The Glass

 

 The glass is described as a non-crystaline amorfous solide. That means thant, in spite of their appearence, it is not a crystal, because the atoms forming the glass are located without any order. That's due to the glass fabrication process. A mixture of some components (composed of (SiO₂), (Na₂O) (Na₂CO₃)) is melted in an oven whit a temperature about 1500 °C. Whit a fast cooling, crystalizatin is avoided, and the molecules get frozen in a non-ordererd location, in a solid phase. Beause of this, there are some others definitions of the glass, as an overcooling liquid, whit a very high viscosity at ambient temperature. This definition has made the common theory of the ''flowing glass'' because of the change of thikness in very old windows, but is it true?

Glass fabrication:

The molecular structure of a common glass is generally isotrope, whitout any order at large range. Nevertheless, some repeated structures can be seen at local order, as shown in the picture, but not ordered enough to create strong forces between them. In a very short range, the strong bondings make a high dregree of  order, as shown below, whit the atoms of O and Si, in a 2-3 armstrong radius.

So, if there is no order, and is fluid at a high temperature, is it a solid or a very dense fluid?

The answer is: glass is a vitreus solid, whit mixes properties of both states, solid and liquid. It mechanical behaviour at lower temperature is solid-like, turning fluid when the melting point is reached and the atoms vibrate so fast that the bounds between them are not strong enough to keep them together. 

                             Viscosity:    μ=Ae^(Q/RT)

This ''vitreus solid'' state depends on the temperature, as shown in the viscosity formule. The viscosity depends strongly on the inverse of the temperature. High temeperatures make the viscosity very low, in an expenential order, making a fluid phase. Low temperatures (lower than 1000ºC) increase exponentially the viscosity, making a solid satate.

If the temperature descend slowly enough, the silicuim can cristalyze in an ordered structure (Quartz):

 

So, do the antique glass really flow? The answer is no. The viscosity does not depends on the time, but on the temperaturee, and if the temperature is much lower than the melting point (500-100ºC) the glass has a solid behaviour. 

Several antique glasses are thicker at the bottom due the fabrication process, in which the glass is stirred by puting it vertically when is still melted, and the glass accumulate on the bottom as gravity does his work. As soon as the glass gets solid, the flowing stops, in a 10-15 secods time range.

 An example of  sheet glass making:

References:

• https://en.wikipedia.org/wiki/Glass
• https://es.wikipedia.org/wiki/Vidrio
• https://en.wikipedia.org/wiki/Structure_of_liquids_and_glasses
• http://html.rincondelvago.com/proceso-de-fabricacion-del-vidrio.html
• https://en.wikipedia.org/wiki/Supercooling
• https://www.youtube.com/watch?v=m2Au_lE6PY4
• https://www.youtube.com/watch?v=tDyeiePort0

Author: Andrés Pinar Solé