There is a long running urban myth about glass; several, actually. Chief among these is the supposition that glass flows, supposedly substantiated by the observation that antique windowpanes are thicker at the bottom.
Back in 1996, Florin Neumann wrote a treatise about these myths, noting that, “the question is a perennial staple of science-oriented online discussions among non-scientists… which has wormed its way even in some high school textbooks, and has managed to fool even apparently intelligent and well-informed laymen.”The Internet is indeed rife with pseudo-scientific discussion about what glass is or so not, and how and why it ‘flows.’
Legendary glass artist and chemist Dominick Labino, who developed glass for the Johns-Manville corporation, had a simple retort to antique windowpane ‘flow;’ “if the windows found in early Colonial American homes were thicker at the bottom than the top because of “flow,” then the glass found in Egyptian Tombs should be a puddle.” The real answer to the antique glass thicker at the bottom query lies in the way it was made, the crown glass process; basically, these windows were blown glass, and it just wasn’t possible to make glass of a uniform thickness. In other words, glass does not flow, or at least, most glass doesn’t anyway.
Glass technology has advanced by leaps and bounds in the twenty first century – Photovoltaic window glass, touch screens, edgeless giant display screens, and much more are in the works. One of these innovations is Gorilla Glass, a product made by Corning, (analog products are made by several other manufacturers as well).Currently in its fourth generation, Gorilla Glass is extremely tough, light, and resistant to damage, with a Vickers Hardness of 620 to 700, it is primarily used as cover material for a myriad of electronics, from cell phones and tablets to display and TV screens. The exceptional strength of Gorilla Glass lies in an ion exchange process during manufacture – The alkali-aluminosilicate sheet glass is immersed in a molten alkaline potassium salt bath, which forces smaller sodium ions in the glass to be replaced by larger potassium ones from the salt bath. These larger ions build up a surface layer that is highly resistant to compressive stress. Gorilla glass has one other interesting attribute, because it turns out that Gorilla Glass does, in fact, flow.
As part of Corning’s ongoing research with the product, the company found that a one meter square of Gorilla Glass shrunk by some 10 micrometers over a period of eighteen months – Not exactly a huge variance, nonetheless, the study confirmed that the glass was somewhat susceptible to deformation at room temperature – it flowed. Now, a group of researchers working out of UCLA’s Physics of Amorphous and Inorganic Solids Laboratory, (PARISlab), have figured out why and how much. Led by Assistant Professor of Civil and Environmental Engineering Mathieu Bauchy, the researchers published their findings in the journal Physical Review Letters this week. What they discovered is that, in essence, the very process that makes the glass so strong also makes it move.
By employing molecular dynamic simulations, the researchers discovered that high performance glass like Gorilla can develop deformation that is proportional to the size of the piece of glass. In other words, our cell phone screens aren’t likely to change perceptibly, but a really big display screen at a sports park or concert venue likely will. The rate of flow is minuscule, about 10 microns per year for that one meter square of glass, yet the fact that the flow occurs at all may dictate some limits on just how big those huge TVs can be.
Professor Bauchy explained that the effect occurs because of, “the coexistence of competitive chemical elements of different sizes in the atomic network of the glass, which is known as the mixed-alkali effect.” Those potassium ions that get exchanged in the manufacturing process are partially to blame – the combination of different sized ions is likely what makes the glass susceptible to creep. Bauchy further explained that, “In a sense, any alkali ion that is not satisfied with the size of its local pocket will want to jump to another one that better fits.”
And in an not resting nod back to that antique glass, the effect that the researchers documented and explained is also the cause of another old riddle, Thermometer Effect. This phenomenon involved thermometers made from glass containing equal portions of sodium and potassium, that became increasingly inaccurate over time, due to deformation of the glass – And now we know why.