Dr. Joe's Brain Sparks

Author Essay

Thixotropy. Who hasn’t been frustrated by the unwillingness of ketchup to emerge from its bottle? But the solution is simple: a good shake, and it pours like a liquid—a thixotropic liquid. Just what happens isn’t completely understood, but the violence somehow eases the molecular entanglements, allowing the gel to liquefy, at least until the motion ceases.
 
Ketchup isn’t the only substance that exhibits such behaviour. Quicksand is another example. Thrashing about in it increases the danger of sinking because the movement causes the sand-water mixture to become more liquid. But perhaps the most curious example of thixotropy is the twice-yearly liquefaction of the dried blood of St.Gennaro, patron saint of Naples.
 
Gennaro was reputedly a third-century bishop who was beheaded by the Romans, although there is doubt about his very existence. In any case, a sample of his supposed dried blood appeared in 1389, along with stories about it miraculously liquefying when taken on a religious procession through the streets of Naples. But the liquefaction did not always occur, and it was purported that when this was the case, some sort of disaster would follow.
 
So, what’s going on here? A spectroscopic analysis of the sample, which involved exposing the closed vial to light and determining what wavelengths were absorbed, has indeed shown that it probably is blood. But what causes the unusual behaviour? Professor Luigi Garlaschelli of the University of Pavia thinks he may have found the answer. The clue was that during the religious procession, the act of checking whether liquefaction has occurred involves repeatedly inverting the glass-walled portable relic case. This suggested that the substance inside may be thixotropic, but that is not a property associated with congealed blood. However, what if some substance had been added to the blood?
 
Garlaschelli and colleagues found that combining solutions of iron chloride and calcium carbonate with salt in a specific fashion yielded a dark brown thixotropic mixture that very closely resembled the “blood” of St. Gennaro. It turns out that ferric chloride can be  found in abundance on active volcanoes such as Vesuvius, which  is near Naples.  Whether the blood of St. Gennaro has been doctored in this way will never be known, because the Catholic Church is opposed to opening the vial. But let’s ask this question: What  is more likely—that there is a God who gives the faithful information about their future through the miracle of the liquefaction of the blood of a saint who may or may not have existed or that some clever alchemist concocted a thixotropic mixture from some readily available substances?
 
The opposite of thixotropy is isotropy. This is when a fluid becomes more firm when agitated. A mixture of corn starch and water can be walked upon—as long as you walk quickly.