Bulge on a Vespasian denarius

[maridvnvm]

The following denarius has been giving me some concern. It is a Vespasian, COS VIII, Mars standing left, holding trophy and spear; corn ear in ground on right, RIC 104.
My concern is with the clear bulge on the coin at 4 o'clock on the obverse and between 2 and 3 o'clock on the reverse. This is the same place on both sides of the coin. I have seen bulges before on fouree coins that had bulges that had exposed the core. I cannot see any evidence of plating of any core exposure in the edge cracks. Could the bulges simple be the result of die breaks? The weight is 3.29 gms.

Please excuse the image. I have just started with a new digital camera and have not sorted my lighting setup yet.

Regards,
Martin

[Numerianus]

Martin, your particular question  leads to a more general one: at this period, how the  silver were  treated to
produce flan  to mint  denarii. It seems that first small  calibrated balls were made (there is a technology how to do this using a tower but I do not know whether it was known by Romans). 
Afterwards,  such a  balls should be hammered  to make a  discus. Finally, the discus was placed
on a die and the coin was mint.  What is intriguing, is the fact that  the metal quite often looks quite
stratified (this is well-seen at the edge) as hammering was  done quite intensively and repeatedly.
If the technology used was  indeed along this line, one can easily imagine that some ``residual" might  penetrate into  the silver body. An eventual chemical reaction could lead to a buldge.   

[mickdale]

if there was a chemical reaction with "some residual" to produce such a bulge, surely the surface of the bulge would show stress/fatigue fractures?
i know silver is malleable but with possible embrittlement over time, probably increased by dissimilar material (residual) in the  coin - and the very large increase in volume (without the addition of more material) at least some minimal surface failure would occour, such as a burst?

mick dale

[Rupert]

Could it be due to an air bubble locked in when the flan was prepared, which then expanded as the flan was heated before striking, then was compressed by striking, and then re-expanded after striking when the flan was still hot?

Rupert