7/30/2019 Glass Doesnt Flow
1/3
No, It Doesn't Flow
Robert H . Brill, Research ScientistThe Corning Museum of
GlassJuly, 2000
Early one spring morning in 1946, Clarence Hoke was holding
forth in his chemistry class at WestSide High School in Newark, New
Jersey.
"Glass is actually a liqu id." the N orth Carolina na tive told
u s in his soft Southern tones. "You can tellthat from the stained
glass window s in old cathedrals in Europe. The glass is thicker on
the bottomthan it is on the top ."
Now , more than half a century later, that is the only thing I
can actually remem ber being taugh t inhigh 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 upin college textbooks on
general chemistry. And now, thanks to th e Internet, our Mu seum
has received
dozens of inquiries about whether or n ot this is true. Most
peop le seem to w ant to believe it.
***
It is easy to und erstand wh y the m yth p ersists. It does have
a certain ap peal. Glass and the glassystate are often described by
noting their similarities with liquid s. So good teachers, such as
Mr. Hokewas, like to quote the story about the w indow s. As is the
case with liquids, the atoms making u p aglass are not arranged in
any regu lar order-and th at is where the analogy arises. Liquids
flow becausethere are no strong forces holding th eir molecules
together. Their molecules can move freely past oneanother, so that
liquids can be pou red, splashed aroun d, and spilled. But, unlike
the molecules inconventional liquids, the atoms in glasses are all
held together tightly by strong chemical bonds. It isas if the
glass were one giant m olecule. This makes glasses rigid so th ey
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 m yth d oesn't
make sense.
Some years ago, I heard a remark attribu ted to Egon Orow an of
the Massachu setts Institute ofTechnology. Orowan had quipped that
there might, indeed, be some truth to the story about glassflowing.
Half of the p ieces in a wind ow a rc thicker at the bottom, he
said, bu t, he add ed qu ickly, theother half are thicker at th e
top. My ow n experience has been that for earlier w indow s
especially,there is sometimes a p ronounced variation in thickness
over a d istance of an inch or tw o onindividual
fragments. That squ ares with the experience of conservators and
curators wh o have han dledhu nd reds of panels. Although th e
individu al pieces of glass in a wind ow m ay be uneven in
thicknessand noticeably wavy, these effects result simply from th e
way the glasses were mad e. Presum ably,that would h ave been by
some precursor or variant of the crown or cylinder m ethods.
One also wond ers why this alleged thickening is confined to the
glass in cathedral wind ows. Whydon 't we find that Egyptian cored
vessels or Hellenistic and Roman bowls have sagged and
becomemisshapen after lying for centur ies in tom bs or in th e
grou nd ? Those glasses are 1,000-2,500 yearsolder than the
cathedral window s.
7/30/2019 Glass Doesnt Flow
2/3
Speaking of time, just h ow long shou ld it take
theoretically-for wind ows to thicken to any observableextent? Many
years ago, Dr. Chuck Kurkjian told me that an acquaintan ce of his
had estimated h owfast-actually, how slowly-glasses would flow. The
calculation sh owed th at if a p late of glass a metertall and a
centimeter thick was placed in an up right position at room temp
erature, the time requiredfor the glass to flow d own so as to
thicken 10 angstrom u nits at the bottom (a change the size of
onlya few atom s) would theoretically be abou t the sam e as the
age of the un iverse: close to ten billion
years. Similar calculations, made more recently, lead to similar
conclusions. But such comp utationsare perhap s only fanciful It is
questionable that the equ ations used to calculate rates of flow
are reallyapp licable to the situation at han d.
***
This brings u s to the subject of viscosity. The viscosity of a
liquid is a measure of its resistance toflow-the opposite of
fluidity, Viscosities are expressed in units called poises. At room
temperature,the viscosity of water, wh ich flows read ily, is abou
t 0.01 poise. Molasses has a v iscosity of about 500poises and
flows like molasses. A piece of once proud Brie, left out on the
table after all the gu estshave d eparted , may be foun d to have
flowed out of its rind into a rou nd ed m ass. In this sad state,
itsviscosity, as a gu ess, wou ld be abou t 500,000 poises.
In the world of viscosity, things can get rather sticky. At
elevated temp eratu res, the viscosities ofglasses can be measured,
and mu ch practical use is made of such measurem ents. Upon removal
froma furnace, ordinary glasses have a consistency that changes
grad ually from that of a thick hou se paintto that of pu tty, and
then to th at of saltwater taffy being p ulled on on e of those
machines you see on aboardw alk. To have a taffy-like viscosity,
the glass wou ld still have to be very hot an d wou ldprobably glow
w ith a du ll red color.
At somewhat cooler temp eratures, pieces of glass will still sag
slowly un der their own weight, and ifthey have sharp edges, those
will become rounded . So, too, will bubbles trapped in the glass
slowlyturn to spheres because of surface tension. All this happ ens
when the viscosity is on the order of50,000,000 poises, and the
glasses are near w hat we call their softening p oints.
Below those temperatures, glasses have pretty w ell set up , and
by the time they h ave cooled to roomtemp eratu re, they have, of
course, become rigid. Estimates of the viscosity of glasses at
roomtemperature ru n as h igh as 10 to the 20th p ower (1020), that
is to say, something like100,000,000,000,000, 000,000 poises.
Scientists and engineers may argue about the exact value of thatnu
mber, bu t it is doubtful that there is any real physical
significance to a viscosity as great as thatanyw ay. As for
cathedral wind ows, it is hard to believe that anyth ing that
viscous is going to flow atall.
It is w orth noting, too, that at room temperature the v
iscosity of metallic lead has been estimatedto be about 10 to
the11th pow er, (1011) poises , that is, perhaps a bill ion times
less v iscousor abill ion times more fluid, if you prefer than
glass. Presumably, then, the lead caming that holdsstained g lass
pieces in place should have f lowed a billion times more readily
than the g lass. While
lead caming often bends and buckles under the enormous
architectural stresses imposed on it, onenever hears that the lead
has flow ed like a liquid .
***
When all is said and done, the story about stained glass wind
ows flowing-just because glasses havecertain liquid-like
characteristics-is an ap pealing n otion, but in reality it just
isn't so.
Thinking back, I do recall another m emorable remark by Mr. H
oke. One day, our selfappointed classclown sat senselessly pou nd
ing a book on h is desk at the back of the room. "Great day in th
e
