Can glass become more brittle over time?

June 3, 2009 10:03 PM

Does glass get more brittle as time goes on, especially if not used?

I've heard that glass, stemware, china, etc., will get more brittle if it is not used regularly. Is there any truth to this?

posted by biwa-shu to food & drink (11 comments total)

There are really three questions here, since what is theoretically true for glass over long periods of time isn't necessarily helpful to what is apparently your real concern about glassware.

So: Yes, theoretically glass gets more brittle over time because subcritical cracks slowly expand while under stress over long periods while exposed to air, moisture, etc. But this is really mostly relevant when talking about BIG glass. Like glass structures or sculptures or whatever, not a glass for drinking out of. So the practical question about your glassware is that it's not something to worry about.

The bit about having to use it regularly is not true. Using glassware is going to stress it far more than if it sits around. That's even ignoring the fact that the odds of you dropping something and breaking it are astonishingly higher than any stress the glass is subject to through gravity while just sitting in cabinet. It's simply inarguable that the more you use glassware, the more likely it is to get broken. If you want glassware to last, you don't use it much. Of course the less you use it the more pointless having it becomes so it's a balance.
posted by Justinian at 10:25 PM on June 3 [2 favorites]

I don't think that glass does, but the manufacturing process has changed over time.

I think that generally the manufacturing process is now more efficient and material science so much more advanced that it is easy to either:
* cheap enough to use more of the material to make an item, thus making it more robust, or
* make a more robust item from the same or less amount of material, again for the same or less cost.
posted by dantodd at 10:26 PM on June 3

(I want to stress again that if your concern is about things like glassware or household glass goods, theoretical stuff like subcritical crack growth under stress is not practically relevant, I just included it because you don't actually limit the question to those sorts of objects)
posted by Justinian at 10:31 PM on June 3

No.

Have you asked the people who claim this regarding their reasoning behind this belief?

Use, on the otherhand, would increase the handling and stresses that would deteriorate glass, stemware, china, etc.

Glass is an amorphic solid. Urban legends of glass "flowing towards gravity over hundreds of years" has cleary been debunked and is not a contraversial issue; glass made at that time was uneven and windowpanes were set with the thickest side down.

China is porcelain; it's not going to change shape over time. In my experience, the more use, the more the gold leaf and colouration fades away.
posted by porpoise at 12:19 AM on June 4 [1 favorite]

glass is a type of fluid. in some scenarios, such as old windows, the glass droops and will become thinner at the top. but that probably isnt related to your question
posted by edtut at 1:36 AM on June 4

glass is a type of fluid. in some scenarios, such as old windows, the glass droops and will become thinner at the top.

This is widely believed but it isn't true. The most likely explanation for why old windows are often slightly thicker at the bottom has to do with the way glass windows used to be made. (spun out in a disc and then cut to the window shape. Since the edge of the disc was thicker, one edge of the window was slightly thicker and it just makes sense to put the thicker edge at the bottom).

Windows manufactured now are not made this way and will always be of uniform thickness.
posted by Justinian at 2:03 AM on June 4 [3 favorites]

glass is a type of fluid. in some scenarios, such as old windows, the glass droops and will become thinner at the top.

This is widely believed but it isn't true.

Do you know, I've believed this for more than 20 years, since my Physics teacher told us in class. Gah! Wonder what else he got wrong...
posted by kumonoi at 2:13 AM on June 4

Theoretically yes, because glass (which is in itself an extremely wide definition) is an amorphous solid which under normal conditions will tend to return to its cristalline form, more or less a 3-dimensional lattice of pyramids joined at the corners, with oxygen atoms being the corners and a silicon atom at the center. This is a spontaneous process.

By almost* all practical means the velocity of this process is virtually zero, so you shouldn't be worried about that.

To prevent (at least partially) this and the effect described by Justinian in the first part of his answer, most glassware is annealed after cooling, and can be* annealed again at any point in its life. And, I second this is mostly noticeable in very large objects.

*an example that comes to mind might be that of extremely precise optics.
posted by _dario at 2:20 AM on June 4

Do you know, I've believed this for more than 20 years, since my Physics teacher told us in class. Gah! Wonder what else he got wrong...

Glass is a fluid, yes, with a viscosity at room temperature quite close to infinite. So, yes, it is possible that over very, very, (very,)ⁿ... very long periods of time, glass will droop, drip, ooze and percolate just like a liquid. Not in 100 or 500 or 1000 years, definitely.

In the pitch drop experiment, a certain quantity of solid pitch is put in a funnel and allowed to freely ooze down. Since 1927 eight drops have fallen and the ninth is expected to fall in a (relatively) short time.

And pitch is extremely less viscous than glass. Proof: the glass funnel is still there :)
posted by _dario at 2:33 AM on June 4 [2 favorites]

To me, the term "brittle" has quite a specific meaning: something that when subjected to increasing strain, breaks before it permanently (plastically) deforms. You can't take any glass object and put a permanent bend or dent in it at room temperature, because it shatters first. You can do this if you heat it up, of course. In that sense glass objects are already completely brittle, so they can't get any more so over time.

If instead the claim is that glass gets weaker over time, meaning less strain is needed to shatter it, I think tempered glass provides good evidence against this theory. As it also provides good evidence against the idea that glass "flows" over time.

Tempered glass has a huge amount of locked-in stress and strain, which is why it shatters into tiny little bits if it gets cracked or broken. If glass got weaker over time, we would have an epidemic of spontaneously exploding tempered glass panes, and we don't. Similarly these locked-in strains could be relieved by just the tiniest amount of flow under the enormous stresses involved. Yet we do not see glass losing its temper over time, either.
posted by FishBike at 5:46 AM on June 4

glass is a type of fluid. in some scenarios, such as old windows, the glass droops and will become thinner at the top.

Here's a link to a post I made on the blue that talks about the fallacy of the beliefs about glass windows (as well as other info about the properties of glass): The simplest example of the truly complex.
posted by amyms at 1:11 PM on June 4