November 24, 2003
Volume 81, Number 47
CENEAR 81 47 p. 27
ISSN 0009-2347


WHAT'S THAT STUFF?
GLASS
Transparent material made from sand plays a role in a wide range of applications

VICTORIA GILMAN, C&EN WASHINGTON

CUTTING EDGE Lead oxide in crystal makes it easier to cut without shattering and increases its ability to refract light.
PHOTO BY VICTORIA GILMAN
From the time I was 12 years old, I knew I would drink a champagne toast at my wedding from a Venetian glass goblet. That’s because I was there when my mom bought six matching glasses from a shop on the Italian island of Murano.

“When you get married, these will be yours,” she told me later as she reverently placed them in our china cabinet. I rolled my eyes at her, a bored preteen who had barely begun to notice boys. But I wondered, what made the glass in these glasses so special compared with our everyday kitchen cups?

From a chemical standpoint, the answer is, “Nothing, really.” The composition of my Venetian goblets is fundamentally the same as my set of $1.99 table glasses. In fact, most of the glass we use today in everything from windows and jars to microscopes and beakers is based on the same key ingredient.

According to Robert H. Doremus, professor of glass and ceramics science at Rensselaer Polytechnic Institute in Troy, N.Y., commercial glass is about 70% by weight silica (SiO2). This compound is obtained from sand and is ideal for making glass because it does not absorb visible light, making the finished product transparent. The glass in fiber-optic cables is made of pure silica because its high transparency speeds the transmission of photonic signals.

Melting pure silica requires temperatures up to 2,000 °C, so most manufacturers save fuel costs by mixing raw sand with about 18% sodium carbonate (Na2CO3), also known as soda. The carbonate brings down the melting point, but the added sodium causes the finished glass to dissolve slowly in water. Adding about 10% limestone (CaCO3) contributes calcium, which helps protect the glass from weathering. Heating these ingredients together yields a molten mix of calcium silicate (CaSiO3) and sodium silicate (Na2SiO3) that, when cooled under controlled conditions, creates what is known as soda-lime-silica glass.

“The chemistry of glass has remained almost exactly the same since man first started making it,” says Robert H. Brill, a research scientist at the Corning Museum of Glass in Corning, N.Y. The earliest glass vessels come from Mesopotamia and are dated around 1500 B.C. The recipe for early glass melts together white stone pebbles and plant ash containing soda and lime (CaO).

Brill notes, however, that in the scientific lexicon, glass does not have to contain silica at all; thousands of compounds can be made into glass. “Glass is a state of matter, not a particular substance,” he says. By definition, glass is an amorphous solid that has cooled to rigidity without crystallizing.

In nature, glass is formed when sand or rocks melt and cool quickly, such as when lightening strikes sand dunes creating fulgurites, or when volcanoes spew lava forming obsidian. Native American and Asian civilizations used natural glass pieces as spearheads and cutting tools, as well as for decorative purposes.

“People probably figured out how to make glass by accident,” Doremus says. Brill says the exact method of discovery is a mystery, but he speculates that glass evolved as a by-product of manufacturing a quartz-based material called faience used by the Mesopotamians.

From Mesopotamia, the technology spread to Egypt, where glass became a luxury item for the rich. Early glass was often colored to simulate gemstones by adding transition-metal oxides to the quartz and ash. Copper and cobalt oxides impart shades of blue, manganese oxide produces a purple, and lead-antimony oxide creates an opaque yellow.

GLASS-BLOWING ENTERED
the scene around the 1st century B.C. and brought glass objects to the common people. The technique, thought to have originated in Syria, was picked up by the Romans and spread across their vast trade routes.

When the Roman Empire declined, the trade routes between the Middle East and Western Europe deteriorated, and glassmaking stagnated outside of the Islamic states. Lacking raw materials from the East, Europeans found they could use quartz stones and ashes from native trees to make a glass that contained potassium oxide (K2O) instead of sodium oxide (Na2O). This potash-lime glass was mostly used to make stained glass windows for medieval cathedrals.

The process for making glass objects was reintroduced to Europe around the 10th century by the Venetians, who regularly traded in the Middle East. In the 14th and 15th centuries, Venice came to dominate the Western glass market, developing a brilliant, colorless glass called cristallo made from imported sand containing silica and soda purified from plant ash. The recipe for making cristallo was a closely guarded secret for centuries. According to Doremus, glassmakers could face a death sentence for attempting to share their skills abroad.

Over time, glassmaking spread from Venice, and in the late 17th century, British glassmakers developed crystal, another luxury item having humble ingredients. Also called soda-lead glass, crystal contains lead oxide (PbO) instead of lime. Adding lead increases the refractive index of the glass, making it sparkle and also making it clearer, heavier, and less likely to fracture when etched or carved.

“Calling it crystal is actually a misnomer,” Doremus notes. “In scientific terms, a crystal is a material that has an ordered structure.” All glass, including what we commercially call crystal, has long-range disorder because of its status as an intermediate state between ordered solid and disordered liquid.

Another glass variation substitutes boric oxide (B2O3) for lime and most of the soda. Borosilicate glass is most often used in bakeware (commonly marketed as Pyrex) and laboratory equipment because it expands less during heating, making it less likely to shatter when exposed to rapid temperature changes.

Knowing more about its rich history gives me a new appreciation for glass, especially for the Venetian goblets used at my wedding. I realize now that a little art and some chemical savvy can make common sand as precious as the rarest of gems.



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