Scientific Notes on Annealing

 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 10^14.5 Poise.   And the annealing point as 10^13.4 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 10^13.4 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 - 10^14.5 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

Soaks Below the Softening Point

There are frequent suggestions that holds in the rise of temperature for glass are required.  Various justifications are given.  A few notes before getting to the explanation of why they are uncessary.

A note is required about the softening point sometimes called the upper strain point. There is a reasonable amount of discussion about the lower strain point.  So much that it is often simply referred to as the strain point.    Below the lower strain point, the glass becomes so stiff and brittle that no further annealing can occur.  Thermal shock can happen though, so the cooling needs to be controlled.

There also is an upper point at which the behaviour of the glass is different.  Above this temperature, no annealing can occur either, because the glass has become plastic and the molecules randomly arranged.  It is only just pliable, of course, but its molecules are no longer strongly bound to one another.  This is the temperature at which much of slumping is done.

It is disputed whether such a point exists.  Still, in practical terms it is where the glass becomes so plastic that it cannot be temperature shocked.  The temperature of this “point” is approximately 45°C above the annealing point, rather than the temperature equalisation soak. 

Note that the temperature at which Bullseye recommends that the annealing soak should occur is a temperature equalisation point, which is about 33°C below the glass transition temperature - the point at which glass can be most quickly annealed.  The average glass transition point for Bullseye is 516°C.  Most other fusing glasses use the glass transition (Tg) point as the annealing temperature for the soak.  They or you could employ the Bullseye technique on thicker slabs of the glass by setting the temperature equalisation point 33°C below the annealing point, and soaking for the same kinds of time used in the Bullseye chart for annealing thick slabs.  In fact, this is what Wissmach has recently done with its W90 and W96 fusing glass ranges.  They now recommend 482C (900F) as the anneal soak temperature.

Now to the point of the post.

The soaks that are often put into schedules on the rise in temperature are justified as allowing the glass to equalise in temperature.  Glass in its brittle phase is an excellent insulator.  This means that heat does not travel quickly through the glass.  Consequently glass behaves best with steady and even rises in temperature (and correspondingly on the reduction in temperature).  Rapid rates risk breaking the glass on the temperature rise, no matter how many or how long the holds are.  

This means a slower rate of advance will accomplish the heating of the glass in the same amount of time, and in a safer manner, than rapid rises with short soaks/dwells/holds.  The slower rate of temperature increase allows the glass to absorb and distribute the heat more evenly.  This slow heating is most obviously required in tack fusing where there are different thicknesses of glass.  

This means that it is possible for thin areas of glass to heat up much more quickly than glass covered by different thicknesses of glass.  It also applies to strongly contrasting colours such as black and white, because they absorb the heat differently - black more quickly than white.

There are, of course, circumstances where soaks at intervals are required – usually because of mould characteristics, in slumping, and in pate de verre.

Sometimes people add a soak at the annealing temperature on the way up in their schedules.  This is unnecessary.  If the glass has survived up to this point without breaking, it is highly unlikely it will break with a further increase in the rate of advance unless it is very fast.  The temperature after all, is above the strain point meaning the glass is no longer in the brittle phase.

Many people add a soak at around 540°C (ca. 1000°F) into their schedule on the increase in temperature, before their rapid rate of advance to the top temperature.  The choice of this temperature relates to the lower strain point.  This also is unnecessary, except possibly for very thick pieces. By this time the glass has reached its plastic stage and if it hasn’t broken by then, it won’t with a rapid rise in temperature either.

Further information is available in the ebook Low Temperature Kiln Forming.

Soaks at various temperatures during the advance to the upper strain points of glass are not necessary.  What is necessary is a knowledge of when the glass becomes plastic in its behaviour, and an understanding of how soaks can overcome characteristics of moulds, or how to achieve specific results and appearances of the glass.