The course from which this information is taken is based on float glass. This is a soda lime glass just as fusing
glass is. The general observations –
although not the temperatures – can be applied to fusing glasses. This is a paraphrase of the course. It
relates these observations to kilnforming.
The course is IMI-NFG Course on Processing in Glass, by Mathieu Hubert,
PhD. 2015
 |
Viscosity vs. Temperature for a borosilicate glass
Graph credit: Schott |
Viscosity Influence on Annealing
Viscosity increases with reduction in temperature. So high viscosity (low temps) cannot release
stress; low viscosity (high temperature) cannot maintain shape – it will deform. The range of viscosity is small. The viscosity must not be so high that the
stress cannot be relieved, nor must it be so low that the glass is unable to
retain its shape. (p.6). This indicates
there is an inverse relationship between temperature and viscosity. This is something we experience each time we
fire.
The mathematical definition for strain point - high
viscosity - is 1014.5 Poise.
And the annealing point as 1013.4 Poise, where if the glass
is all the same temperature, the stress can be relieved in about 15
minutes. (p.7-8)
As kilnformers we talk of the annealing range in terms of
temperature, because that is what we can measure. The annealing occurs within a
small range of viscosity. This has a relation to temperature that is not the
same for all glass compositions.
The definition of the annealing as the range of viscosity at
which annealing can occur is important.
First, the viscosity value remains the same over many types and styles
of glass. The temperature required to
achieve that viscosity varies, leading to different annealing temperatures for
different glass.
Second, there is a range of viscosity - and therefore
temperature - during which annealing can occur.
The annealing point is 1013.4 Poise, at which viscosity the
stresses in glass can most quickly be relieved (generally within 15 minutes for 3mm
glass). However, the stress can be
relieved at greater viscosities up to almost the strain point - 1014.5 Poise.
(p.8). At higher temperatures, the
glass becomes more flexible and cannot relieve stress. At lower temperatures (beyond a certain
point) it becomes so stiff that stress cannot be relieved. Again, those temperatures are determined by the viscosity of the glass.
Annealing Soaks
Annealing can take place at different points within the
range. Bullseye chose some years ago to
recommend annealing at a higher viscosity, i.e., a lower temperature. This has also been applied by Wissmach in
their documentation although initially the published annealing point was almost
30°C higher.
The closer to the strain point that annealing is conducted,
the longer it will take to relieve the stress.
Annealing at the strain point is possible, but it is impractical. Apparently, it would take at least 15 hours
for a 6mm thick piece (p.8).
However, the trade off in annealing a few degrees above the
strain point – requiring longer annealing soaks – is reducing the amount of
time required by the annealing cool, especially for thicker or more difficult
items.
A further advantage to annealing at lower temperatures and
slower rates is that it results in a denser glass – one with lower volume (p.3).
Arguably, a denser glass is a stronger one.
Annealing Cool
After annealing, the glass should be cooled slowly and
uniformly to avoid formation of internal stresses due to temperature
differentials within the glass. Stresses
that are unrelieved above the strain point are permanent. Stresses induced during cooling below the
strain point are temporary, unless they are too great. To avoid permanent stress, the cooling should
be slow between anneal soak and strain point (p.9). Although glass can be cooled more quickly
below the strain point, care must be taken that the temperature differentials
within the glass are not so great as to cause breaks due to uneven contraction.
Annealing cool factors for flat pieces are about three times
that for cylinders and five times that for spheres (p.26). Or the other way
around – spheres can be annealed in one fifth the time, and cylinders in one
third of the time as flat glass of the same volume. This indicates how much more difficult it is
to anneal in kilnforming than in glass blowing.
The industrial cooling rate for float glass of 4mm is 6
times the rate for 10mm although only 2.5 times the difference in thickness
(p.27). This indicates that the thicker the glass, the slower the rate of
cooling should be. But also, that there is not a linear
correlation between cooling rate and thickness.
Glass with no stress has a uniform refractive index. Stresses produce differences in the
refractive index which are shown up by the use of polarised light filters.
Source: IMI-NFG Course on Processing in Glass, by Mathieu
Hubert, PhD. 2015 (available online www.lehigh.edu/imi).
https://www.lehigh.edu/imi/teched/GlassProcess/Lectures/Lecture09_Hubert_Annealing%20and%20Tempering.pdf
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