🚀 go-pugleaf

    Assuming that glass is an ideal (Newtonian) fluid, then the
shear strain rate dg/dt = (1/n) * T, where g = shear strain, t = time,
n = viscosity, and T = shear stress.

    For standard soda-lime-silica glass, n = 10**19 Pa-sec.  To estimate
T, I estimate the density of the glass at 2.5 g/cc.  For a 1 meter wide
pane of glass, that gives a shear stress of about 2.5 Pa (assuming that
gravitational acceleration is 10 m/sec/sec).

    The integration of dg/dt is simple and gives g = (2.5 x 10**-19) * t.
To bracket the amount of time required for a pane of window glass to
sag noticeably, let's pick g = 0.1 and g = 10**-8 as the high and low
shear strain ranges.  This gives a range of time 10**11 sec < t < 10**18 sec.
10**11 sec equals 1.2 x 10**6 days or 3200 years.  10**18 sec equals
3 x 10**10 years.

    The choice of g's is based upon a 1 cm thick pane of glass which
gives a thickness change of 1 mm (g = 0.1) and 1 Angstrom (g = 10**-8),
respectively.

    As for whether or not glass is a supercooled liquid, look up the
definitions.  Would a rapidly solidified, amorphous, melt-spun ribbon
also be called a supercooled liquid or would you say that it is a
solid with the non-crystalline structure of the liquid phase frozen-in
due to kinetics.

-Julius