Last Updated: Aug 09, 2017     Views: 254

From Dr. Robert Brill, Research Scientist Emeritus:

The main property that sets glass aside from other materials -- and allows it to be formed by some means that cannot be used for other materials -- is its viscosity-temperature curve. 

It is not easy to visualize from just a word description (please see the attached article from the Journal of Glass Studies).  You will notice from the curve shown somewhere toward the end of that article that as the temperature of a glass is raised, the viscosity slowly decreases.  This is a marked difference from most crystalline materials.  With the other materials, the viscosity stays constant as the temperature is increased and then, when the material reaches its melting point, the viscosity drops abruptly and the solid collapses into a liquid state.  This is because in a crystalline material all the chemical bonds holding the material together tightly in a crystal lattice are identical and equally strong.  Therefore, they all manage to hold together until the melting point is reached, and the input of a little more heat energy causes them to break simultaneously.  This explains why crystalline materials have sharp melting points.

On the other hand (referring to the diagrams in the same text), notice that in a glass the chemical bonds are not identical, because the atoms have become fixed in space before they have had a chance to arrange themselves into the regular crystal lattice that the laws of thermodynamics say they should form.  Consequently, some of the chemical bonds are distorted from what they should be and are weaker than their neighbors.  As a glass is heated, the bonds that are most distorted (the weakest ones) break first.  Then the next weakest bonds break, and so on.  The result is that the glass softens gradually from a rigid state, and its viscosity gradually decreases as its temperature increases.  Hence, glasses do not have any sharp melting point such as crystals have.  In fact, in the strict sense of the word, glasses do not have any melting point at all.

In the long run, this lies at the heart of glass blowing and explains why one cannot (in the same sense of the word) blow bronze or salt or ice.

What differentiates glass from other substances that might have similar melting patterns, such as waxes, is, I think, the strong secondary forces that hold together the clusters of "silicate molecular groups," their possible physical entanglements, and the strong primary chemical bonds that hold the individual atoms to one another.  I have always thought that things like waxes could not withstand the pressure differences imposed to produce the expansion of blowing (as in glassblowing) and would burst before they expand.  It seems to me that internal cohesive forces might be a controlling factor, but I am sure a physicist could better explain the forces involved.

Anticipating what might be a follow-up question, the basic difference between pottery and porcelain, as opposed to glass, is that in the former cases the starting materials are never heated to a high enough temperature to take them into a completely melted (liquid) state.  If a clay is heated to a high enough temperature that it does melt to a liquid state and then cooled relatively quickly, it will, indeed, form a glass.

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