Crystal and Glass: A Few Clear Differences

Question: I have wineglasses that are “glass” and wineglasses that are “crystal.” They all look like clear glass. What makes them different?

Answer: Less than you’d think. The principal ingredient of most everything we call glass is silicon dioxide--ordinary old sand--which you may also know under its highfalutin alias of quartz.

Sand alone makes a passable sort of glass. But it has a melting point around 3,100 degrees F, an inconvenient temperature for flammable mortals. So over the centuries, glassmakers have found additives that lower the melting point. The two most common are soda and lime. A basic household water glass is made from this.

The familiar kitchen and laboratory staple called Pyrex glass, however, is a bit different. Common glass has a nasty propensity to expand or contract a lot when heated or cooled; change its temperature too hastily and it shatters. But a mix doped with a little boron oxide results in a glass with a much lower thermal expansion rate--a trait much prized by cooks and the finicky flask-and-beaker set.

“Crystal” glass is produced by yet another formula. But first, a cruel truth: No matter how much you pay for it, it’s almost certainly not a crystal. That term refers to materials in which the constituent atoms and molecules arrange themselves into orderly geometric patterns, which usually happens when molten things cool slowly.

Commercial glass, which is cooled quickly, is not crystalline; it is amorphous. Its constituent molecules are connected in random arrays with no regular spatial patterns, much like the arrangement of molecules in a liquid.

Amorphous glass is easy to dope with compounds that influence color or other optical properties. One of those is lead, added in the form of lead oxide, which dramatically increases the refractive index of the glass. That is, it bends light more, which enhances its sparkle.

*

Q: Why does a wet finger stick to icy surfaces? If you wash your hands and touch something in the freezer, it can be difficult to detach yourself without losing skin.

A: Grab one end of an ice cube with each hand and try to pull the cube in half. You’ll notice one of the big differences between liquids and solids. The molecules of solids are much more tightly bound to one another.

So if your wet finger freezes to the object, pulling it off is no easier than tearing an ice cube into pieces and will probably result in an involuntary skinectomy.

Why doesn’t the ice just pop off your finger? Because it’s wedged into your skin. The reason lies in the nifty way water molecules attach themselves to one another. Each has two atoms of hydrogen and one of oxygen. But the hydrogen atoms both sit on one side of the oxygen atom, sort of like Mickey Mouse ears. That makes water molecules electrically polar: The hydrogen end is slightly positive, and the bare oxygen end slightly negative.

In the liquid state, the oxygen side of each molecule typically binds itself to the hydrogen side of one or two other neighbors with an electrostatic tug called a hydrogen bond. The strength of those bonds accounts for water’s relatively high boiling point.

As water freezes, there is less thermal motion to wiggle the atoms. Now each oxygen atom forms as many as four hydrogen bonds with adjacent molecules, locking it into a rigid crystalline structure in which four hydrogen atoms form a pyramid-like tetrahedral shape around each oxygen atom.

That arrangement contains a lot more empty space than its liquid counterpart. So water expands when it freezes. If it has worked its way into tiny crevices of skin, it anchors itself by swelling as it turns to ice. And you’re stuck.