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HOT!
NSF fellow Amy Barnes makes glass. Photo
by James Collins.
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Catching
the Melting Bug
by
Dana Bauer
I’m
dressed for the deep freeze of January in central Pennsylvania
when I enter the glass-making lab. But Amy Barnes, a
graduate student in the materials science department,
has already fired up the "Rapid Temp Furnace"
and it’s just a couple of ticks shy of 1200 degrees
Celsius, our melting temperature for the day. Heat radiates
from the furnace. I shed my winter layers quickly.
1200
degrees Celsius is as hot as lava—the kind that
emerges deep from the Earth in places like Hawaii and
Iceland to form glassy obsidian, porous scoria, and
dense black basalt. Making glass is sort of like making
volcanic rock: It’s the same process of superheating
following by supercooling.
In
the Hot Zone
Barnes, an NSF fellow at Penn State who studies with
glass-expert Carlo Pantano, has agreed to give me a
crash course in the science of glass making—a condensed
version of the undergraduate lab that she teaches. After
a careful explanation of the procedures, she hands me
a thick silvery apron and what looks like a welder’s
mask. "You’ll want these," she says smiling.
Next, she gives me a pair of heavy canvas-covered gloves
that smell like sweaty gym mats. Barnes wrinkles her
nose in sympathy.
She
points to the glass beaker resting on the black counter.
It’s filled with a gray powder called batch—a
mix of silicon oxide, sodium carbonate, boric acid,
and cobalt oxide. "I’ll get the crucible out
of the furnace," she says, fitting her own mask
onto her head, "and you add more powder from the
beaker."
Barnes
opens the furnace door. The inside glows a deep reddish-orange.
Seemingly impervious to the heat, she takes a pair of
long metal tongs, extracts the crucible—a small
cup made of a silicate clay (similar to the stuff flower
pots are made of)—and puts it on a metal stool.
I slowly tap the gray powder on top of the molten batch
already in the crucible. The liquid sucks in the powder,
and burps out small gas bubbles. "Great,"
she says, when I fill two-thirds of the cup. "What
you just did is called charging the batch."
A
few minutes later, we charge the batch again. This time,
it’s my turn to man the furnace. Barnes talks me
through each step. She advises me to keep my head down,
to look through the dark visor of my mask. I open the
furnace door and instantly I’m overwhelmed by the
heat. I forget to keep my head down and discover that
looking into a 1200-degree furnace is like gazing at
the sun. My exposed chin feels like it has been sunburned.
I
persevere. Grabbing the crucible with the tongs looked
so easy for Barnes, but for me it’s a cruel test
in depth perception. I close the tongs too close and
almost knock it over. I begin to sweat. Barnes waits
behind me with the patience of a tee-ball coach. Finally,
the tongs close firmly around the fat part of the cup,
and I pull it from the furnace (more praise from Barnes),
so that she can pour more powder into it. We repeat
this two more times, and by my second turn, Barnes makes
me feel like a pro.
Glass
Chemistry: A Very Brief Primer
The building block of glass is the silica tetrahedra
molecule: four atoms of oxygen arranged around one atom
of silicon. When the glass is still in its hot, liquid
form, the molecules move randomly, bumping into each
other, forming weak bonds, separating, then bonding
again. When most liquids cool to form solids, the molecules
snap to attention, lining up in a perfect repeating
order. But glass is different. The melt is cooled so
fast—from 1200 degrees Celsius (or higher) to around
500 degrees, which is below the material's freezing
point—that the molecules never get a chance to
line up. As it cools, the consistency of glass changes
quickly from water to honey to "molasses-in-January"
to silly putty to solid. The glass "freezes"
in solid form, while its molecules are still arranged
as randomly as the molecules in a liquid.
Barnes’
students spend the semester studying how changes in
the glass recipe affect how fluid the glass is over
a range of temperatures. But the part of the recipe
that’s most fun to play with is color. That glass
we’re making today is blue. Barnes selects an old
crucible from a tray of crucibles coated with thin layers
of colored glass (her palette). Blue is made from adding
a very small amount of cobalt oxide to the batch. Manganese
makes purple glass. Iron oxide or chromium oxide will
make green glass.
Pouring
the Melt
An hour later we’re ready to pour the melted batch.
Barnes pulls the crucible out of the furnace (again
with expert use of the tongs), and pours a nickel-sized
pool of melt onto a graphite plate. "We have very
fluid glass today," she says. "Sometimes it’s
like honey, and hard to pour." The liquid cools
quickly—changing from orange to deep red to a reddish
purple to dark blue. I take the crucible and pour a
button, then another, and soon the plate is covered
with half a dozen small blue buttons that are still
glowing with heat around their edges. As I pour, Barnes
uses a glass stirring rod to catch the bits of glass
that start to flow down the sides of the crucible. She
pulls the rod away, twirling cooled strands of glass
that look like hair. "This is fiberglass,"
she says.
The
supercooling causes a lot of stress inside the glass.
So we put the buttons directly into another furnace,
this one at 500 degrees Celsius, for the annealing process.
Heating the glass again just a little bit, but not enough
to melt it again, gives it a chance to relax, to "repair"
some of the internal damage. Later, the glass will be
cooled very slowly to prevent the stresses from redeveloping.
Barnes
shows me several kinds of molds that I can use for the
rest of my melt. With a paperweight in mind, I choose
one that is an inch-thick and disk-shaped.
"What
we do in this lab isn’t art. It’s science,"
says Barnes. "Although, sometimes the undergrads
try to make things with glass. One guy tried to make
a glass heart for his girlfriend at Valentine’s
Day."
Barnes
smiles. "Usually, my students catch the melting
bug," she says. It happens, I realize as I pour
molten glass into my mold, the moment you discover just
how cool glass-making can be.
******
"Catching
the Melting Bug" by Dana Bauer was orginally published
on R/PS Online, www.rps.psu.edu.
Amy
Barnes is an NSF Fellow in the deparment of materials
science, College of Earth and Mineral Sciences. Her
adviser is Carlo Pantano, Ph.D., is professor of materials
science and engineering.
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