glassesPhoto, by Wikimedia user UFA66, of an artwork titled “Hip, Hip, Hurrah!” by Danish painter P.S. Krøyer, 1888, at the Gothenburg Museum of Art in Sweden. CC BY-SA 3.0.!_Konstn%C3%A4rsfest_p%C3%A5_Skagen_-_Peder_Severin_Kr%C3%B8yer.jpg 

As we anticipate the approaching new Gregorian year, those of us who are taxonomists are looking forward with renewed anticipation to the taxonomic challenges that certain kinds of words bring. Take “glass”, for example.

“Glass” is one of those words that contain an abundance of possible meanings. Ironically, this poses the potential for ambiguity. What makes this particular situation even more ironic is that this ambiguity clouds the very clarity that the word often symbolizes. Ambiguous words are tricky to work with in constructing and developing taxonomies and thesauri. Moreover, they make the writing of effective indexing rules challenging. Taking care in the crafting of those rules becomes all the more important, because of the need for disambiguation.

Another basic challenge is posed by words representing concepts that fit (sometimes neatly, and sometimes, not so neatly) into a variety of categories, and that can be subdivided (again, sometimes neatly, and sometimes, not so neatly) into a variety of sub-categories or sub-concepts. Glass is one of those words, too.

When we raise a glass, or see one as half full or half empty (your choice), that glass is a drinking vessel. A looking glass is (or at least used to be) a mirror. When we venture into the plural form, “glasses” are often understood to be eyeglasses. And search engines have their own opinions, it seems; when I decided to google “glass”, the search engine presented a very different interpretation in the top hit, reflecting a certain bias by a well-known search engine. Glass also has its artistic side, especially with stained glass. Getting down to basics, and to what many of the various meanings have in common, glass is a kind of material.

A glass – a drinking vessel

halfglassPhoto by Derek Jensen, 

Looking glass – a reflection on ourselves

tennielIllustration by John Tenniel, for Lewis Carroll’s Through the Looking-Glass,

Magnifying glass – the bigger to see with


Glasses/eyeglasses/spectacles (Spectacles? There’s some big ambiguity there.)the better to see with


Google Glass – for when what you normally see (and hear) isn’t enough by itself

googleglassPhoto by Dan Leveille,, CC BY-SA 3.0

Stained glass – For when stained means pretty

stainedglassPhoto by Kate Jewell, of a stained glass window designed by Ronald Whiting and constructed by Chapel Studios of Kings Lynn,, CC BY-SA 2.0

Glass – a kind of material (getting down to basics)


But what kind of material? Glass as a material presents several categorization challenges. For one thing, artists, art collectors, historians, engineers, and materials scientists deal with a large variety of glass materials. What most of us think of simply as glass, scientists classify as soda-lime glass. The Wikipedia category page on glass types lists forty-two separate articles, and it is by no means exhaustive.

And then there is the matter of state of matter. Is glass a liquid or a solid (presumably at room temperature)? The consensus has gone back and forth. For a long time (before the complications of solid-state physics), it was assumed to be a solid. Then some scientists speculated that it was a supercooled liquid. Evidence was supposedly provided by antique windowpanes, many of which were thicker at the bottom; this supposedly proved that the glass was flowing downward, ever so slowly. This persistent urban myth was solidly debunked only twenty-five years ago, in an article in the Journal of Chemical Education, although it still pops up in textbooks and science classes. Later, in 2000, the same journal published an article with the wonderful title “Glass Doesn’t Flow and Doesn’t Crystallize and It Isn’t a Liquid.”

Well, if glass doesn’t crystallize, it isn’t a solid, either, at least not in the traditional solid-state physics sense. So what is it?

Here’s one answer (of sorts) that I found on a University of California, Riverside webpage:

“There is no clear answer to the question “Is glass solid or liquid?” In terms of molecular dynamics and thermodynamics it is possible to justify various different views that it is a highly viscous liquid, an amorphous solid, or simply that glass is another state of matter that is neither liquid nor solid. The difference is semantic. In terms of its material properties we can do little better. There is no clear definition of the distinction between solids and highly viscous liquids. All such phases or states of matter are idealisations of real material properties. Nevertheless, from a more common sense point of view, glass should be considered a solid since it is rigid according to everyday experience.”

Well, that was kind of iffy. But then I found that a team of scientists recently tried to settle the question by performing a series of experiments and measurements on a chunk of amber (yes, amber turns out to be a kind of glass) that was twenty million years old.

amberPhoto of Dominican amber by Brocken Inaglory,, CC BY-SA 3.0

Their report, at, has been summarized in an io9 blog post by George Dvorsky as follows:

“The team performed a series of calorimetric and stress relaxation experiments on the Dominican amber. They measured its relaxation times (intermolecular rearrangements) at various temperatures, including above its fictive temperature. The team observed that the amber relaxation times did not diverge — meaning that it couldn’t possibly be a kind of fluid.”

Okay, so it’s a solid, I thought. But then, I came across a fairly recent news article with this comment:

“The solids catalog used to be pretty straightforward. Solid stuff was either a crystal or a glass. Crystals fill up space with atoms or molecules in specific, fairly rigid patterns. The positions of the atoms are fixed such that if you take any section of pure crystal and slide it up, down, in, out or sideways a given distance, it will fit perfectly in the new position. That’s translational symmetry. You can also spin the crystal through certain angles and the atoms also will line up; that’s rotational symmetry.

“Glasses have neither symmetry. They’re just a random arrangement of their components, as if you’d taken a liquid and suddenly frozen everything in place without giving the atoms a chance to get in order. Which, in fact, is how metallic glasses are made.”

Okay, so glass isn’t a liquid, and it isn’t a crystalline solid. So what is it, really?

The modern scientific consensus seems to be that glass is a special kind of non-crystalline solid. As explained in Wikipedia’s “Amorphous solid” article:

“In condensed matter physics and materials science, an amorphous (from the Greek a, without, morphé, shape, form) or non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal. In some older books, the term has been used synonymously with glass. Nowadays, “amorphous solid” is considered to be the overarching concept, and glass the more special case: A glass is an amorphous solid that exhibits a glass transition. Polymers are often amorphous. Other types of amorphous solids include gels, thin films, and nanostructured materials.”

So non-crystallinity is a defining characteristic of glass. Here we encounter another irony, and another potential source of ambiguity. Fine drinking glasses used to be made of “lead crystal”, but paradoxically, lead crystal is actually a kind of glass, and not one bit crystalline. Nor are its modern replacements that replace the lead oxide with a safer oxide, and that are known as crystal glass or lead-free crystal. So when you’re raising a fancy glass, be assured that it’s really a glass (unless it’s plastic or some such).

Let’s raise that glass to the wonderful richness and complexity of words! Cheers!

Barbara Gilles, Taxonomist
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