Edge properties of broken glass?

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AmadeusM

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Why does broken glass always result in keen edges? When I look at it, I doubt the edge is less than 60 degrees, and it still cuts like crazy.

What is the cause for this interesting property of glass?

Thanks.
 
Yes. glass-like things like flint, glass and the like are hard and inflexible so the edge can only be a fraction of a micron thick. Metals are malleable and flex so that it is virtually impossible to hone an edge that thin. Some plastic surgeons often use obsidian scalpels as they are much sharper than metal scalpels and so leave a finer cut that heals more quickly with less scarring... there are also small scalpels with ruby or sapphire blades.
 
I checked this awhile back, I broke a bunch of glass, yes you could cut things with it but it was *MUCH* duller than a sharp piece of steel, not even in the same class. Not knapping to be clear, just breaking it. For reference a sharp edge can cut light cotton under 200 grams of tension in about 0.25 cm, with much more force (pounds) it took several times as much edge length with a piece of beer bottle.

-Cliff
 
I read a report once on obsedian (volcanic glass) and how it compared to a knife edge. This was a piece that was flint knapped into a cutting edge. Supposedly it cut so cleaning on tissue that there was no bruising of the cells in the cut. That is a pretty clean cut I would say. The problem was it was not precise in the cut it made so it isn't like surgery could be performed with it except maybe in a pinch, but it did skin out a hide and cut the tissue it was tested on with relative ease.

STR
 
I read a report once on obsedian (volcanic glass) and how it compared to a knife edge. This was a piece that was flint knapped into a cutting edge. Supposedly it cut so cleaning on tissue that there was no bruising of the cells in the cut. That is a pretty clean cut I would say. The problem was it was not precise in the cut it made so it isn't like surgery could be performed with it except maybe in a pinch
Click to expand...

Umm... surgery is performed with it on a regular basis! Eye surgery mostly.

Cliff, I could have sworn that broken glass was sharper then that, weird.
 
There may be differences among types, but I don't recall ever handling broken glass which was as sharp as a knife. There is a limit to how thin you can get an edge and not have it fold over immediately anyway, a crystal a few atoms across isn't going to cut anything as it won't be strong enough to overcome the action/reaction force during the cutting, so the "one molecule thick" edge isn't a reality, aside from the fact that it isn't distinct molecules but atoms in a crystal formation.

-Cliff
 
Make sure of what I said. Surgery may be performed with obsedian just fine but not flint knapped primitive ones like an arrow head with a curved every which way edge and jagged uneven surfaces. They ain't cuttin' on me with one of em like that!:D

STR
 
Also look up "glass knives" These are microtome blades made by breaking glass so as to form an ultra-sharp edge that can be used to make slices of samples as little as 1 micron thick (0.15 microns for transmission electron microscopy) Diamond, tungsten carbide and sapphire/ruby are also used to make ultra-sharp microtome blades.
 
I use to run a shoe repair shop and then took a video course on custom bootmaking. The folks being taped in the videos where from South America and they used broken glass for trimming the edges of soles and ruffing up leather for glueing. I tried it and it worked slick as can be. A lot cheaper than sandpaper all was needed to handle the broken peices was a peice of tape.
 
I've been an amateur flint knapper for years, making arrowheads mostly. I took several flint knapping courses with Errett Callahan, who is considered one of the best flint knappers in the world (he's won many awards, mostly in the 1990's, see his work at: http://www.errettcallahan.com/awards.htm

Errett also used to make some very high quality obsidian (volcanic glass) scalpels. When I had to have minor surgery performed on my arm, I had the doctor make one incision using one of Errett’s obsidian scalpels and another incision with a steel scalpel. I wanted to observe the differences between the blades. For one thing, I opted not to be injected with pain anesthetic, because I wanted to see if the blades felt different, and I found unequivocally that the answer is no…they both sliced though my arm with an equal level of searing pain. Secondly, I wanted to see if the incisions would heal differently and leave different kinds of scars, but unfortunately, the doctor didn't sew the wounds tightly closed, so both scars ended up looking the same. However, I have seen photos of Don Crabtree, a famous flint knapper from Idaho, after he had a lung removed using an obsidian scalpel, and the long scars on his chest were amazingly small and minimal--just thin hairline red lines (compare that with scar tissue from a patient that has undergone open heart surgery with a steel scalpel, where the scar tissue is a prominent bulging line).

I’ve learned some really interesting things about obsidian (or any kind of glass) as a cutting edge:

- Because the edge of an obsidian cutting blade is FRACTURED, not ground like a steel blade, it can be much sharper than a steel blade. According to the American Medical Association (AMA News 2 Nov 1984), obsidian is up to “500 times sharper than surgical steel” and may fracture to the last molecule. Therefore, obsidian scalpels often leave less scar tissue, and are frequently used for cosmetic surgery. However, they are more fragile, so you have to be careful not to torque the blade (which can break it), or to knick it against hard surfaces like bone. There is a great article about the sharpness of obsidian cutting blades on pg. 81 of the book “Primitive Technology: A book of Earth Skills” edited by David Wescott (http://www.primitive.org/sptbook.htm). It also includes an amazing photo taken at 10,000x magnification of an obsidian blade versus a steel scalpel. The obsidian blade edge is an absolutely smooth straight line, whereas the surgical blade edge looks like a jagged, uneven line.
- Because obsidian is a form of glass, it actually flows like a liquid very, very slowly (if you ever look at windows in an old house, the glass is thicker at the bottom than the top because it has actually flowed down because of gravity over time). Therefore, an obsidian or glass blade is sharpest immediately after it has been made. It dulls over time. Errett Callahan recommended that the scalpels he makes be used within about a year of being manufactured. (As an interesting side note, one of the guys I took a flint knapping course with worked in a scanning electron lab and had to dissect small biological samples to micron thicknesses using flint knapped blades he made. He found that if he didn’t use the blade within a couple hours of making it, it would be too dull to dissect his samples)
- Unlike an obsidian arrowhead that has a cutting edge made up of many small fractures that look a little like a serrated edge, an obsidian scalpel like the ones that Errett makes have only one long, continuous blade made by a single fracture. They are very hard to make, requiring full body weight to break a long, thin blade off of a “core” of obsidian, and only a few people know how (Errett is one of them, and to this day, I don’t think he’s shared the knowledge with anyone else).
- Some of the best quality (in terms of purity and color variations) in the world comes from Glass Buttes, Oregon (according to the Bureau of Land Management, I think you can legally collect something like 2000 pounds per day). Errett’s scalpels and most of his knives are made from obsidian from this site (which covers at least several square miles, and is criss-crossed with meandering dirt roads).
- I don’t know if these are any good, but here’s some modern obsidian scalpels available on the internet: http://www.finescience.com/fst/ScalpelsKnives/10110-01.html
- Other related articles:
http://www.umich.edu/~urecord/9798/Sep10_97/surgery.htm
http://www.christianforums.com/t58401
http://www.pubmedcentral.gov/articlerender.fcgi?artid=1273673#reference-sec
http://www.worldandi.com/public/1991/february/cl4.cfm

Hope you find this interesting…although I would never EDC a stone age knife, it sure is fascinating to see how technology that was developed and used tens of thousands of years ago in some way surpasses modern technology.

BTW, here are the photos of the obsidian versus steel blade from the “Primitive Technology” book, as well as a photo of the obsidian scalpel and surgical steel scalpel used to cut open my arm:
http://public.fotki.com/CliffVolpe/miscellaneous/surgical_obsidian/dsc_6587c.html
dsc_6587c.html

http://public.fotki.com/CliffVolpe/miscellaneous/surgical_obsidian/dsc_6588c.html
dsc_6588c.html
 
I have seen obsidian scalpels for sale in a medical tools catalog. They were lashed to little wooden handles, just like a native American might do. I think the handles were epoxy coated so they were readily able to be sterilized for surgery (as the previous post said . . . ). The benefit of obsidian being that it cuts through individual cells, and does not tear them apart. Basically what was said earlier about cutting without bruising. Low scarring being important for eyeball surgery.

The obsidian scalpel used to cut up xdmountain= AMAZING

My understanding about glass slowly flowing over time is that, if it occurs, it occurs of geologic time scales, thousands of years, if at all. The old houses having windows with thick bottoms comes from the glass panes being cut from blown glass, which ends up with a much thicker portion at the end it was attached to the blowpipe. When installing the window the thicker portion was generally installed at the bottom, more stability, or simple aesthetic notion that thick should be at the bottom. (I do believe. . . . )

As for the apparent dulling of glass edges with time. Perhaps this is an effect of surface tension, if there is such a property in glass. Maybe the edge surface is trying to get back to a low surface energy or something. So maybe there is some truth to “flowing” glass in this particular condition/scale? IIRC the edge on broken glass is on the order of 10s of atoms across, while steel is an order or two of magnitude higher, i.e. 100s-1000s.

Material scientist please?
 
All materials relax with time once deformed, you could check the effect of gravity trivially by storing the edge up vs down and seeing which one went blunt faster, I doubt this is actually the relevant factor here. I also doubt the scalpels are cutting through cells vs the steel ones tearing, or the edges are molecules in thickness (it isn't a molecule anyway they are crystals).

The above pictures are interesting but both Lee and Verhoeven have shown steel edges which are much more highly refined than the steel edges shown. Verhoeven also noted the importance of shots from both sides as well as directly into the edge. I would be curious about what the 500 refers to and would wonder if anyone ever actually measured the sharpness of the edge directly which isn't difficult to do.

-Cliff
 
Cliff Stamp said:
I checked this awhile back, I broke a bunch of glass, yes you could cut things with it but it was *MUCH* duller than a sharp piece of steel, not even in the same class. Not knapping to be clear, just breaking it. For reference a sharp edge can cut light cotton under 200 grams of tension in about 0.25 cm, with much more force (pounds) it took several times as much edge length with a piece of beer bottle.

-Cliff
Click to expand...

Sorry, but I have to disagree here. I've been cut by glass that's been much sharper than any knives I've had.

We all know that Spyderco's are very sharp out of the box. I've had cuts from glass light bulbs and even glass windows out of a house. Both of those types of broken glass were much sharper than any Spyderco I've ever had.
 
Light bulbs are an interesting idea, I'll note their sharpness next time one burns out. Of course if you get jagged fractures which end in points you can get punctured easily, or small shards will splinted easily, but in terms of long pieces acting as a knife I have not found just breaking glass makes a sharp knife. Most people that do make glass knives have to knapp them, similar to rock to get a decent level of cutting ability.

-Cliff
 
True, a knapped edge like those found on typical arrowheads and stone axes etc would have many jagged points but an obsidian microblade is not knapped that way, and a hunk of busted window would have pretty much like a knife sharpened with a 45 degree bevel on each side... which wouldn't be especially good at cutting to any depth.

Obsidain microblades are a different matter. A long (up to 20cm) rounded core is made and then the blades are snapped off the core by applying a great deal of pressure along the top edge of the core with a wooden or bone tool. With the right angle and amount of pressure, a blade will pop off the side of the core. These long thin pieces with a triangular cross-section having a slightly convex hypotenuse and sort of resemble a double-edged 2"-6" razor blade with a chisel or pukko edge. http://www.fs.fed.us/r10/tongass/forest_facts/resources/heritage/technologies.html The edges are fragile and easily damaged because of their thin edge bevel but a fresh blade has a straight, smooth, super-sharp edge with a fairly small bevel angle.

http://www.umich.edu/~urecord/9798/Sep10_97/surgery.htm

Other microcrystaline ceramic materials, like pressure stabilized zirconia, can be formed into blades with an edge thickness of only 1/10 micron (probably not on your Kyociara ceramic kitchen knives though) and are used in industrial slitting applications where metal blades would not last.
 
Having worked with cut glass before, I can attest to its sharpness. Score it & snap it, and if you try to handle it with bare hands prior to sanding it, you will mave many slices on your hands. (Don't ask me how I know)

Now, if you hold the glass firmly but not too tightly, and IF it does not slip in your hands at all, then you should be ok. But if it moves a bit, watch out for those telltale blood trails. I have cut my hands several times and not even known it until somone saw the blood. Much akin to a frsh razor blade.

Of course, we are talking about the 90° edge, and of course, that is not practical to use as a knife blade. But it IS sharp if mishandled.


Ok, that is my first post, great site thus far, I have enjoyed the reading immensely, and hope to learn a lot more in my stay here over the coming months/years.

Cheers!
 
yuzuha said:
... a hunk of busted window would have pretty much like a knife sharpened with a 45 degree bevel on each side... which wouldn't be especially good at cutting to any depth.
Click to expand...

Yes but we were discussing sharpness not cutting ability, even very obtuse edge angles on steel can be very sharp, try some standard tests of sharpness on a piece of broken window, it will score very low. As for handing glass bare handed, yes you can get cut, now imagine handling a bunch of knives from Fikes, R.J. Martin, Dozier, etc. by the edge barehanded and having them move at all in the hand.

-Cliff
 
xdmountain said:
Because obsidian is a form of glass, it actually flows like a liquid very, very slowly (if you ever look at windows in an old house, the glass is thicker at the bottom than the top because it has actually flowed down because of gravity over time).
Click to expand...

"A new study debunks the persistent belief that stained glass windows in medieval cathedrals are thicker at the bottom because the glass flows slowly downward like a very viscous liquid.

Edgar Dutra Zanotto of the Federal University of Sao Carlos in Brazil calculated the time needed for viscous flow to change the thickness of different types of glass by a noticeable amount. Cathedral glass would require a period "well beyond the age of the universe," he says."

http://www.robinsonglass.com/analysis.htm

maximus otter
 
I believe that glass is an amorphous solid, some people refer to this as a supercooled liquid, which is why many people think it can flow appreciably in human time frames. This means that it has no regular crystalline structure or array. In other words, the smallest unit of the material could be a single molecule(and a relativly small molecule at that), and not some larger unit cell which requires dozens or hundreds of molecules to assemble. This is all of course a gross simplification and I hardly remember the details myself.

Intermolecular forces would probably not allow a single molecule of SiO2 to stick way about the others. However, since glass does not have a normal crystalline structure, edge thickness is not dependent on such crystalline properties such as unit cells etc. . . ad nauseum. So the edge would probably not be a single molecule thick, but could certainly approach this (10s-100s of molecules). Several orders of magnitude thinner then a steel knife edge.


I would be curious to see the cutting ability of a 45 degree steel edge compared to a 45 degree glass edge. Specialized breakage (knapping) may be necessary to realize its edge potential, but it is still friggin sharp in most cases.
 
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