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Finding the Viscosity of Pitch
ColdChrist writes "The University of Queensland has a page about a 72-year-old experiment on the fluidity of pitch. There's a webcam where you can try to become the first person ever to see a drop of the pitch fall; eight drops have fallen since 1930 and the ninth is now forming. The experiment 'demonstrates the fluidity and high viscosity of pitch, a derivative of tar once used for waterproofing boats. At room temperature pitch feels solid - even brittle - and can easily be shattered with a blow from a hammer', but it does flow, as the pictures demonstrate." I know this is going to bring up glass comparisons, so we'll head those off: glass is not a fluid.
Lord Kelvin (William Thomson) created an older pitch experiment: one which had a variety of objects lying on a tray of pitch that are slowly sinking in.
Its usually on show in either the Hunterian Museum or the Department of Physics and Astronomy at Glasgow University.
As I recall, this is considered the oldest continuously running scientific experiment, with the exception only of a wheat-breeding experiment in England? (I can't find references on that, just remember it from back in the mists of time)
BTW: it is more fun to watch paint dry - its faster...
Reminds me of a cornstarch and water experiment we used to do. Mix it together and you get a weird substance that exhibits properties of solids and liquids. Try it if you're bored...
Considering that the 8th drop fell in Nov. 2000 and the one before that dropped in 1988, we have only spent the first two years. I would expect that it would take at least 5 years before the next one drops. It will require more thant the students there to keep us entertained for that much time.
...who when reading the article - and looking at the picture of the smashed pitch - finds it hard to get images of a slow motion T-1000 out of my head?
besides if so much glass did actually shift from the top of an elaborate stain to the bottom, the picture would be long blurry, as they love making a big deal about how obvious the thickness difference is.
typically the colored pieces of stained glass windows are separated by a border of lead and tin i believe. this would prevent them from blurring. i once saw a presentation on this, and the lady giving it said people who make glass look at glass from ancient rome. evidently they provide good data points.
-- john
It's also strange to pur it onto a table - itpours out of the glass like treacle would, but then it breaks on contact with the table. Then, it liquifies again, very reminiscent of Terminator, when the shattered metal melts.
I have a page about telescope making that should give you some jumping off points, but I haven't yet got to the polishing stage of the mirror I'm working on.
One reason for using pitch is that you can press a mirror into it and get a very close fit. Another is that if the mirror is not perfectly spherical, the pitch will flex as the mirror moves across it. And finally, the polishing abrasive (ferrous oxide or cerium oxide) will set in the pitch and have a planing action rather than rolling around and chipping little flakes off as in ordinary grinding.
Pitch is nasty stuff to work with. It takes a lot of practice before a novice telescope maker can make a pitch lap they're happy with.
-- Could you use my software consulting serv
Robert H. Brill, Research Scientist
The Corning Museum of Glass
July, 2000
Early one spring morning in 1946, Clarence Hoke was holding forth in his chemistry class at West Side High School in Newark, New Jersey.
"Glass is actually a liquid." the North Carolina native told us in his soft Southern tones. "You can tell that from the stained glass windows in old cathedrals in Europe. The glass is thicker on the bottom than it is on the top."
Now, more than half a century later, that is the only thing I can actually remember being taught in high school chemistry. I didn't really believe it then, and I don't believe it now.
In the years that followed, I came across the same story every now and then. Most often it popped up in college textbooks on general chemistry. And now, thanks to the Internet, our Museum has received dozens of inquiries about whether or not this is true. Most people seem to want to believe it.
***
It is easy to understand why the myth persists. It does have a certain appeal. Glass and the glassy state are often described by noting their similarities with liquids. So good teachers, such as Mr. Hoke was, like to quote the story about the windows. As is the case with liquids, the atoms making up a glass are not arranged in any regular order-and that is where the analogy arises. Liquids flow because there are no strong forces holding their molecules together. Their molecules can move freely past one another, so that liquids can be poured, splashed around, and spilled. But, unlike the molecules in conventional liquids, the atoms in glasses are all held together tightly by strong chemical bonds. It is as if the glass were one giant molecule. This makes glasses rigid so they cannot flow at room temperatures. Thus, the analogy fails in the case of fluidity and flow.
***
There are at least four or five reasons why the myth doesn't make sense.
Some years ago, I heard a remark attributed to Egon Orowan of the Massachusetts Institute of Technology. Orowan had quipped that there might, indeed, be some truth to the story about glass flowing. Half of the pieces in a window arc thicker at the bottom, he said, but, he added quickly, the other half are thicker at the top. My own experience has been that for earlier windows especially, there is sometimes a pronounced variation in thickness over a distance of an inch or two on individual
fragments. That squares with the experience of conservators and curators who have handled hundreds of panels. Although the individual pieces of glass in a window may be uneven in thickness, and noticeably wavy, these effects result simply from the way the glasses were made. Presumably, that would have been by some precursor or variant of the crown or cylinder methods.
One also wonders why this alleged thickening is confined to the glass in cathedral windows. Why don't we find that Egyptian cored vessels or Hellenistic and Roman bowls have sagged and become misshapen after lying for centuries in tombs or in the ground? Those glasses are 1,000-2,500 years older than the cathedral windows.
Speaking of time, just how long should it take theoretically-for windows to thicken to any observable extent? Many years ago, Dr. Chuck Kurkjian told me that an acquaintance of his had estimated how fast-actually, how slowly-glasses would flow. The calculation showed that if a plate of glass a meter tall and a centimeter thick was placed in an upright position at room temperature, the time required for the glass to flow down so as to thicken 10 angstrom units at the bottom (a change the size of only a few atoms) would theoretically be about the same as the age of the universe: close to ten billion years. Similar calculations, made more recently, lead to similar conclusions. But such computations are perhaps only fanciful It is questionable that the equations used to calculate rates of flow are really applicable to the situation at hand.
***
This brings us to the subject of viscosity. The viscosity of a liquid is a measure of its resistance to flow-the opposite of fluidity, Viscosities are expressed in units called poises. At room temperature, the viscosity of water, which flows readily, is about 0.01 poise. Molasses has a viscosity of about 500 poises and flows like... molasses. A piece of once proud Brie, left out on the table after all the guests have departed, may be found to have flowed out of its rind into a rounded mass. In this sad state, its viscosity, as a guess, would be about 500,000 poises.
In the world of viscosity, things can get rather sticky. At elevated temperatures, the viscosities of glasses can be measured, and much practical use is made of such measurements. Upon removal from a furnace, ordinary glasses have a consistency that changes gradually from that of a thick house paint to that of putty, and then to that of saltwater taffy being pulled on one of those machines you see on a boardwalk. To have a taffy-like viscosity, the glass would still have to be very hot and would probably glow with a dull red color.
At somewhat cooler temperatures, pieces of glass will still sag slowly under their own weight, and if they have sharp edges, those will become rounded. So, too, will bubbles trapped in the glass slowly turn to spheres because of surface tension. All this happens when the viscosity is on the order of 50,000,000 poises, and the glasses are near what we call their softening points.
Below those temperatures, glasses have pretty well set up, and by the time they have cooled to room temperature, they have, of course, become rigid. Estimates of the viscosity of glasses at room temperature run as high as 10 to the 20th power Scientists and engineers may argue about the exact value of that number, but it is doubtful that there is any real physical significance to a viscosity as great as that anyway. As for cathedral windows, it is hard to believe that anything that viscous is going to flow at all.
It is worth noting, too, that at room temperature the viscosity of metallic lead has been estimated to be about 10 to the11th power, poises, that is, perhaps a billion times less viscous-or a billion times more fluid, if you prefer than glass. Presumably, then, the lead caming that holds stained glass pieces in place should have flowed a billion times more readily than the glass. While lead caming often bends and buckles under the enormous architectural stresses imposed on it, one never hears that the lead has flowed like a liquid.
***
When all is said and done, the story about stained glass windows flowing-just because glasses have certain liquid-like characteristics-is an appealing notion, but in reality it just isn't so.
Thinking back, I do recall another memorable remark by Mr. Hoke. One day, our self-appointed class clown sat senselessly pounding a book on his desk at the back of the room. "Great day in the mawnin', son! " shouted Hoke. "Stop slammin' your book on the desk. Use your head!" That was good advice-no matter how you read it.
Reprinted with permission from Dr. Robert Brill, brillrh@cmog.org
That is an example of a non-Newtonian fluid. Normal Newtonian fluids' viscosity is a function of temperature: the colder it gets, the thicker it gets. Non-Newtonian fluids' viscosity is a function of something else, in this case, force. That is, the more force you apply to it, the thicker it gets. If you want a really good and simple 'goop' recipe, try this:
-white glue, mixed with water, 50:50
-tablespoon of borax (from laundry section) in a few cups of water
-(optional) food coloring mixed with glue
pour the glue/water mix into the borax solution and it with thicken up. You'll pull out a slimy, goopy mass that is too watery to play nicely with but if you work it in your hands for a bit to get the excess water out, you'll have some fun. Bounce it around, slap it, tear it and it's more like a solid. Let it sit on your hand and it flows like a liquid. Plenty of fun.
In Soviet Russia, hot grits put YOU down THEIR pants.
I rememebr this, it used to sit at the front of the old Kelvin Lecture theater before the remodelled it, in fact it sat out in the open and it was pretty much gathering dust.
It was more like a little series of steps, pitch had been placed in a reservoir at one end and had flowed down the steps into the reservoir at the other end. In fact it had started overflowing at the bottom.
www.thinkgeek.com is reselling a goo they labeled "smart mass." The original product is Crazy Aaron's Thinking Putty. I'll leave it to google to provide links. Crazy Aaron has quite a few mpeg's of the product being shot from a potato gun.
It's similar to your cornstarch putty, though a bit more involved. It exhibits different properties on four different time scales. It will drip on its own weight slowly, will bounce firmly if dropped, will tear and shear if pulled too quickly, and will shatter if struck with a hammer.
Kinda like the force shields in the Dune movie and books. You can dent it easily with a fingertip if you move slowly, but it will repell your fist if you try to punch it.
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If not, and you find exampls where say the top of the glass is thicker pretty frequently, then the idea that glass flows isn't as compelling as the idea that only in modern times have we been able to mass produce industrial quality, evenly flat panes of glass.
But even if the panes are generally thicker on the bottom, what does that mean? Maybe it was easier / safer / more reliable to set the thick end of the glass at the bottom. Maybe it's easier to install that way. Maybe experience showed that glass set that way held up longer. Who knows?
Either way, "melting glass" is only one of several explanations, with others including "no difference" and "difference can be explained by work practices", and it isn't clear which if any explanation is the valid one.
DO NOT LEAVE IT IS NOT REAL