Physical
changes of Glass at the Annealing Point
What
happens at the annealing point and what is its relevance to
compatibility? There are two main changes that occur – physical and
chemical. They both affect the temperature of the annealing point,
but in different ways. These notes are an attempt to understand
these changes and how they affect compatibility.
The
first requirement is to understand what the annealing point is.
First it is a range of temperature during which the glass transforms
from a liquid to a solid. It has a definition:
The
annealing point is the point at which the material reaches the glass
transition temperature. It occurs in a temperature region at a point
where stresses can be relieved in a very short time. It is defined
mathematically by a specific viscosity. In
simple terms, this is the temperature below which viscosity prevents
any further configurational changes.
Any
contraction beyond the transition temperature range is due only to
the lower kinetic energy of the groupings of the tetrahedra
molecules. Thus, the compatibility of the glasses is determined at
the annealing range as a combination of expansion/contraction and
viscosity at the annealing range of temperatures rather than at the
lower CoE which is more suited to crystalline solids. The
transition temperature of a given “glass composition” depends
both on its constituents and upon the rate of cooling.
The
physical changes of glass during the transition/transformation range
of temperatures are various:
- Viscosity has a very large increase with temperature reduction, but without any discontinuity. Viscosity has an enormous effect on the activity of molecules in glass. As the glass cools below its transition temperature it causes the progressive immobility of the molecules.
- The expansion rate (CoE) shows a relatively sudden change around the annealing temperature. Below the annealing point, the glass expansion and contraction behaves much like the CoE at the lower, measured temperatures. This means viscosity may be the most important element in creating a stable fusing compatible glass.
- The amount of heat required to increase the glass temperature rises quickly rather than the previous regular heating rate needed to achieve unit changes.
- The shear modulus changes rapidly, making the glass much more brittle below the annealing point.
- The rate of heating or cooling can affect the exact temperature at which the glass transition point occurs.
The
annealing phase (glass transition) is a dynamic process where time
and temperature are to some extent exchangeable. This allows
annealing to occur at the lower part of the range of the transition
phase, but the glass then needs a slower cool from there. From the
(higher) annealing point temperature - as defined by viscosity - the
cool can be a little more rapid than at the lower temperature range
of the transition phase. The anneal at the lower part of the
transition saves annealing and cooling time for thick slabs, but for
thinner pieces (less than 9mm), soaking at the annealing point and
cooling from there is the simpler process.
Slow
cooling results in a lower transition range because the tetrahedra
forms of the molecules have more time to rearrange (to the degree
that this is possible). This slower cooling results in tighter
packing of tetrahedra as the mass reaches its transition range. When
the glass reaches room temperature, its volume will be smaller when
cooled slowly than glass melt which has been cooled rapidly. Hence,
slower cooling from the melt results in a denser glass.
Reference:
http://glassproperties.com
With low temperature annealing, does the glass become more dense than its original state and does density have any effect on the ease or difficulty of cutting?
ReplyDeleteYes according to the research an anneal at the lower part of the annealing range with a long soak does make for a denser glass. However, I do not know, nor does the research I have found, indicate whether the glass is more or less difficult to score and break. My experience leads me to believe that there is no significant effect on cuttability of denser glass. The fractures will run in the same way between the tetrahedra molecules in more and in less dense states. But that is my view. It is unsupported either way by scientific evidence.
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