Tuesday, January 19, 2016

Didymium: The Unreal Element in Glassworking Glasses

Glasswork is an ancient technology—though it's perhaps more apt to describe it as an art—that's been subject to refining and improvement over countless generations. While you might not expect new developments to emerge very often in such an old field, technology has a way of surprising us with its potential for synergy.

What you'll see below is a material called didymium at work. Occurring naturally as a combination of the elements Praseodymium and Neodymium (which you might be familiar with thanks to ultra-strong rare earth magnets), didymium was originally hypothesized to be one element—a mistake that gave Mendeleev no small amount of trouble when trying to construct his periodic table. Despite this, didymium has proven uniquely useful in a number of applications, not the least of which is glassworking.


As the glass tube is blasted with heat from the welding torch, atoms of sodium in the surface of the glass are shaken loose at a molecular scale, flying off in the withering gout of flame. As that happens, the electrons on those sodium atoms are kicked up into an excited state, momentarily occupying higher-energy orbitals before cooling off and dropping back down to their ground energy levels. As the electrons make that downward transition, they give off a very certain amount of energy—2.1 electronvolts (about 320 zeptojoules) each—in the form of a yellow-orange photon (roughly 589 nanometers in wavelength). This gives rise to the lovely but difficult-to-see-through flare of orange light that obscures the glass when the torch is turned on it.

As it happens, 320 zeptojoules is just the right amount of energy to excite certain electrons swirling around the nuclei of neodymium and praseodymium atoms. Thanks to that fact, the photons coming from the sodium "tail" are almost completely absorbed by the glasses, which have didymium distributed throughout. Unlike welder's glasses, which are absorptive across much of the visible spectrum and therefore obscure the wearer's vision, didymium glasses are almost perfectly transparent at wavelengths other than 590 nanometers, rendering only the sodium flare invisible and allowing the glassworker to see his subject clearly.

Unfortunately, the same property which makes didymium so useful in this context makes it relatively unhelpful in others. While it's appealing to imagine special lenses for firefighters that allow them to see clearly through an inferno, the narrowness of didymium's absorption spectrum means that it would only block out a tiny fraction of the yellow light from a typical fire. However, future advances in materials science could allow the engineering of special substances that do offer such selective vision—potentially promising materials as miraculously useful in other fields as didymium is for glasswork.

For more news on recent progress in this ancient field, check out MIT's glass lab, where they've produced a 3D printer that works with molten glass!

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