Glass workers have been
shaping and coloring
glass to make art for millennia.
3,000 years ago, for
example, ancient Egyptians
made colored glass beads
that they used in jewelry.
A thousand years later,
Romans developed techniques
for making the first hollow
shapes and clear glasses.
And there have been
countless advances
in the science and engineering
of glasses since then.
Most notably, developments
in glass processing
over the past two centuries have
transformed glass from an art
into an indispensable
part of modern life.
Nowadays, glass is a common
engineering material,
used in everything from
bottles to microscope slides
to the screen on
your smartphone.
It's especially
useful because it's
transparent to visible light.
The only real disadvantage to
glass is that it is brittle.
And cracks can easily
propagate through it.
For example, when we
bend glass like this,
cracks, the top surface,
are pulled in tension.
The tensile stress
opens the crack
and pulls it through the
slide, resulting in fracture.
Metals, on the other hand,
are generally ductile,
meaning that when we
bend them, the atoms
can reshuffle to accommodate
the high stresses.
Consequently, cracks have a hard
time traveling through metals.
Metal's intrinsic ductility
and related toughness
means we can drop metal objects
and be confident they'll
survive the impact.
Glasses, on the other hand, will
normally shatter when dropped.
[MOAN]
[GLASS BREAKING]
You don't want your
smartphone screen shattering.
So materials engineers have
developed ingenious tricks
for making glass more
fracture resistant.
One technique for
toughening glass
is to trap compressive
stresses in its surface
by rapidly cooling the
glass from the molten state.
We can illustrate this principle
with the Prince Rupert's Drop,
where we take a gob of molten
glass and quench it in water.
The thick end of the
Prince Rupert's Drop
is incredibly
difficult to fracture.
You can even hit it with a
hammer, and it won't break.
But damage the fragile thin
end, and the whole drop explodes
into a fine glass dust.
The compressive surface
stresses toughen the glass
because any bending
load has to overcome
the quenched and compressive
stress before there
is a net tensile
stress at the surface.
Since it is difficult to
generate a net tensile surface
stress, we can do things like
hammer the drop without pulling
open any cracks.
A glass' density
at room temperature
is a function of how fast it is
cooled from the molten state.
Cooling a glass faster
results in a lower density.
We use this property of glasses
in a temperature profile that
develops when glass is
rapidly quenched to generate
the surface stresses in
a Prince Rupert's Drop.
We can illustrate the
temperature profile
and its effect on
the glass density
by representing the molten glass
gob as three separate plates.
Two of the plates represent
the surfaces of the drop.
And one represents the inside.
The two external plates
solidify very rapidly
when the drop hits the water.
The core, however,
takes longer to solidify
so that when the
drop is fully cooled,
the outer plates are less
dense than the inner plate.
This difference in density
means the outer plates
want to occupy a larger
volume than the inner plate.
Because the glass is
a continuous drop,
the end result is that the
middle plate, representing
the core of the drop,
gets pulled in tension,
while the two external plates,
representing the surface,
are compressed.
We can actually see
the stress profile
trapped in the glass using a
device called a Polaris Scope.
The square chunk of glass
at the top of the screen
is stress free as evidenced
by its uniform color.
The Prince Rupert's
Drop, on the other hand,
is extremely colorful.
And the closely spaced
lines along its axis
reveal the steep residual stress
gradient stored in the drop.
We can also see bubbles
inside the glass
that form due to shrinkage
during solidification.
The Prince Rupert's
Drop is a great example
of the impact the
materials processing can
have on its properties.
In particular, we
showed how you can
turn a normally brittle
glass into a tough,
fracture-resistant one.
Manipulating the
material so that it
has extraordinary properties
is all in a day's work
for a materials engineer.
