[taregg (OP)]

Why is broken glass often sharper than a knife?

[Soul Surfer]

Glass is technically a liquid although it is very viscous so it does not have any defined crystal structure or faces to crack along when it breaks it exhibits what is known as a conchoidal (shell shaped or curved) fracture.  see http://en.wikipedia.org/wiki/Conchoidal_fracture   this curved fracture leads to facets that are initially sharp right down to the atomic level although wear can rapidly blunt them a bit.

[syhprum]

Although broken glass can produce really sharp cutting edges ceramics are even worse.

[Bored chemist]

"Glass is technically a liquid"
No it's not.

[CliffordK]

A crystalline structure that has fracture planes can essentially be sharpened down to a single atom/molecule edge.  For example, flint.

It is difficult to get steel to sharpen to such an extent with traditional sharpening methods.

I'm seeing questions on whether glass fractures in a similar pattern.  I suppose it would also depend on whether it is tempered or not.  Certainly a method of fracturing to sharpen an item can create very fine edges on the shards.

[Soul Surfer]

BC  I always understood that things like glass and pitch were in effect supercooled viscous "liquids" because they had no defined melting point but slowly softened, had no crystal structure right down to the atomic level,  exhibited conchoidal fracture and flowed (albeit very slowly) when put under stress.   What is now considered to be the structure of glassy substances with these properies clearly they are not normally or microcrystalline.  I suppose you could call them amorphous solids but then there is the problem of defining precisely what is a solid and what is a liquid. for example is pitch a solid or a liquid  I believe there is a famous experiment where it is running through a funnel and dripping once every ten years or so.

[yor_on]

Very interesting SoulSurfer. Where can I find that experiment?

Because it reminds me of the discussion I had with a coin on a window pane, 'wandering' through it. If that experiment exist then the coin just might be able to 'wander through'? Which would be quite fun to prove, as everyone seems to find it impossible. And I also read about it somewhere, but without being able to find the source for it.

[CliffordK]

I know that steel, for example, will soften to some extent, and become more malleable as it is heated.  Then, at some point when welding, one gets distinct puddles of liquid steel.  I'm not sure if glass is similar.

Not all materials become crystalline.

I've read that some old glass panes appeared to have runs, or a taper, but it is now believed that they are manufacturing defects, and not a sign of aging.

It would be an interesting experiment to try glass in a funnel.  Build a cone shaped piece of glass, and put it in a funnel, then add, say 100 tons of force on the top.  Then wait 10 years at room temperature.  Would it slowly run out?

Anyway, it is probably best described as an amorphous solid.

Perhaps one of the differences between glass and other solids is that when it fractures, there aren't discrete fracture planes.  So, when it fractures, you can get very acute angles.  Whereas, salt (NaCl), for example, will fracture into cubes.  These non discrete fracture planes would mean that one can get things like needle shapes, something that would be difficult with NaCl.

As mentioned above, fractures likely extend all the way to the atomic/molecular level so you could get extremely sharp, but fragile edges. 

The glass is relatively non reactive, so it is not quickly blunted by oxidation as other blades might be.

[Geezer]

I thought I might as well pitch in.

http://en.wikipedia.org/wiki/Pitch_(resin)

[syhprum]

Surely the pitch drop experiment is much influenced by temperature, if it had been set up in Baghdad or Anchorage the results would have been much different

[Geezer]

Surely the pitch drop experiment is much influenced by temperature, if it had been set up in Baghdad or Anchorage the results would have been much different

I think so. Same thing with glass - it's just a matter of temperature.

[Bored chemist]

When successfully someone does the pitch drop experiment with glass all that will become relevant.

In the meantime, ancient obsidian blades are still sharp. If they were liquid they would be dull after all these years. Also, old telescopes wouldn't work because the mirrors would have "run".
Up to the glass transition temperature, glass is a solid.
http://en.wikipedia.org/wiki/Glass_transition

"It would be an interesting experiment to try glass in a funnel.  Build a cone shaped piece of glass, and put it in a funnel, then add, say 100 tons of force on the top.  Then wait 10 years at room temperature.  Would it slowly run out?"
No. Glass doesn't flow like that.
On the other hand, some metals do.
http://en.wikipedia.org/wiki/Extrusion

[CliffordK]

Of course, one can also do extrusion forming of glass, although I believe it is done hot.

[Bored chemist]

True, but if it's that hot it doesn't make sharp blades so it hasn't got a lot to do with the original question.

[Soul Surfer]

Thanks for the glass transition ref BC I hadn't looked things up in that area for a very long time most of my chemistry was done when I was at school but reading degree standard text books. Since then it has been mostly physics, astronomy, electronics, communications and information theory at the limits with chemistry only at the popular science level.  I have ben catching up on my chemistry theory recently

[yor_on]

Extrusion was also interesting to read. Ah well, but some day.
I will find that coin, and then..

maybe not

[Geezer]

Extrusion was also interesting to read. Ah well, but some day.
I will find that coin, and then..

You do realize that copper wire was invented by two Scotsmen who found a copper penny at the same time?

[yor_on]

Ach Sooo.

So, that's where its gone?