Metallic ink was used to inscribe scrolls regarded as an archaeological wonder, according to scientists.
The discovery pushes back the date for the first use of metallic ink by several centuries.
The Herculaneum scrolls were buried by the eruption of Mount Vesuvius in AD79 and are charred and fragile.
Previous efforts to read them, over many centuries, has damaged or destroyed some of the scrolls.
The task of reading the surviving scrolls has fallen to scientists using technology such as the European synchrotron, which produces X-rays 100 billion times brighter than the X-rays used in hospitals.
Last year, physicists used the 3D X-ray imaging technique to decipher writing in the scrolls.
Now they have gleaned that the papyrus contains high levels of lead, which they say could only have come from its intentional use in the ink.
“We found some metal – some lead – in the ink, which is supposed to come four centuries after,” said Dr Emmanuel Brun of the European Synchrotron in Grenoble, France.
“The common belief is that the Romans introduced metal in the ink in the fourth century.”
Until now, it was thought that the ink used for the manuscripts was carbon-based.
The work, which appears in the journal Proceedings of the National Academy of Sciences, will help further investigations of the scrolls using synchrotron light.
“The discovery is interesting for the historical aspects but also for us for the papyrus scroll imaging,” Dr Brun said.
“The different phases of the present study on the ink will allow us to optimise the next experiments on the reading of the invisible text within papyri. ”
The eruption of Mount Vesuvius buried the resort town of Herculaneum in ash along with its larger neighbour, Pompeii.
A library of about 2,000 scrolls was excavated from one of its villas in the 18th century, of which about 600 remain unopened.
Most are philosophical works in Greek, but other works include a comedy in Latin.
Commenting on the study, Dirk Obbink, Professor of Papyrology and Greek Literature, said: “These are startling findings, if confirmed, charting the wave of the future.
“Until now, I hadn’t expected to be able to read any of these scrolls from the inside, without damage to them, in my own lifetime. But now I do.”