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I was reading the internets and I saw an image of of random facts. Yes, I know, its a humor site, but based on the image item #23 I have some questions.

Does glass really get stronger the longer its submerged? If so, why and whats the upper limit of its strength? Does depth matter?

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My gut feeling tells me that it is a nice myth, but I cannot give you a solid proof –  Bernhard Dec 8 '12 at 10:00
    
May be more appropriate for Skeptics –  Sklivvz Dec 8 '12 at 21:11
    
@Sklivvz would you like this migrated? Right now the answers seem theory-oriented, not reference-based. –  David Z Dec 9 '12 at 2:04
    
@DavidZaslavsky it should be up to the OP, really. The answers would be cleaned apart from the top one. –  Sklivvz Dec 9 '12 at 11:25
    
@Sklivvz ok then. I'll leave it alone unless Justin808 asks for it to be migrated. Justin808, if you want that to happen, just flag this for moderator attention and ask to have it migrated. –  David Z Dec 9 '12 at 20:38

4 Answers 4

up vote 15 down vote accepted

As far as I remember, there is some truth behind this statement: glass is inherently extremely strong, but it is fragile in practice because of microcracks on its surface. In water, glass dissolves to some extent. As a result, microcracks partly disappear, and glass becomes much stronger (at least for a while). I remember reading that A. F. Ioffe (I guess his name was sometimes written as Joffe) held a thin glass stick in water for a few days, and the stick became flexible. Maybe I'll try to find a reference later.

EDIT: the following article confirms that holding glass in water can make the glass stronger, but the authors used hot water:

Stockdale, G. F., Tooley, F. V. and Ying, C. W. Changes in the Tensile Strength of Glass Caused by Water Immersion Treatment. Journal of the American Ceramic Society, vol. 34, no. 4, p.116, 1951. doi: 10.1111/j.1151-2916.1951.tb11618.x

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This is why the process of optical fibre drawing must be tightly controlled - to exclude microcracks. Optical fibres are very flexible, and bare fibres dissolve in water. –  WetSavannaAnimal aka Rod Vance Dec 3 at 12:16
    
Would that apply even more to water that is slightly corrosive eg alkaline content? –  Dirk Bruere Dec 3 at 12:35

Please see this short paper as well: Influence of Water on Crack Growth in Glass

In the mid-1890s, Brodmann conducted experiments on the strength of glass and found that specimens whose surfaces were etched in HF solution were significantly stronger than specimens tested without etching. That result provided the first reported evidence that glass fails from surface defects.

As early as the 1920s, the strength of glass and other brittle materials was understood to be limited by the presence of small cracks. Under stress, the small cracks would grow into larger cracks until reaching a critical size, at which point the material would fracture catastrophically. It also was well known that humid environments reduced the strength more severely than did dry environments. Furthermore, a glass loaded at a stress below its ultimate strength did not show infinite life, but rather failed without warning after some finite time.

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I think what they mean is that glass is stronger whilst it is underwater, owing to the density of the water around it, as opposed to air under normal circumstances. Glass won't react with water at all. It is inert, impermeable, remains unchanged over time irrespective of chemical contact and does not react with substances including oxygen. That's why we use it to store stuff.

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The linked image reads it is only true for glass. I think your reasoning can hold for more materials? –  Bernhard Dec 8 '12 at 10:30
    
Glass is only mostly inert. Hydrofluoric acid will react with glass and cannot be stored in it, as an example. –  tpg2114 Dec 8 '12 at 10:55

Dissolution of microcracks sounds like a plausible mechanism. However, the "the longer its submerged" part is likely inaccurate: I would expect the strengthening to reverse after a time, as more soluble components leach out of the glass. It may become less brittle, though, and it of course depends on the type of glass (saying "glass" is kind of like saying "metal", there's actually a huge variety of different types).

It's not simple immersion in water, but a somewhat similar process is used by Corning to produce a porous glass with higher silica content then usual, which can be further processed to produce a type of glass with high resistance to high temperatures and thermal shock: http://en.wikipedia.org/wiki/Vycor

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