Glass formers: the main features of the amorphous state
Transparency, lightness and resistance are some
of the main features that justify the wide purpose of glass both in art and in manufacturing
of artisan products for daily life.
Generally speaking, if a system at the liquid
phase is cooled down below the melting temperature Tm, it would tend
to crystallize; under particular pressure and temperature condition, the system
does not crystallize, but it gradually increases its viscosity, without
acquiring the long-range structural order, typical of crystalline solids; it
undergoes the glass transition. Therefore,
glass is a state of matter, which shows some characteristics typical of solids
(e.g. mechanical rigidity) and some of liquids (e.g. structural disorder). According
to this definition, pottery, plastic, epossidic resin and polymeric materials
belong to this class.
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| Glass window in York Cathedral. |
Although archaeological remains of the most
ancient glass manufacturing are situated in Mesopotamia, wherein a piece of
blue glass dated back to the late sargonid period (XXIII century B.C.) was
discovered, the gothic glass windows are one of the most magnificent
manifestation of these materials.
Glass windows, indeed, have represented a
fundamental turning point in architecture, preferring the transmitted light,
meant as the light of God, to the reflected (frescoes) or reflected/refracted
(mosaics) light, going inside the church.
The majesty of this production has aroused a
great interest also in the scientific community: as it observed a greater glass
thickness in the lower part, Professor Hoke (West Side High School in Newark,
New Jersey) asserted that the glass is actually a liquid, that slowly flows due
to the gravity. Indeed, more recent studies showed that glass inhomogeneity can
be found at the top, at the bottom or to the side of the windows with the same
probability. It depends on the glass manufacturing: before the XIX century,
glass sheet were realised from a sphere that was gradually grown thin into a
plane by means of percussion, inevitably determining inhomogeneity. It is
likely that it was preferred to set the windows, orienting the thickest part on
the top in order to have a better load bearing.
To understand better the differences between
crystalline, glass and liquid state, it considers the cooling down procedure
and it monitors the behaviour of the system volume as a function of
temperature. Crystallization is characterized
by a sharp discontinuity of the volume at melting temperature Tm.
Conversely, if the glass transition takes place and the temperature decreases,
the absence of crystallization is followed by the transition into supercooledliquid: molecular dynamics slows down and at the glass transition temperature Tg
it is no more possible to reach thermodynamics equilibrium within experimental
timescale. Unlike Tm, Tg depends on the cooling down
speed and it is observed that if the process is faster, Tg
increases. After the transition takes place, the ability to flow, typical of
the liquid, decreases and the timescale characterizing the diffusive motion
significantly increases. This effect can be quantified in terms of viscosity
measured in poise. Water’s viscosity measures 0.01 poise, a glass at room
temperature 1020 poise, several orders of magnitude greater than the
lead. So, going back to “the legend of the gothic glass windows”, as the lead in the frame do not show signs of
failure due to structural stress, it is confirmed that the thickness inhomogeneity
depends, not on the nature of the glass state, but on the manufacturing.
Bibliography
-T. Scopigno, La transizione vetrosa, KOS - Periodico dell' Istituto Scientifico Universitario San Raffaele 232-233 (Gennaio-Febbraio 2005)
- P. Mazzoldi, Dalla leggenda dei materiali alla magia del vetro, Il nuovo saggiatore, vol26, no5-6, (2010)
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