A very popular urban myth is that window glass is a
liquid. This apparently originated by
the recognition that old European cathedrals had windows with the glass being
thicker at the bottom than the top. The
actual cause of this is not attributable to gravity pulling the glass downward
in a slump but rather the early window manufacturing techniques followed by a
common practice of mounting window glass with the thicker side down.
Prior to modern manufacturing techniques, glass windows
could be made by spinning molten glass to stretch it out into a thin layer from
which the final product was cut. With
the outer edge of this raw window glass then being thicker due to this
manufacturing process was then mounted in the window on the bottom similar to any
statue or lamp where the heavy side is placed down.
Glass does not flow and so is not a liquid but it is not a
crystal either. In general, all metals,
minerals and other solids are crystalline in form. To understand what this means requires
looking at the atomic structure. A
crystal has all the atoms arrayed in three dimensions in a periodic
structure. This effectively is the very
definition of a crystal, periodic atomic repetitions. This can be multiple combinations of
different elements so long as the pattern repeats over and over again in all
three dimensions.
A liquid of any material has no such order in the location
of its atoms. The liquid effectively has
no correlation in the location of one molecule or atom to another just a few
molecular diameters away. This stands in
stark contrast to a crystal where if you know the location of one atom, you
know with high confidence the probability that any given atom will be located
in a fixed volume many tens of molecular diameters away. The key difference here is that the atoms in
a liquid have chaotic positions but in a crystal, the positions are strictly
ordered and predictable.
A glass is defined as a solid (a true solid) but having this
disordered interatomic bonding structure similar to a liquid. All glass is solid but not having crystal structure,
the locations of atoms are randomly distributed so knowing where one atom is
does not allow you to accurately predict the location of others only 3 or 4
bond lengths away (like a liquid).
A metal is typically a crystal in that the atomic structure is
aligned in a predictable periodic structure.
this applies also to alloys (mixtures of different metals such as bronze
and solder) although this can be in the form of only crystal domains such that
the materials nature is to have multiple separate crystals adhering to each other
to build up the full alloy material.
If a metal is in its liquid form and cooled extremely fast,
the chaotic liquid atomic structure can be frozen into the metal leaving it as
a glass rather than the more familiar crystalline form with which we are all so
familiar. Metal alloys are generally
easier to cool rapidly from a liquid into a glass as the mixtures of different
elements take longer to assemble into a crystalline lattice than a pure element
by itself. Basically you have to take
liquid metal and cool it so fast that it is not able to coalesce into a crystal
lattice of atoms but the atoms are frozen in the distribution of the liquid.
These bulk metallic glasses can be created by injection
molding of the molten alloy into a cooling chamber so that the product does not
need to be cut into shape afterwards.
These materials generally have greater strength and elastic limits than
simple metals but they are also more brittle and difficult to cut, grind or
drill. They retain their conductivity
and can be more conductive than their crystalline counterparts. Their properties are still being investigated
and many commercial applications have already been found in such things as
prosthetics, miniature radiofrequency antennae and electronics.
One elusive property with all metals is transparency
although there is at least one close second.
The mixture of gold and cesium in the molten state can result in a semi
transparent solid. This crystal is not actually a metal but it is considered a
salt as the two metals actually have to go through an oxidation, reduction
process leaving the gold in the -1 state and the cesium in the +1 state.
Still, any metal can in theory be cast into a glass and
every glass is a solid. A glass is simply
defined to be a solid with disordered atomic bonding.
