Shakeup over sacred blood.

Thixotropy -- the property that lets toothpaste ooze when squeezed out of its tube and yet not drip off the toothbrush -- may explain a centuries-old miracle.

Blood, once congealed, tends to stay that way. But when religious leaders handle a vial believed to contain the blood of St. Januarius, the dark brown substance begins to flow. Periodic demonstrations of this effect have drawn crowds to Naples since 1389, notes Luigi Garlaschelli, an organic chemist of the University of Pavia in Italy.

In the Oct. 10 NATURE, Garlaschelli and two other Italian researchers propose that medieval alchemists could have created a thixotropic substance that looked like blood by mixing water and salt with a mineral called molysite. Thixotropic materials exist as gels until a mechanical stress -- such as picking up or tilting their containers -- makes them flow.

To explore this possibility, Garlaschelli searched through the scientific literature and discovered that about 70 years ago, researchers demonstrated thixotropy in an iron hydroxide alloy. He reproduced their work by mixing a ferric chloride compound with calcium carbonate in water, then separating out the iron hydroxide that formed. By adding salt to a solution of this alloy, he created a dark brown gel. "It looks exactly like the samples in Naples," he told SCIENCE NEWS.

All of these materials were available five centuries ago, including ferric chloride, found near Mt. Vesuvius in the form of molysite, he says. While noting that the Catholic Church forbids opening the sacred vials and analyzing their contents, Garlaschelli and his colleagues write: "Our replication of the phenomenon seems to render this sacrifice unnecessary."