Kalium-40 famously has one too many neutrons, β-decaying to argon-40. Not quite radioactive enough to give you argon burps; so it comes up from the mantle with normal potassium in eruptions. How radioactive, is the trick; if it's too long, as with bismuth-209 (2.01×1019 years) the halflife gets imprecise.
Pumice is telling us more-exactly how radioactive is 40K: from Vesuvius. This is done in tandem with recent work verifying the Plinian Eruption against the first months of Titus' imperium.
The latest coins in situ predate 24 June AD 79. I guess the Romans wanted to wait a bit in case someone assassinated the likely heir. It turned out his bro Domitianus liked the guy, better than he liked anyone else in Latium anyway. I don't know any coins got minted in Campania for a year or so after the eruption; Roman coins after 8 September get the "IMP XV" sigil, the fifteenth acclamation
of their new imprah. Also "P P", pater-patriae.
The town with the pumice in question was Oplontis. With the assumption of a July-August eruption - Pliny says 24 August - the halflife is narrowed. 1.2044×1010 years, twelve billion. Give/take 88 million.
Which they then overlay back on the eruption for 1,938 ±13 years before 2025. Which is AD 87. So uh. I guess they added these calcs to some others to get the halflife. The precision is still not what we'd prefer, but 14C is also a pest.
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