Catching the Melting Bug

Dana Bauer
June 01, 2001

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.

To learn more about the science of glass-making, read "Flowers Out of Glass" by Nancy Marie Brown.

Last Updated June 01, 2001