Will the saint save Naples from another calamity?

Molecule of the month: thixotropes
Today is the feast day of Saint Januarius, patron saint of Naples. In the city's cathedral worshippers will gather in the hope that a miracle will occur, as it has for the last 600 years. If it does, Neapolitans can rest easy for another year. The miracle is the liquefaction of Januarius's blood, held in two vials in an orb-like reliquary. When the archbishop lifts the relic from the altar and tilts it, the normally solid blood should flow.

Sometimes this happens on the first tilting, sometimes it needs hours of clamorous devotion. Sometimes it does not happen at all and this is seen as a bad omen. When the miracle failed in 1527, some 40,000 died from plague. Failures also previewed calamity in 1569 (famine), 1836 (24,000 died from cholera), 1941 (defeats in north Africa) and 1980 (earthquake that killed 3,000).

Januarius was a Naples bishop who was thrown to the bears on 19 September 305 AD, during the persecutions of Christians in the reign of the Emperor Diocletian. In the arena, a miracle occurred: the animals refused to harm him. Instead, he was beheaded. A serving woman, Eusebia, collected his blood from the execution block. By another miracle, this survived for 1,000 years, although its whereabouts was not recorded. It reappeared in Naples in 1380.

Not everyone believes that the holy relic is real blood, because a clot of blood will liquefy, but only once. This happens when the globin component degrades. It will not then re-solidify. A Victorian sceptic, J Timbs, suggested in his book Things Not Generally Known that Januarius's blood was a mixture of spermaceti wax and a red dye. This would melt under the heat of the altar candles.

Professor Luigi Garlaschelli, a chemist at the University of Pavia, has come up with a better theory: the "blood" is a thixotropic gel. He has not been allowed to analyse the holy relic, but has produced a material that behaves in the same way, and has made it using chemicals available in the 14th century.

A thixotropic gel is a viscous fluid which has set, but which becomes liquid if shaken, pressed or jarred. Left to stand, it sets again. Non- drip paints are thixotropic gels, as are some heather honeys and iron hydroxide.

Thixotropy depends on a network of hydrogen bonds, a weak type of chemical bonding, which is strong enough to support the bulk of the material when set, but easily collapsed by shock or pressure. Hydrogen bonds are formed when two oxygen atoms are attracted to the same hydrogen atom. Although the hydrogen can only form a strong chemical bond to one of them, it forms a temporary bond to the other.

In 1991, Garlaschelli and colleagues published their findings in the science journal Nature and sparked off a controversy among chemists about the holy relic. Garlaschelli made his "blood" by adding calcium carbonate to iron (III) chloride hydrate dissolved in water. The reaction produced a deep red precipitate of iron hydroxide which was dialysed for four days with distilled water to draw out the calcium ions. A pinch or two of sodium chloride and the iron hydroxide sets to a thixotropic gel in about an hour. Gentle shaking will cause it to liquefy and resolidify on standing. This sequence can be repeated over and over again.

The Italians claimed the materials they used would have been available to alchemists of the 1380s. Iron (III) chloride hydrate occurs naturally as the mineral molysite, found on active volcanoes such as Vesuvius. Marble is calcium carbonate, and sodium chloride is just common salt. Animal gut could have been used as the dialysis sac.

Thixotropic iron hydroxide is not consistent with all the curious observations reported down the centuries, but it does explain why the holy blood has unexpectedly liquefied without the intervention of a priest. This has happened a few times when workmen have moved the cabinet in which it was stored. But it does not explain why sometimes the blood has been observed to bubble and even to expand in volume.

The Church's claim that spectroscopic analysis of the relic has proved it really is blood has also been questioned. Passing a beam of light through the vial and analysing the wavelengths absorbed seemed to confirm the presence of haemoglobin. That was done in 1902, but the apparatus was rather primitive. All it showed was that iron appeared to be present in the vial. Not surprisingly, iron hydroxide "blood" gives a similar absorption spectrum.