A Bit About Glass
Has anyone ever told you that glass is actually a liquid? That, ever so slowly, everything made of glass is actually flowing, and will eventually reduce itself to a glossy puddle on some futuristic landscape? It's not an uncommon tale, even though it's absolute horsepucky.
Some glass buffs think the notion of glass as a liquid got started in Europe a century or so ago, when very old sheet glass was removed from very old buildings under renovation. The workers might have noticed that the sheets were much thicker on one end than the other, and that the thicker end was always down. Drawn by centuries of gravitational pull, they speculated, the bottoms of the windows had become thicker than the tops. Since people were already accustomed to fairly uniform sheet glass by that time, it's no surprise that the ancient hand-blown sheets raised some eyebrows, and possibly, the faulty conjecture.
A more likely explanation for the misconception arises from the definitions of "liquid" and "solid." The truth is, glass exhibits characteristics of each. Webster defines "liquid" as "a substance which exhibits a readiness to flow." Of a "solid" the dictionary says: "of definite shape and volume; not liquid or gas." Clearly, what we call "glass" better fits the latter than the former. But science doesn't look to Webster to draw their distinctions. Science-types will tell you that a material is a solid when its molecules are motionless and lined up in flawless geometric fashion, in perfect little rows and columns, like thousands of tiny soldiers at attention. This molecular configuration is called "crystalline". A liquid, on the other hand, is quite the opposite. Liquid molecules are in a constant state of movement and entirely random in their configuration. Scientifically, then, cold glass is neither liquid nor solid, because its molecules are motionless (like a solid) but random in configuration (like a liquid). This structure is characteristic of all vitreous (glassy) substances.
Many materials, like water and iron, are common in both liquid and solid states. At any given moment, their state depends on their temperature. Water molecules are disorderly and mobile only to a point, which is thirty-two degrees Fahrenheit. There, the water molecules "crystallize" - they line up in perfect lattice-like order and cease moving altogether, until the warmer side of thirty-two shines once again. The liquid has become a solid.
Thirty-two is said to be the "freezing point" of water (or the "melting point", depending on which direction temperature is moving). But this is a "point"only in temperature, not in time. When water reaches thirty-two degrees, it stays at thirty-two degrees until crystallization is complete. This may take a split second (a snowflake) or a great deal of time (Lake Omygoochie). Only when every molecule has taken its place in the lattice and come to a standstill will the temperature continue its decline.
One of the most fascinating things about vitreous substances is the conspicuous absence of any freezing point or melting point. There is no "point" in temperature where glass naturally maintains itself while its molecules reconfigure. As temperature decreases, the free-flowing molecules in molten glass simply move more and more slowly, until they are no longer able to move at all. But they maintain their random configuration; crystallization does not occur.
If, on the other hand, we hold glass at a given temperature for a long enough time, crystallization, to some degree, will take place. The crystallized areas will no longer be glass, of course, because lack of crystallization is how we define glass. The crystallized areas are non-glass; they are de-vitreous—devitrification has occurred.
Think about what happens as a piece of cold glass is heated. The randomly configured "frozen" molecules slowly begin to awaken and agitate. As the glass becomes warmer there's more freedom of movement. The higher the temperature, the faster and freer the molecules spin about. What is happening? In a practical sense, the glass is moving through different degrees of viscosity. From cold and hard it becomes warmer and softer, then pliable, then soft enough to slump under its own weight. Eventually it will puddle and finally, become liquidous. There are no such stages or degrees between water and ice; it's either one or the other. Such is the unique magic of vitreous substances.
Glass, then, is really neither a liquid nor a solid; it exhibits definite characteristics of each. In fact, some schools of thought find it more clear and convenient to classify matter into four states instead of the traditional three. So don't be surprised if your kids come home from school one day and tell you the Four states of Matter are liquid, solid, gas, and glass.