2-58. 5 Glass Facts or Myths

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After our last episode with ghosts and glass, I got thinking about the nature of glass itself and how interesting it is. But I also came across some “facts” about glass that aren’t actually true. Here are five common facts or myths about glass:

1) You can shatter a glass with your voice.

Maybe you can, but I sure can’t. And while this trope is seen in countless movies and tv shows, it has only happened a few times in reality. But it has happened, as the Mythbusters proved in a 2005 episode. Not only do you need a very strong voice, you also need to find the resonant frequency of the glass, which varies from glass to glass. And you also need a glass that has some defect in it, as a perfect glass with just go on vibrating without shattering.

2) Glass flows very slowly, and that’s why glass in old windows is thicker at the bottom.

This one is a myth. While it’s true that glass in very old windows is thicker at the bottom, it’s not because the glass flowed that way. It’s because that’s the way it was installed. Before plate glass was invented, that is, the process by which glass could be made perfectly flat, there was always some variation in thickness. As such, glaziers would look at each pane and find the thickest end, and put that on the bottom.

3) Glass is a liquid.
Well, not in any way that makes sense in the real world. Glass does not flow at room temperature. That makes it a solid, but it’s a specific one. The vast majority of solids have a crystalline structure, and while we may think of “crystal” glass, that’s a mixing of terms. Glass itself has no crystalline structure, making it an amorphous solid. This property allows it to soften and melt gradually rather than all at once like most crystals. If you’ve ever soldered anything, you can see the difference in how metals melt vs. how glass or sugar candy or plastic or some other amorphous solid melts.

4) Lead Crystal Glass contains lead.
Yes, this is true. Most glass is silicon dioxide, also known as quartz or most simply as sand. But it order to impart specific properties like clarity or color, other substances are added. Lead is added to some glass to make it more workable and to increase “brightness.” This brightness is a result of the change of refractive index in the glass, and it can be best demonstrated when this glass is formed into “crystal” prisms which create bright rainbows. But there is no crystal here: this type of glass was used in Italy to create crystal shapes.

And yes, there is a chance of ingesting some lead if you drink from a lead crystal glass. It’s a small risk so long as the glass was cleaned properly, but it’s enough of a risk that lead glass is reserved mostly for artwork these days.

5) Glass is stronger than concrete.

This is true! And there’s no reason that glass can’t be used structurally. But “strength” isn’t all that matters. Cost of materials, weight, workability and repairability are all important concerns, and concrete wins out in all of these categories.

So there are five things about glass worth considering. Imagine all the things around you right now which wouldn’t be possible without glass. Those include the screen on your device, the fiber optic cables that got the signal to you, and in my case, the desk I’m typing this script on. Now if we could only get it not to break so easily…

Removing the putty on an old piece of glazing. (Photo by Andy Dingley.)

Removing the putty on an old piece of glazing. (Photo by Andy Dingley.)

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