The Liquefaction of San Gennaro's Blood

Januarius, in Italian San Gennaro, was a bishop of Benevento traditionally said to have been martyred near Naples under the emperor Diocletian's persecution around 305 CE. Two sealed vials kept in Naples Cathedral are venerated as containing his blood. On the saint's feast day, 19 September, on the Saturday before the first Sunday in May, commemorating the translation of his relics, and on 16 December, the vials are brought out before the archbishop and a public congregation, and the dark solid material inside is reported to liquefy, and at times to increase in visible volume or bubble.

The event has been recorded at Naples since at least the fourteenth century, and local tradition has long read a failure to liquefy as an ill omen for the city, a belief strong enough that its absence has historically drawn public anxiety independent of the ceremony's religious meaning. The relic is not tested chemically as part of the ritual; the vials remain sealed throughout, opened only to be turned and observed. Naples is not unique in this regard: several other Italian relics, generally called blood miracles, are reported to liquefy on their own feast days, though San Gennaro's is the most widely attended and the most closely studied. The cathedral does not restrict attendance, and the moment of liquefaction, when announced, is traditionally met with applause and a white handkerchief waved by the crowd.

In 1991 the chemist Luigi Garlaschelli, with Franco Ramaccini and Sergio Della Sala, published a study in the journal Nature proposing that the relic's contents could behave as a thixotropic gel, a substance that is solid or semi-solid at rest and turns liquid when agitated or disturbed, then returns to its resting state. Using chalk, iron chloride and other materials that would have been available in fourteenth century Naples, they produced a mixture with the correct dark brown color that stayed solid when left still and turned fluid when shaken or stirred, closely resembling the described behavior of the relic.

Because the Church has never permitted direct chemical sampling of the sealed vials, Garlaschelli's result is a demonstration that a mundane substance can be made to reproduce the reported effect using only medieval materials and techniques, not a laboratory finding about what the vials actually contain. Both proponents and skeptics agree on that distinction; they disagree on how much weight a successful reproduction should carry when the original material itself remains untested. Garlaschelli has since applied similar thixotropic reasoning to other reported blood relics in southern Italy, arguing the recipe was plausibly available to medieval custodians even if it was never written down as a formula.