28 DISCOVERMAGAZINE.COM
FROMTOP:^COURTESYOFELLERYFRAHM;BEN^ WALKER;^FRAHMcolors occur — may have made it attractive for
symbolic or aesthetic reasons as well. Whatever
its appeal, early humans sought it out. Despite
its frequent use throughout prehistory, the raw
material is not common.
“Obsidian is rare most places in the world,
and it takes the right kind of volcanic eruption
to create it,” says Frahm. “The lava needs to be
highly viscous — or sticky — to form glass. A high
amount of silica is required to get that sticky lava.”
While this high-silica lava is still below ground,
minerals from surrounding rock leach into it,
adding different elements specific to that location.
When it finally erupts, if it reaches the surface fast
enough, the lava will cool quickly and turn into uni-
form glass before its unique chemical mix has a chance
to organize itself into crystals.
“When that happens, all the elements are frozen
inside that glass like a snapshot,” says Shackley. “That’s
why we can distinguish all these different sources.”
X MARKS THE SOURCE
Determining the source of a piece of obsidian means
analyzing the chemistry of the artifact and then match-
ing it to a database of geochemical signatures unique
to individual sources, called flows.
For decades, archaeologists have had few options
for that analysis. Most of the equipment is expensive
and limited to lab settings. The most precise methods
also require destroying the material to be analyzed
and, as Shackley notes dryly, “for some reason, people
in museums get really upset when you break their
artifacts into tiny little pieces.”
That’s why non-destructive X-ray fluorescence
(XRF) emerged as the most popular method for
sourcing obsidian. Though not the most exacting
analysis, it’s particularly good at identifying strontium,
rubidium and other elements that are frequently key
ingredients in a flow’s signature.
“XRF is a beam technology,” says Zipkin. “It hits
the target atom, knocks electrons out of orbit, exciting
them, and then, as they drop back down [into normal
orbit], they emit characteristic energy that the device
measures.”
Lab-based XRF devices have gotten smaller over
the years: “The first XRF [machine] I used in the ’80s
filled a room,” says Shackley, who is director of the
Geoarchaeological XRF Laboratory. “Now the equip-
ment sits on a desktop, a little bigger than a breadbox.”
Outside the lab, the technology has seen even more
impressive downsizing.
The first commercial portable XRF (pXRF)
devices arrived in the 1990s, intended for industrial
applications such as exploratory mining and quality
control in cement manufacturing. They soon found
their way into the hands of many an archaeologist. In
those early adoption years, enthusiasm for the new
tool’s potential sometimes outpaced scientific rigor,
and results were not always reliable.
Methods and standards across the field have since
improved, as have the portable devices, which continue
to get cheaper and more precise. Today’s pXRF devices
— which “look like a combination of a ray gun and a
hair dryer,” says Zipkin — run less than $40,000.
“One can put it in a carry-on bag
and bring it almost anywhere in
the world,” says Frahm. “No longer
do we have to request permission
to export a few artifacts to a distant
lab. Instead, we can work where the
artifacts are, whether in a museum,
in a field lab, or even at an archaeo-
logical site itself.”
He adds: “In only 10 or 20
seconds, we can know where a
particular obsidian artifact origi-
nated, hundreds, thousands, or
even hundreds of thousands of
years ago.”DATES THAT DON’T
HOLD WATER
While the tools to determine the
source of obsidian become ever more user-friendly,
obsidian hydration, a method used to date the mate-
rial, remains problematic.
Developed in the mid-20th century, the method
takes advantage of the fact that an obsidian surface
starts absorbing atmospheric water as soon as it’s
exposed to the air. To date the artifact, researchers
take a cross-section and look at the hydration rim,
a gauge of how far water has penetrated into the
volcanic glass.
“It’s a great idea, but it’s almost impossible to put
into action,” says Zipkin, who calls the results “really
just an educated guess.”
That’s because the speed of moisture penetration
is affected by numerous factors, including tem-
perature and humidity. A difference in temperature
of 1 degree Celsius, for example, can change the
hydration rate by 10 percent; a single event such
as a forest fire roaring past the artifact’s
location might reset the hydration rim
completely.
Most archaeologists instead date obsidian
artifacts by context — what else was found with itORIGIN STORY
Archaeological
scientists Ellery
Frahm (top, with
an obsidian hand ax
unearthed in Armenia)
and Andrew Zipkin
(above, collecting
obsidian samples in
Arizona) use portable
“ray gun”-like devices
(below) that provide
information about the
geographical origin
of individual pieces
of the material.