The Alan Sondheim Mail Archive

To: sondheim@PANIX.COM
Subject: Physics News Update 868

The American Institute of Physics Bulletin of Physics News
Number 868 August 7, 2008
by Phillip F. Schewe, James Dawson, and Jason S. Bardi

The layers in old-master paintings can now be mapped non-invasively.
The beauty of a painting-its design, subject matter, and color---is
the first thing that strikes a museum visitor.  But art historians
want to know more than what can be seen on the surface.  They want
to know the stratigraphy.  This is the scientific word for the
succession of layers composing the total painted work-including a
preparatory layer right on the raw wood or canvas medium, an under
drawing, the actual layers of paint above, and possibly a layer of
varnish.  A new viewing method, borrowed from the kind of medical
resonance imaging (MRI) used in hospitals, will now allow this
detailed study of deep layers in paintings to be made non-invasively.
Why delve?  Why not stay on the surface and enjoy the artwork for
what it is?  Because decoding the properties of the materials in the
layers can establish such things as the age, origin, and
authenticity of the work.  Therefore, like geologists studying past
eras in Earth’s history by looking at strata exposed in a roadcut,
curators will sometimes probe the deep layers of a painting by
removing a tiny sample in order to inspect its layers with
microscopes employing electron beams or visible light.
Can one study a painting without wrenching out tiny portions of the
artwork?  Yes, by using other techniques, such as by shining x rays
on the surface and looking at the fluorescent light coming back
out.  However, these techniques generally allow only a shallow
mapping of layers into the body of the painting.
The new profiling technique, pioneered by a group of Italian and
German scientists, uses nuclear magnetic resonance (NMR), the basis
for MRI medical scanning.  The NMR process works on a microscopic
level.  First a powerful magnet helps to get all the hydrogen atoms
in a sample to point in the same direction-sort of like getting a
thousand soldiers to all look at a flag and then salute at the same
moment.  Then the atoms are exposed to a bath of harmless radio
waves.  These waves oblige the hydrogen atoms to swivel about in
place, as if they were doing exercise.  Eventually the atoms (or
more particularly the protons at the heart of the atoms) re-emit a
radio wave.
It is these emitted waves that are detected by a nearby sensor.
With the help of a computer the wave information can be turned into
an internal map of the whereabouts of the hydrogen atoms.  In the
case of medical imaging, the hydrogen atoms are usually found in
water molecules throughout the body.  By subtle analysis of the
resultant map a trained doctor can spot the location of a tumor
since its water content will be slightly different from that of the
surrounding healthy tissue.
In a study of a painting, by comparison, these same techniques can
provide information about the binding agents used in the painted
layers.  Historically these binders consisted of such things as egg
yolk or oil.  Learning the nature of the binding agent is often
enough to distinguish a naturally aged master-work from an
artificially “aged” fake.
One of the researchers, Federica Presciutti, a chemist at the
University of Perugia, says that the thicknesses of the layers in
the painting, even at the deepest level, can be determined.  And
although the age of the layers cannot be determined absolutely, it
is possible to tell which layers are older than others.   The whole
process is non-invasive.  Furthermore, the magnet used in the new
approach is single-sided.  Unlike the massive MRI magnets used in
hospitals, which usually surround the patient, or at least her limb,
the scanner can be brought right up to-but not touch-the painting.
(See the accompanying photo of the device being used to study the
painting “Madonna with Child” by Gentile da Fabriano, 1411.)
The operation of the new painting scanner were reported in 21 July
issue of the journal Applied Physics Letters.

Scientists study hearing in polar bears as the Arctic acoustic
environment changes.  As global warming melts Arctic ice and
pressure mounts for oil exploration, scientists worry that
increasing noise might interfere with polar bear reproduction.
In the frozen tundra on the northern edge of the Arctic National
Wildlife Refuge in Alaska, solitary polar bears live in a world that
is both bitterly cold and eerily quiet.  No leaves rustle in the
breeze. No insects buzz through the sub-zero air.  The only sound is
the hiss of the wind-blown snow, and when the wind fades, it is one
of the quietest places on Earth.
Several years ago, San Diego-based research biologist Anne Bowles
wondered what would happen if retreating ice sheets and intensified
oil exploration and drilling changed the nature of this silent
"acoustic environment" of the polar bears. The impact of increasing
noise on how polar bears hunt and their other little-understood
behaviors was a concern, she said. But perhaps the most crucial
question was whether an increase in noise would affect mating habits
and disturb female polar bears in their dens. Alaska's Prudhoe Bay,
where Bowles is focusing her work, is a prime area for female polar
bears to construct their dens.  They spend five or six months in the
dens before emerging in the spring, typically with one or two cubs.
What kind of noise might be a problem for the bears? Will noise from
human activity bother bears in the open, but not females in their
dens? Should there be limits on noise allowed in the vicinity of the
bears, and exactly what kind of noise would be a problem?  "If you
want to mitigate noise, you first have to know what the bear can
hear," Bowles said.  "That's step one.  That is disturbance ecology
So Bowles, of the Hubbs-Sea World Research Institute in San Diego,
set out to test the hearing of polar bears.  Given the hostile
environment, and equally hostile bears, trying to conduct hearing
tests on bears in the wild was clearly impractical, she said.  Polar
bears are perhaps the largest land carnivore, with males weighing as
much as 1,400 pounds. Although females are much smaller, they still
can top 500 pounds. And they are smart, dangerous and unpredictable,
Bowles said.
Bowles began her hearing tests at the San Diego Zoo with two female
polar bears who had been raised in captivity.  For the tests, the
bears were trained to hold their noses against a button, or station,
on the inside of a cage.  A tone of a certain frequency was played,
and when the bear heard it, she would move her nose to another
station and be rewarded with food, Bowles said.
Before the testing could begin, the cage was insulated against
outside sound with thick, lead-lined blankets.  Even with the heavy
insulation, in the urban setting of San Diego it was impossible to
make the cage quiet enough to test for extremely low frequencies,
which may be a critical range of hearing for polar bears.
"Low frequency sounds are a signal of power and size, so wild
carnivores growl and rumble at low frequencies to maintain their
territories, defend themselves, and threaten," Bowles wrote recently
in an article for the Polar Bear International organization, which
funds some of her work. "Low-frequency sound is also important
because it travels over long distances, so large, wide-ranging
animals like whales and elephants often use it to communicate."
She also noted that there is usually more environmental noise at low
frequencies, including noise from "human-made machinery."
Bowles conducted 4,000 hearing tests, first on the San Diego Zoo
polar bears, and then on two polar bears at SeaWorld San Diego.
Hearing thresholds were measured for 19 different frequencies, and
the results suggest that hearing in polar bears is indeed shifted
toward the lower frequencies.  How low remains uncertain, she said,
because it was impossible to make the test facilities quiet enough
to test below 14 kilohertz (humans typically can hear down to 20
"As far down as you can measure, they can hear it," Bowles said.
"They got down below what a cat can hear."  Conversely, she noted
that Polar bears don't hear well at high frequencies, with their
sensitivity to the high tones being much less than that of  dogs and
The sensitivity to low frequency sound, apparently at the expense of
high frequencies, might merely be a result of size, Bowles said.
Polar bears are big, and so are the structures in their ears.  In
other large mammals, such as horses and cows, the upper limit to
what frequencies the animals can hear that seems to be based on
size.  "It could just be an outcome of scaling," Bowles said.
But the lack of high frequency hearing might be adaptive, because
polar bears mostly eat seals, not small rodents that have
high-frequency squeaks.  Bowles said she would like to do a
comparison study of land-based brown and black bears that do eat
small mammals to see if their hearing is attuned to higher
frequencies than polar bears.
The concern with the polar bears, she said, is that as the Arctic
ice melts, the bears will move onto land and encounter human
civilization, which is noisy. "Just as for humans, there is noise
you can deal with and noise you can't, and we don't know what that
is for the bears. Our job is to find out what they can tolerate and
what we are looking for is not 'win-win,' but 'okay-okay' for both
bears and people."
There is enormous political pressure for the oil industry to get
into these areas, she said, and with the oil activities come people
and more use of the land for recreation. From her time in the
region, Bowles said, the oil industry workers are treating the bears
and other wildlife well.  "You wouldn't believe the care these guys
are giving to the wildlife," she said. "I wish we could get people
in the cities to behave that way."
After three years of testing the bears in San Diego, Bowles and her
co-investigator, Megan Owen, a researcher with the zoo's
Conservation and Research for Endangered Species facility, are
writing several scientific papers, including one for the Acoustical
Society of America, about their results and looking toward what they
hope is the next phase of their research.
In December or January, Bowles hopes to go to Prudhoe Bay and
construct a polar bear den, then measure outside noise from the
inside.  "We need to create a den to bear specifications, and have
it accessible enough that we can have vehicles nearby so we can
measure the sound," she said.  "We have to get information on what
they can hear inside the den, how much noise gets into the den.  If
they can't hear it, it isn't an issue, but we have to make the
There won't be a real bear in the den, for as Bowles noted several
times, polar bears are very dangerous.  Even the San Diego bears who
have been raised around people remain unpredictable carnivores.
"Most of the time they are sweet and glad to see you, and glad to
have the food your bring them," Bowles said of the bears. "Then
there is the day when you are the food. They are very opportunistic
For images to go with this story, please go to:

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