Re: GLASS Flows? (getting way OT!)

["Hank Marzina" <prasco at icubed_com>]

Please  Please   Somebody send the three pairs
Hank Marzina
Couldn't resist
----- Original Message -----
From: "Wright Huntley" <jwwiii at pacbell_net>
To: <killietalk at aka_org>
Sent: Friday, August 15, 2003 2:39 PM
Subject: Re: GLASS Flows? (getting way OT!)


>
>
> Hladky, John wrote:
> > I did a little research yesterday, AFTER (unfortunately) I sent the
e-mail
> > on glass flow.  Although it is a well known scientific fact that glass
does
> > flow if given enough time, it is apparently a WRONG fact.  Does anyone
out
> > there have a good recipe for crow?  Does it go well with killies?
> > John
>
> Hey John,
>
> We all make misteaks, once in a while. ;-)
>
> I posted a little fun response and Charles thought I was accusing him of
> originating your original glass-flow post. Guess I should apologize to
> him, and anyone who misunderstood my poorly-worded note. I knew that
> Charles was posting *no-flow* information, but re-reading my answer I can
> see where it wasn't obvious what I meant by it. Sorry.
>
> Another little goof. I said glass was stable to a part in ten to the
> eighth, which is nonsense, as stated. It was better than a part in ten to
> the eighth *per year*, according to the engineers (at -- I think it was --
> Ferranti of England), who did those studies. That is, it was more stable
> than we knew how to measure it, if left unstressed.
>
> We took advantage of the fact that glass is way softer than steel -- it
> has a modulus of elasticity only about 1/3 that of steel. By attaching
> both ends of a glass measuring scale to a steel mount that was at least
> three times as big in cross section as the glass scale, the steel
> dominated (by 9:1) so the glass just stretched and compressed to follow
> whatever the steel did. El cheapo way to make a glass scale with exactly
> the same thermal expansion coefficient as steel, for measuring bearings
> and other critical automotive parts.
>
> Unfortunately, in that nit-picky business, it was well known that most
> steel shinks at about 1ppm/year, due to metallic crystal phase changes. We
> had to nitride and grind the steel mount in a process similar to that used
> to make guage blocks, to assure the scale wasn't changing, too. Then we
> had to have the centroids of the glass and steel mount exactly aligned, so
> we didn't create the equivanent of a bi-metallic strip that would bend
> with temp. changes. Nutty business!
>
> Why so fussy? Our Holograf scales gave an analog sine/cosine signal pair
> that had a period of 10 microinches (or a quarter of a micron in the
> metric version). One customer, who made optical-disc-mastering equipment,
> divided that signal into 256 parts, for a digital least count down around
> 1 nanometer, which is pretty small. Such accuracy is why you get so many
> MB on your DVD (original tracks written accurately) and why they get so
> many transistors on an IC (good, accurate photomasks).
>
> Look John, just send be three pairs of any really rare killifish and I'll
> forget the whole thing. Otherwise, expect a long treatise on metallic
> crystaline phase change. :-)
>
> Wright
> >
> >
> >
> > -----Original Message-----
> > From: Wright Huntley [mailto:jwwiii at pacbell_net]
> > Sent: Thursday, August 14, 2003 9:45 PM
> > To: killietalk at aka_org
> > Subject: Re: GLASS Flows?
> >
> >
> > There is probably some limited cold flow, under enough stress, of almost
> > any amorphous material (i.e., glassy solid) but the mythology of windows
> > slumping with age is just that -- mythology.
> >
> > Most big store windows, and other large sheets, were made with float
> > glass, on float tables that were deliberately tilted. This gave a
thicker
> > end that was thought to be stronger as the load-bearing base.
[Ill-trained
> > grad students probably applied impecable post-hoc-ergo-propter-hoc logic
> > to look at old windows, as old builings were torn down, and start the
> > myth. It continues, obviously, to this day.]
> >
> > No big telescope has ever been refigured due to cold flow, AFAIK.
> >
> > About 40 years ago, extensive studies on the stability of glass for
> > optical scales and gratings was done, and the final conclusion was that
> > glass is inherently more stable than our ability to know the velocity of
> > light was at that time (about a part in ten to the eighth).
> >
> > Acrylic (i.e., "Plexiglas") does not usually warp unless uneven moisture
> > is involved. Look in your rubber book, Charles and see just how much it
> > expands when exposed to moisture. It stays very flat over a terrarium if
> > the humidity is the same inside and out. [Few other plastics have that
> > problem, BTW.]
> >
> > Wright
> >
> > Charles Harrison wrote:
> >
> >>>I believe the reason the covers are bending is that despite it's solid
> >>>appearance, plastics have some properties of liquids, i.e. they flow.
> >>>Even
> >>>glass exhibits this behavior.  It can be observed in the windows of
> >>>200 year
> >>>old houses.  They are slightly thicker at the bottom than at the top.
> >>>Also,
> >>>the big optical telescopes lose their focus after a few decades unless
> >>>something is done to compensate for the glass flow.  I suspect that if
we
> >>>leave our aquariums set up long enough, in a few 1000 years the tops
> >>>will be
> >>>so thin that they will break!
> >>>John
> >>
> >>
> >>It is strange from a physical chemistry point of view, considering the
> >>*non-solid* nature of glass, but the windows in many of the oldest
> >>churches in Florida and Cuba - put into place by the Spanish
> >>missionaries back in the 1500's have been measured to determine the
> >>amount of flow which can occur over a period of four centuries.
> >>
> >>I will have to go back into the Chemical & Eng News to find the issue,
> >>it has been a dozen years ago -  but, the results of the survey was that
> >>there was no measurable flow. Actually, this was a complete surprise to
> >>me and many scientists who have some training in glass blowing and
> >>constructing glass devices.
> >>
> >>I wonder if anyone has laid lengths of these various plastics over empty
> >>aquariums or shelves or whatever to determine: does the stuff bend
> >>without water involved???
> >>
> >>Charles H
> >>---------------
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> >>
> >
> >
>
> --
> Wright Huntley -- 760 872-3995 -- Rt. 001 Box K36, Bishop CA 93514
>
>
> Physics is the study of frictionless elephants whose masses may be
ignored.
>
>
>
>
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>


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