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May 23, 2006 > TechKnow Talk

TechKnow Talk

by Todd Griffin

Is glass a liquid or a solid? A very old window, like hundreds of years old, is thicker at the bottom than the top. This proves that glass is a liquid, right? It just flows very, very slowly.

When the TechKnow Guy was in high school (and dinosaur meat was on the cafeteria menu) he was taught that glass is a viscous fluid, specifically a supercooled liquid. However, most scientists now agree that glass is in fact an amorphous solid. Nonetheless, the myth of the liquid nature of glass has become so widespread that some science textbooks still include it as fact, and many people still believe it.

Let's start by addressing the question of fluidity or flow in glass. It is true that some glass windows in very old buildings, European churches for example, are thicker at the bottom than at the top. Some tour guides in these buildings have propagated the "flowing glass" theory to explain this phenomenon. After all, it does make a nice story. To understand the true cause, we will need a brief primer on medieval glass craftsmanship.

Most window glass manufactured prior to the 20th century, when the modern float glass process was commercialized, was produced using the crown glass process. Glass was blown, then rolled and worked in its molten state, flattened out, and finally spun into a disc shape. This yielded glass with a rippled, uneven surface. More importantly to this topic, it resulted in glass somewhat thinner and more uniform in thickness near the edge of the disc than in the center or crown, near the "umbilical" used to spin it.

The best quality windows were made of small, usually diamond-shaped sections cut from near the edge of the disc, and assembled into the desired window dimensions using a lead lattice. When larger, rectangular panes were cut, they were necessarily thicker at one end (typically the end nearer the center) than at the other. When such single panes were framed and mounted into a window enclosure, they were installed with the thicker end down, presumably to better bear the weight.

If you are not convinced, try thinking about it this way. If the glass had flowed over the centuries to such an extent as to noticeably thicken at the bottom, it would also have flowed over and around the frame at the bottom. Yet these aged windows, some nearly 1000 years old, show no signs of flowing onto their frames. If you want to make the tour guide uncomfortable, ask about that.

Scientists have also examined the oldest surviving examples of man-made glass, ancient Roman and Egyptian figurines dating back 2000 years. They exhibit no indications of flow. Yet another convincing argument involves very sophisticated optical instruments such as telescopes. The lens of a telescope or interferometer is ground and polished to an exceedingly fine tolerance. If glass flowed over time, even very slightly, large telescopes would soon go out of focus. But in fact, this does not happen.

So let's agree that glass does not flow like a liquid. That doesn't specifically address the question of its state of matter. Despite its lack of flow, glass is a sufficiently unique material to defy easy categorization as solid or liquid. Most solid materials are crystalline, meaning their atoms or molecules are arranged in a very orderly manner. Think of filling a can with marbles. The marbles position themselves in a structured pattern, touching each other in the same ways, and leaving spaces that are of the same size and shape.

But glass is quite different. Its atoms arrange themselves in a haphazard, random manner, with no repeatable, orderly structure. Think of filling a can with pennies. They pack themselves in a disorderly and irregular fashion, touching each other in different ways, and leaving spaces of varying size and shape. Thus the arrangement of atoms in glass resembles a very cold liquid more than a solid. Many plastics have similar molecular structures. This is known as an amorphous solid, opposed to a crystalline solid such as ice, metal, or stone.

Again, most scientists agree that, at least for practical purposes, we can categorize glass as an amorphous solid. However, some argue it is a highly viscous liquid (that does not flow!). This is known as a viscoelastic liquid. Others claim glass is sufficiently different from both solids and liquids to merit a category all its own, neither solid nor liquid. The bottom line is that glass behaves very much like a solid material. Dropping a wine glass on the kitchen floor is an easy way to confirm this.

People talk about giving something the "acid test," which means finding out if it will really work. Is there a real acid test, and if so what does it test?

The expression derives from the California gold rush of the mid-19th century, if not earlier. One dictionary defines it as "a crucial, final test of the value or quality of a thing or person." As you pointed out, it is used to refer to the ultimate or most telling measure of something, often its practical application, as opposed to the theoretical or conceptual stages. For example, a design for a new mousetrap may look great, but the acid test is whether it catches mice.

As implied above, the original acid test was to verify the authenticity of gold. In those days, a prospector or assayer may have used simple nitric acid, which will not harm gold but will strongly oxidize copper, present in nearly all minerals that appear similar to gold. Copper oxide is a distinctive blue color, and readily signals false gold. Today, jewelers and assayers use more sophisticated techniques. A modern acid test uses a combination of nitric and hydrochloric acid called aqua regia. The reaction of the material in question is sometimes compared to a known sample of gold to refine the test.

By the way, the widespread myth that gold is not affected by bleach, such as the common household product Clorox, is an unfortunate lie. Do not put your jewelry into bleach in an attempt to determine if it is solid gold; it can be seriously damaged. White gold is especially susceptible to attack by bleach. Even worse than harming your jewelry is getting an acid burn. Never try the acid test yourself. Take your jewelry to a reputable jeweler who is trained to evaluate it accurately, safely, and inexpensively.

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