DEVITRIFICATION: Part I, The structure of glass

DEVITRIFICATION: Part I, The structure of glass

Post by Michael Olse » Fri, 13 Dec 1996 04:00:00



Comments to a private email (which may have been posted as a follow-up)
from Mike Firth:

Mike,

Thanks for your comments -- all of them -- about my writings.  I want to
apologize up front for what is often considered a character flaw on my
part -- I have a frequent tendency to be pedantic, to direct my
conversations 'down' to others rather than to accept their positions and
discuss them.  I tend to monologue rather than dialogue.  Email is good in
that dialogs can be *constructed*, and yet at the same time they are weak
because they *are* constructed.

I get the impression that you have an interest in philosophy, and will
understand the following:  Much of my 'pedantic' approach derives from an
appreciation for what I call the first 'holy Trinity'.  I have for many
years now had an interest in comparative mythology (yeah, I know -- what
the hell does this have to do with glass, but...), and the concept of
'Trinity' appears in nearly all of them.  I've theorized that the very
first holy Trinity likely occured when the first of our early ancestors
capable of abstract thought peered down into a still pool of water to take
a drink.  As always, he saw his reflection in the water, but this time it
occured to him that the face peering back at him was *his own*.  I have a
distinct mental picture of this early homonid taking his hand and
alternately reaching down to the water to touch his reflection and then up
to touch his face, and simultaneously *understanding* that the water was
neither himself nor his reflection, yet the water was somehow involved.
The water and his body are objects, the reflection is a precept, and the
connection between the two -- the 'understanding' -- is a concept.
Object, precept, and concept: that was the first holy Trinity.  My
teaching method emphasizes imparting an understanding of objects and
concepts -- I can have no _direct_ effect upon a student's perceptions.
Yet, buy giving an understanding of the transcendental 'reality' which
lies behind the 'merely physical', the student becomes an 'adept' -- how
they will 'see' the universe is changed forever!

Quote:
>> A glass is by definition a liquid at all temperatures.
> (snip)
>> When glass devitrifies, it doesn't necessarily revert to its former,
>> solid, crystalline constituents.
> These two statements in your posting seem to contradict themselves.

I'm not going to further support the first statement -- I'm not convinced
that I have to.  To clarify the second statement, what I'm describing is
the process of the conversions of solid, crystalline starting materials
into a liquid vitreous glass, and then devitrifying the glass to form a
solid, crystalline material, although not necessarily the same as the
starting materials.

Quote:
> Further, if glass can be modelled as a long chain polymer, it is by
> definition, not a liquid, which has the capability of moving freely in
> any direction.

Liquids have structure.  This is a bizarre concept when first considered,
but it is in fact easy to demonstrate.  The phase diagram was originally
devised (by the British physicist Sir James Rutherford) in order to
illustrate the American, Josiah Willard Gibbs' discovery of the
differences in enthalpy (a measure of internal order) between the solid,
liquid and gasseous phase of any particular substance (as described in his
1874 paper, "Equilibrium of heterogeneous substances").  It is the
understanding which underlies the progressive loss of order (structure)
which we observe in the differences (the 'behavior' or 'nature') between a
crystalline solid, a liquid and a gas.  With more modern analytical
methods of examining the *surfaces* of liquids we reveal something
completely counterintuitive -- there is a 'boundary' layer at the
liquid-gas interface (what may be 'seen' as a liquid-air interface) where
the degree of order of the liquid actually increases to such a degree that
it can be considered as essentially a *solid*!  This order is imposed upon
the boundary layer of the liquid by the chemical binding constraints
consequent to the geometry of the surface -- in other words, the molecules
at the surface cannot reach 'up' to bind, as there's nothing 'up there' to
bind to, thus they either reach 'down' or else bind to nothing (which will
cause the surface to be what is considered 'active' or 'reactive').

The structure of liquids can be illustrated by considering the constituent
molecules to be tiny magnets -- polar liquids (like water) have a
permanent intrinsic polarity, while nonpolar liquids (like benzene) will
have a transiently induced, weak polarity.  If you were to examine a
beaker full of tiny magnets you will find that all of the 'north' ends
will be stuck to the 'south' ends of adjacent magnets.  Seen from a
distance a beaker full of magnets will seem to be a jumbled, random mess,
but examined closely they are in fact constrained by an imposed order --
it is structured!

Quote:
> to explain that the solid appearing glass is not like other solids,
> especially not clear crystalline solids, but has a random structure more
> like a liquid may be fine, but when it leads to questions about glass
> sagging in windows over a thousand years but just getting thicker at the
> bottom, things have become silly.

I take it that you might consider glass to be a liquid *if* this is the
only explanation for ancient glass panes to be thicker at their bottoms
than at their tops, otherwise it is only 'common sense' that glass is a
solid?

First point:  'Common sense' has time and again led scientists into
dead-ends, for 'common sense' is that intellectual faculty which informs
us, with an *unusually high* degree of certainty, that the earth is
*flat*.

Second point:  Ancient glass is thicker at the bottom than at the top
because that was how the glass was made!!  I know what the offhand
technique was for making flat glass, but I will leave it to some historian
of ancient glassmaking technique to start a thread on this.  Hint:  Check
out (I'm pretty sure this is the title) Denis Diederot's "Illustrated
'cyclopedia of the arts and technology".  Dover has reprinted it in an
English translation, at a very affordable price.

Quote:
> Glass is a glass, a special form of material.  Not all substances behave
> the same and glass behaves as glass.  

That was a non sequitur, and can only be countered with another:  No two
pieces of glass are, or will behave the same.

Quote:
> Some solids cleave on sharp planes, some do not, plastics (thermosetting
> or thermoplastic) do not, glass does not.

Plastics are, at all temperatures, liquids.  That is why it is called
'plastic'.

Michael Olsen