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Metals And Their Corrosion

Silver coins produced in the 16th century. Courtesy of Daniel Frank Sedwick, LLC.

There’s a double hit of interesting metal artifact research news this week.

The first discovery comes from researchers in Lyon, France, who wanted to answer a contentious question in European historical economics: Did an influx of silver coins from Central and South America cause a period of crazy high inflation in Europe during the years 1520-1650–an episode known as the Price Revolution?

Yeah, I hadn’t heard of it either. But according to Lyon’s Anne-Marie Desaulty, it’s a seriously controversial topic in historical circles.

Remedying this controversy could seem relatively simple: 1. Find some coins used Western Europe during this inflation era. 2. Check their chemical make-up to see if they originated from mines in Europe or mines in the Americas. If the metal originated in the Americas then it would be safe to blame Europe’s inflation on Spain’s accumulation and distribution of silver from the colonies.

In principle, science can answer this question because metals from mine ores in different geographical regions often have their own unique chemical fingerprint. This fingerprint is called the isotope ratio: For silver, it would be the relative abundance, in the mine ores, of silver atoms that have 60 versus 62 neutrons in their nuclei. Unfortunately for this inflation controversy, different silver isotopes are really hard to tell apart analytically.

OK, fine. Instead researchers could try comparing isotopes of lead, which is another element present in silver ores and is thus is also found–in trace amounts–in silver coins. Except unfortunately lead isotope ratios are quite similar in Europe and the Americas—a tad too similar to conclusively distinguish the provenance of coins through lead isotopes alone. Oi.

So researchers might then be tempted to look at isotope ratios of copper which was added to the silver coins to improve their hardness. But scientists have to be wary of drawing too many conclusions from copper isotope analysis of silver coins because the copper might not have originated from the same place that the silver was mined.

With so many caveats, I might be tempted to give up on the project. Undaunted, Desaulty’s team first figured out how to use a recently improved technology to distinguish silver isotopes (which is incidentally called Multi-Collector Inductively Coupled Plasma-Mass Spectrometry, oh yes!). And then they combined these silver results with their copper and lead isotope measurements.

Examined together, the data allowed the researchers to conclude that Spanish money originating in the Americas did not cause the inflation. Desaulty says that it wasn’t until 1700s, after the Price Revolution, that Spanish coins from the Americas exploded into the European marketplace. I imagine there’s a group of economic historians somewhere in this world that are currently in a tizzy.

So while Desaulty figured out the geographical provenance of a metal, this week’s second metal-related discovery is on metal dating.

It turns out that figuring out the age of a metal is hard; there aren’t many scientific options for dating many archaeological artifacts made of copper, iron, tin, gold or lead, according to Antonio Doménech-Carbó, an analytical chemist at the University of Valencia, in Spain. Of course there are some ways around the problem: For example, if an iron artifact has some charcoal on it because the artifact was, for example, used as a pot, then you can do radiocarbon dating on the charcoal. But not every metal artifact has charcoal on it.

So Doménech-Carbó and his colleagues developed a technique called voltammetry to measure the age of metal artifacts containing lead–which is a heck of a lot of artifacts since lead was used widely in antiquity. For example, “lead was produced for fishing nets, anchors and sling bullets, fastening iron clamps in the walls of buildings, water pipes, jewelry and cult figures,” Doménech-Carbó notes.

Voltammetry involves first applying a voltage to a small sample of the artifact, and then measuring the current of electrons that get subsequently ripped off the sample–in this case, electrons from two components of corroded lead called lead oxide and lead dioxide. There’s a mathematical relationship between current, voltage and time of corrosion that can be used to date the sample.

All this inspired me to track down the song This Corrosion, by the English band Sisters of Mercy, who are not precisely a metal band per se, but are definitely a fascinating artifact from the 1980s music scene.

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