was still in circulation. If a hoard was burned long after being hidden, perhaps then corrosion is a more likely factor.

[jmuona]

Indeed, I was thinking of hoarded stuff burnt recently!
Jyrki Muona