REHOVAT, Israel, April 15 (UPI) -- Israeli investigators said Tuesday that they have found a new technique to measure the age of ancient metal artifacts.
The method parallels radiocarbon dating, which has been used by archaeologists for decades to determine the age of bones and other organic material.
Until now, scientists had no direct way of determining the age of archaeological finds made of stable metals, including lead. But researchers have found that turning lead into a superconductor by super-freezing the metal, then looking at the level of oxidization -- rust -- made distinct by the process, could provide a new way to peer into the past.
"There is no other method to date the age of lead, which is quite often found in archaeological excavations. So it's better than nothing," researcher Shimon Reich, a materials scientist at the Weizmann Institute of Science, told United Press International.
Lead was one of the first metals used by humans. The Egyptians manufactured coins from the metal as far back as 5,000 B.C.
"In antiquity, they used to sheathe ships in lead. The idea was to avoid fouling by the sea, keep creatures like barnacles from accumulating on the hull, and (the barnacles don't build up) on the lead sheets because it's poisonous," Reich said.
Ancient Romans ran miles of lead pipes to bring water into their burgeoning cities -- a practice that had led historians to speculate that lead poisoning caused the downfall of the Roman Empire. Although that notion no longer is in favor, the word plumber, derived from the Latin word for lead, "plumbum," remains, Reich added.
When it comes to measuring the age of organic remains, all of which contain carbon, scientists use a technique known as radiocarbon dating. The method is based on the fact that all carbon atoms contain six protons in their nuclei.
When it comes to measuring the age of organic remains, which all contain carbon, scientists often use a technique known as radiocarbon dating. This method is based on the fact that all carbon atoms contain six protons in their nuclei. Nearly 99 percent of all carbon atoms in nature possess six neutrons as well, for 12 particles altogether, which is why such atoms are known as C-12. The other 1 percent or so of naturally occurring carbon atoms contain seven or eight neutrons in their cores. These variations on the carbon atom, or isotopes, are named C-13 and C-14 respectively.
Although carbon-12 and carbon-13 are stable, C-14 is not, decaying continuously to become nitrogen. The timeline of the decay, measured in intervals named half-lives, is well known to scientists and can be determined with useful precision. Carbon-14's half-life is about 5,730 years, and by measuring carbon isotope ratios inside organic matter like wood, scientists can roughly determine the age of the wood.
Dating the age of metal relics remained another matter. No natural unstable isotopes of iron, gold and silver are known. When it comes to lead, its only natural unstable isotope, Pb-204, has a half-life of at least 140 quadrillion years -- a span about 10 million times longer than the age of the universe.
Instead of radioactive decay, the research team decided to measure chemical decay. When exposed to air, soil or water, the normally lustrous, silvery-blue metal develops a dull, gray film of lead oxide and lead carbonate. This tarnish seldom dissolves in water at room temperature and serves to protect lead objects from further rusting -- so they corrode extremely slowly, Reich explained. In soil neither too acidic nor alkaline, lead objects would only corrode about a half-millimeter per millennium.
Reich knew lead became superconductive when super-cooled to about -450 degrees Fahrenheit (about -270 degrees Celsius). Superconductive materials produce no resistance to electricity.
However, "only the lead superconducts -- all the corrosion products do not," Reich said. Moreover, superconducting lead's magnetic signature is 100,000 times stronger than its corrosive compounds. Therefore a magnetic scan of a super-cooled lead artifact can determine how much lead has corroded and how much has not.
Over the past year, Reich and his partners, archaeologist Sariel Shalev and metallurgist Gregory Leitus, tested their technique on about 50 lead samples from 2,400-year-old Persian artifacts, 1,350-year-old Byzantine pieces and 750-year-old Crusader-era items -- objects of known ages based on other research. In findings presented online and submitted to the New Journal of Physics, they found the age of the objects matched roughly as hoped with the level of corrosion detected.
"This can serve as a way to now measure the archaeological age of an object without destroying it," Reich concluded.
The method is not without problems, however. Because it is based on chemistry instead of radioactivity, scientists always would have to worry about temperature, acidity levels and other environmental conditions that might affect corrosion rates. The researchers said they hope to test more samples to refine their technique.
"Such unusual application of superconductivity is a nice surprise. What is exciting for me is an elegance of approach, its beauty," physicist Vladimir Kresin at Lawrence Berkeley (Calif.) Laboratory, told UPI. He added he thinks the method is "very promising" for other applications, such as studying metal fatigue in various alloys.
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(Reported by Charles Choi, UPI Science News, in New York)
