There aren’t very many substances which have been both manufactured in the sands of ancient Egypt and in outer space. In fact, off-hand, I know of only one: glass.
Glass is an “amorphous solid”– its molecules don’t form a strict pattern, like the molecules of steel or granite, but are jumbled together like more like the molecules in a fluid. You might think of glass as a very stiff liquid.
The glass we’re familiar with consists primarily of sand, heated with other materials such as soda and limestone to about 1,300 degrees Celsius. (The additional materials lower the melting point of pure sand to a temperature achievable with a wood fire.)
Since the principle requirements for making common glass are sand and fire, it’s not surprising the ancient Egyptians, who had plenty of both, started using glass (as a decorative glaze) around 3,000 B.C. The discovery of glassblowing, probably about 50 B.C. in Phoenicia, made possible, and affordable, many new glass objects.
In medieval times the Venetians created new compositions, colours (through the addition of various impurities) and techniques. In the late 1600s the English invented “crystal,” which had a unique sparkle thanks to the use of very pure raw materials and the addition of lead oxide, which makes glass refract light more sharply than usual, resulting in rampant rainbows.
Most commercial glass is soda-lime glass, made from inexpensive ingredients–sand, soda and lime–that can be worked at a relatively low 700 degrees Celsius. Borosilicant glass, such as Pyrex, is a distant second in commercial importance. Because it doesn’t expand or contract as much when heated or cooled, it’s ideal for use in kitchens and laboratories everywhere.
Glass is brittle because it has many microscopic flaws in its surface. Putting stress on the glass causes these flaws to grow at the tip until the glass shatters (you see this in slow motion when a crack spreads across your windshield). “Tempered” glass is more resistant to breaking because its surface is cooled first, becoming rigid, so that as the interior then cools and shrinks, the whole piece of glass is compressed.
Glass doesn’t have to be made of sand (silica). Any material that can be melted can theoretically make glass. The trick is to cool the melted mass quickly enough to prevent its jumbled atoms from joining up into the strict patterns that define crystals.
In Earth orbit, molten liquids don’t crystallize as easily as they do on Earth, so it’s easier to make glass out of other materials. Delbert Day, the Curators’ Professor Emeritus of Ceramic Engineering at the University of Missouri-Rolla, who has been experimenting with glass melts on space shuttles for the last 20 years, theorizes that this is because of an effect called “shear thinning.” Stirring a thick liquid–say, syrup–makes it more fluid. On Earth, gravity alone is enough to “stir” a melted substance and instigate shear thinning. The increased fluidity allows atoms in the melt to move more rapidly into their preferred crystalline patterns. In space, in the absence of gravity, the melt remains more viscous, making it harder for the atoms to form crystals before the material hardens.
In experiments with test melts of fluoride glass (a mixture of zirconium, barium, lanthanum, sodium and aluminum) aboard a NASA transport plane that provides short bursts of near-zero gravity interspersed with periods of high gravity researcher, researcher Dennis Tucker found it easy to pull out fibers in low-gravity, but that the melt quickly crystallized in high gravity.
In upcoming experiments, Day will melt and cool identical glass samples on Earth and on the International Space Station, then count the number of crystals in each sample.
If his theory pans out, he may be able to use the knowledge to improve glass-making techniques on Earth, allowing the production of some of the remarkable forms of glass that are currently very difficult to make in gravity.
Those include bioactive glasses that can be used to repair human bones, then dissolve when no longer needed; glasses so insoluble they an be used to treat cancer by delivering high doses of radiation directly to a tumor; strong, corrosion-resistant glasses made of metal from which complex objects like engine parts could be cast (instead of machined); and the aforementioned fluoride glass, a hundred times more transparent than ordinary silica-based glass. (A fluoride-glass fiber optic fiber would be so clear that a light shone in one end in New York could be seen on the other end in Paris.)
Even after 5,000 years, it seems, glass is still on the cutting edge (sorry) of technology.
