Glass 101: Glass Processing Temperatures

 

Glass 101: Glass Processing Temperatures

Posted Rebecca Molt on Aug 14, 2019

Glass is an amorphous solid with no long-range order. This lack of order is what differentiates a glass from a crystalline solid. For example, when silicon dioxide is cooled slowly through the crystallization temperature, it is allowed to form crystals, giving the solid a geometric structure throughout the material. When silicon dioxide is heated and then rapidly cooled, it’s ordered crystalline structure is unable to reform and it becomes an amorphous solid (glass). [1] 

Glass goes through different transitions during melting. The glass transition temperature, softening point, and crystallization temperature are all part of the glass forming process. Careful maneuvering through these steps is critical to the formation of a stress-free glass product.

Glass Transition Temperature

The glass transition temperature (Tg) characterizes a range of temperatures where an amorphous material transitions from a hard brittle state to a viscous state relative to increasing temperature.[2] The solid begins to exhibit viscoelastic properties above Tg. When disordered molecules are below Tg, they have less energy, and the molecules aren’t able to move into new positions when stress is applied. When above Tg, the molecules have more kinetic energy, allowing them to move in order to alleviate applied stresses.[3] The annealing temperature is selected based on the glass transition temperature, allowing any stress to be released before completely cooling the glass.

Littleton Softening Point

The Littleton softening point (Ts) of glass is the temperature at which the glass moves under its own weight. As a glass is heated, the glass flows more easily. The resistance to flow is known as viscosity. At the softening point, the glass has a viscosity of 107.6 poise.[4] This point is often used to define the working range of the glass. Once the glass has reached the softening point, it is malleable without melting.

Crystallization Temperature

The crystallization temperature (Tx) characterizes the onset of crystallization. Crystallization is the process of forming a solid. The molecules become highly organized into a geometric structure known as a crystal. This occurs in two steps. The first step is the nucleation or “seed” formation. Nucleation can be influenced by the initiation of a secondary phase formation within the matrix or the introduction of an outside substance, such as particles from the crucible. The second step is crystal growth, which is the growth around the original nucleation sites in layers.[5] Crystallizing brings the melt down to a lower energy state. If a melt crystallizes it will not become a glass since glass is a disordered solid. Crystallization is avoided by rapidly quenching through the glass transition region. 

Coefficient of Thermal Expansion

The coefficient of thermal expansion (CTE) describes a material’s change in shape, area, and volume in response to temperature change.[6] Heat is a type of kinetic energy. When a material is heated, its kinetic energy increases which causes the molecules to vibrate at a higher frequency. The molecules then take up more space than usual. The reverse is true when cooling a material. When cooled, the molecules have less kinetic energy, and they contract, taking up less space....

Mo-Sci Corporation’s extensive glass knowledge and research experience make it a perfect candidate for custom development needs. Mo-Sci has partnered with companies across a wide variety of industries, creating custom glass solutions for unique applications. Contact us for more information on how we can help with your next glass product.[8]

References

1. NDT Resource Center, http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/solidstate.htm, Center for NDE, Iowa State University.
2. ISO 11357-2: Plastics – Differential scanning calorimetry – Part 2: Determination of glass transition temperature (1999).
3. The Glass Transition. https://pslc.ws/macrog/tg.htm. Accessed: 21 June 2019.
4. Technical Glasses: Physical and Technical Properties | SCHOTT Technical Brochure. https://www.us.schott.com/d/tubing/ffed51fb-ea4f-47d3-972e-5a2c20f123f5/1.0/schott-brochure-technical-glasses_us.pdf. Accessed: 21 June 2019.
5. Crystallization |Reciprocal Net http://www.reciprocalnet.org/edumodules/crystallization/. Accessed: 24 June 2019.
6. Tipler, Paul A.; Mosca, Gene (2008). Physics for Scientists and Engineers – Volume 1 Mechanics/Oscillations and Waves/Thermodynamics. New York, NY: Worth Publishers. pp. 666–670. ISBN 978-1-4292-0132-2.
7. Novel Sealants to Significantly Improve the Lifetime and Performance of Solid Oxide Fuel Cells | Mo-Sci Blog https://mo-sci.com/novel-sealants-Improve-solid-oxide-fuel-cells. Accessed: 25 June 2019
8. Mo-Sci Corporation Website https://mo-sci.com/en/custom-development. Accessed: 24 June 2019.

Annealing Range

NOTE: completely revised 31 December 2021

After Bullseye published annealing tables for thick slabs, some people feel they need to use the lower part of the annealing range for all their glass. To determine whether or when to use these tables needs some understanding of the annealing range.

Range

The annealing range of a glass is approximately 40ºC/72ºF on either side of the annealing point, but for practical kiln forming purposes it is normally taken as 33ºC/60ºF. The annealing point is around 510ºC/950ºF for System 96; 516ºC/962ºF for Bullseye and Uroboros for example. The range for a fusing glass will be around 549ºC to 477ºC/1020ºF to 890ºF for fusing glasses. Although the upper half of that range is merely theoretical. The lower end of the range is the strain point.

The annealing soak is to equalise the temperature throughout the glass to within 5ºC. Once the annealing soak is complete, the first stage of cooling begins. This first 55ºC/100ºF below the annealing soak is essential to the adequate annealing of the glass.  And this illustrates the impracticality of annealing in the upper part of the range.  The first cool rate needs to be maintained to at least 55ºC/100ºF below the low end of the annealing range.

To exemplify this. It would be possible to start the annealing at about 550ºC/1020ºF for any of these glasses. But the slow rate of decline in temperature, following the equalisation soak, would need to be maintained for the whole range of 550ºC/1020ºF to 429ºC/805ºF, rather than just the 55ºC/100ºF from the anneal soak point. This would more than double the annealing cool time. This high temperature anneal is a much slower process, which – together with the more rapid relief of stress at the annealing point – is why the top of the range is never used for the temperature equalisation point. It is also why the Spectrum 96 soak above the annealing point was not essential.

Soak

The annealing point is the temperature at which, if all the glass is at the same temperature, the most rapid cooling can take place. To achieve that equalisation temperature (+ or – 5ºC throughout), the glass needs to be soaked at the annealing point for varying lenghts of time relating to thickness and other variables. To complete the anneal and keep the glass within that tight range of temperature, the anneal cool needs to be continued at a steady slow rate.

Lower part of annealing range

Bullseye now recommends the use of 482ºC/900ºF for  the temperature equalisation soak, but have increased the soak time from 30 minutes to one hour. Choosing to start the annealing process at the lower part of the annealing range speeds the process for thick slabs and is very conservative for thinner glass. Bullseye have not changed the composition of their glass so the anything annealed at 516ºC/960ºF for things 6mm/0.25" or less is still properly annealed.

Using the bottom end of the annealing range for thick items, means there are a fewer number of degrees of very slow cooling to the strain point. But this lower soak, or temperature equalisation point, requires a longer soak to equalise the temperature within the glass before the slow steady decline in temperature to maintain the temperature differentials within the glass to less than 5ºC.

Bullseye have found that using a temperature a bit above the bottom end – 482ºC/900ºF – with a long soak reduces the total time in the kiln, but continues to give a good anneal. In the case of Bullseye, 461ºC/863ºF is the bottom end of the annealing range according to the calculations indicated above. 

Strain Points

A critical range is the temperature around the annealing point. The upper and lower limits of this range are known as the softening and strain points. The higher one is the point at which glass begins to bend.  It is also the highest temperature at which annealing can begin. The lower one is the lowest point at which annealing can be done. Soaking at any lower temperature will not anneal the glass at all. This temperature range is a little arbitrary, but it is generally considered to be 55C above and below the annealing point. The ideal point to anneal is thought to be at the annealing temperature, as annealing occurs most rapidly at this temperature.

Annealing Range
However, glass kiln pyrometers are not accurate in recording the temperature within the glass, only the air temperature within the kiln. The glass on the way down in temperature is hotter than the recorded kiln atmosphere temperature. A soak within the annealing range is required to ensure the glass temperature is equalised. If you do a soak at 515°C for example, the glass is actually hotter, and is cooling and equalising throughout to 515°C during the soak. The slow cool to below the lower strain point constitutes the annealing, the soak at the annealing point is to ensure that the glass is at the same temperature throughout, before  the annealing cool begins.

Strain Point and Below
No further annealing will take place below the strain point. If you do not anneal properly, the glass will break either in the kiln or later no matter how carefully you cool the glass after annealing.

It is still possible to give the glass a thermal shock at temperatures below the lower strain point, so care needs to be taken.  The cool below the anneal soak needs to be at a slow controlled rate that is related to the length of the required anneal soak. Too great a differential in contraction rates within the glass can cause what are most often referred to as thermal shock.  The control of the cooling rate reduces the chance of these breaks.

Softening Point

The glass is brittle below the softening point temperature, although it is less and less likely to be subject to thermal shock as it nears the softening point.  It is after the softening point on the increase in temperature that you can advance the temperature rapidly without breaking the glass.  So, if you have a glass that gives its annealing temperature as 515C, you can safely advance the temperature quickly after 570C (being 55C above the annealing point).

Characteristics of Some Glasses

This information has been taken from various sources. Some manufacturers may change the composition of their glasses or the published information about them from time to time. Therefore, this information can only be used as a guide. If the information about strain, annealing, and softening points is important, contact the manufacturer for the most accurate information.

The temperature information is given in Celsius.

Strain point – the temperature below which no annealing can be done.

Annealing point – the temperature at which the equalisation soak should be done before the annealing cool.

Softening point – the temperature at which slumping can most quickly occur.

Armstrong – Now made by Kokomo

Typical Borosilicate – nominal CoE 32

Strain point – 510 - 535C / 951 - 996F

Annealing point – ca. 560C/1041F
Softening point - ca. 820C/1509F

Blackwood OZ Lead – nominal CoE 92

Annealing point - 440C/825F

Blenko – nominal CoE 110

Annealing point – 495C/924F

Bullseye – nominal CoE 90

Transparents

Strain point - 493C/920F

Annealing point - (532C)  Note that Bullseye has changed this to 482C/900F for thick items

Softening point - 677C/1252F

Opalescents

Strain point - 463C/866F

Annealing point – (501C)  Note that Bullseye has changed this to 482C900F for thick items

Softening point - 688C/1272F

Gold Bearing

Strain point - 438C/821F

Annealing point - (472)   Note that Bullseye has changed this to 482C/900F for thick items

Softening point - 638C/1182F

Chicago – nominal CoE 92

Desag  Note that this glass is no longer produced

Artista – nominal CoE 94

Strain point – 480 - 510C / 897 - 951F

Annealing point – 515 - 535C / 960 - 996F

Softening point – 705 – 735C / 1302 - 1356F

Fusing range – 805 – 835C / 1482 - 1537

Float Glass (Pilkington UK)

Optiwhite

Strain point – 525 - 530C / 978 - 987F

Annealing point – 559C/1039F

Softening point – 725C/1338F

Optifloat

Strain point – 525 - 530C / 978 - 987F

Annealing point – 548C/1019F

Softening point – 725C/1338F

Float Glass (typical for USA) nominal CoE 83

Strain point - 511C/953F

Annealing point - 548C/1019F

Softening point – 715C/1320F

Float Glass (typical for Australia) nominal CoE 84

Strain point - 505-525C / 942 - 978F 

Annealing point – 540 -560C / 1005 - 1041F

HiGlass “GIN” range – nominal CoE 90

Annealing point - 535C/996F

Gaffer colour rod – nominal CoE 88

Gaffer NZ Lead – nominal CoE 92

Annealing point - 440C/825F

HiGlass

Annealing point - 495C/924F

Kokomo – nominal CoE 92 - 94

Cathedrals

Strain point - 467C/873F

Annealing point - 507C/946F

Softening point - ca. 565C/ca.1050F

Opal Dense

Strain point - 445C/834F

Annealing point - 477C/891F

Softening point – ca. 565C/1050F

Opal Medium

Strain point - 455C/834F

Annealing point - 490C/915F

Softening point – ca.565C/1050F

Opal Medium Light

Strain point - 461C/863F

Annealing point - 499C/931F

Softening point – ca.565C/1050F

Opal Light

Strain point - 464C868F

Annealing point - 502C/937F

Softening point – ca.565C/1050F

Kugler – nominal CoE

Annealing point - 470C/879F

Typical lead glass – nominal CoE 91

Lenox Lead – nominal CoE 94

Annealing point – 440C/825F

Merry Go Round – nominal CoE 92

Moretti/Effetre – nominal CoE 104

Strain Point: 448C/839F

Annealing Range: 493 – 498C / 920 - 929F

Softening Point: 565C/1050F

Pemco Pb83 – nominal CoE 108

Annealing point – 415C/780F

Schott Borosilicate (8330) nominal CoE 32

Annealing point - 530C/987F

Schott “F2” Lead – nominal CoE 92

Annealing point - 440C/825F

Schott “H” & “R6” rods - nominal CoE 90

Annealing point – 530C/987F

Schott “W” colour rod – nominal CoE 98

St Just

MNA

Strain point - ca.450C/843F

Annealing point – ca. 532C/ca. 991F

Spectrum

System 96 – nominal CoE 96

Transparents

Strain point – 476C  +/- 6C  /  890F +/- 11F

Annealing point – 513 +/- 6C  /  956C +/- 11F

Softening point – 680 +/- 6C  /  1257F +/- 11F

Opalescents

Annealing point – 505 -515C  /  942 - 960F

Spruce Pine 87 – nominal CoE 96

Annealing point – 480C/897F

Uroboros system 96 – nominal CoE 96

Transparents

Strain point - 481C/899F

Annealing point - 517C/964F

Opalescents
Strain point - 457C/855F
Annealing point - 501C/935F

Uroboros - nominal CoE 90

Transparents

Strain point - 488C/911F

Annealing point - 525C/978F

Opalescents

Strain point - 468C/875F

Annealing point - 512C/955C

Wasser - nominal CoE 89

Annealing point – 490C/915F

Wissmach
Wissmach 90
Annealing point - 483C/900F
Softening point - 688C/1272F
Full Fuse - 777+

Wissmach 96
Annealing point - 483C/900F

Softening point - 688C/1272F

Full Fuse - 777+ / 1432+

• 
  

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.

Bubble Squeeze Temperature

“My bubble squeeze temperature is higher than my slumping temperature.“ The writer goes on to say that their bubble squeeze is at 1250°F/676°C vs. a slumping range of 1150°F/620°C - 1175°F/634°C.

I applaud the writer for doing the slumping at the lower range of slumping temperatures. This allows the glass to relax into the mould with fewer marks being picked up.  The temperatures might require significantly long soaks depending on the span, depth, shape, weight of glass, etc.   But it is a good practice to get work done at as low a temperature as practical.

 But...

There does seem to be a misunderstanding on how a bubble squeeze works. Like most things with glass, any process works over a range of temperatures.  Bullseye glass begins to soften about 540°C. This continues to about 680°C where the transformation range begins – that is, the glass is behaving more like a viscous liquid than a softening solid.  A bubble squeeze or a slump can begin anywhere in this 540°C to 680° range.  At the lower end of the range, any slump will take “forever”.  At the top end, some slumps may occur too quickly and have mould and stretch marks on the bottom. 

Credit: Fusedglass.org

However…

This note is about the relation of bubble squeeze to slumping temperatures.  If you can slump an item at 620°C, you can also perform a bubble squeeze at that temperature.  Both processes rely on the glass becoming “soft” enough to relax into the shape below it.  It may be that you will need a very long soak to press out air in a bubble squeeze at 620°C, but it can be done if you are willing to wait a long time.  

Many people begin their bubble squeeze at 620°C for fusing glass with a soak.  I am not sure that a soak is required at this point, as slowing the rate of advance over the next 50°C will have the effect of increasing the heat work the glass receives without the need of a soak at the beginning of the bubble squeeze ramp – unless you have a rapid rate of advance toward the bubble squeeze.  

They then progress slowly (maybe 50°C or less, depending on thickness) for the next 55°C to 60°C and soak at that higher temperature for half an hour, or more for difficult shapes.  This additional heat work allows the glass to gradually become more plastic and deform more slowly than at a higher temperature bubble squeeze.  This is often called a cautious bubble squeeze, since it starts at a lower temperature and moves gradually to the top of the bubble squeeze range.  It removes the single shot bubble squeeze at a higher temperature, when air might already be trapped. 

In general terms, the slump can be carried out at or below the softening point of the glass.  This softening point is also the maximum temperature for a bubble squeeze. For example, float glass has a softening point of about 720°C, so a bubble squeeze and slumping can be in the 660°C to 720°C range.  Some glasses have even higher softening points, and others have much lower softening points than Bullseye or Oceanside.

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

Float annealing

As a result of various memory failures, I've done a bit of searching on the annealing of float glass.  There are now various compositions of float glass and with different coatings for various applications.

This leads to a variety of annealing points for Pilkington float glasses. The search led to various hard to find documents, which indicate a range of annealing temperatures between 548°C and 559°C. This is not a huge range, so anywhere between 548°C and 560°C can be taken as the annealing point. Pilkington indicate that optifloat has an annealing point of 548°C

The strain point seems to be mostly between 525°C and 530°C for all the varieties.  This indicates the temperature equalisation soak should not be less than 535°C.

The conclusion seems to be that annealing should have a temperature equalisation soak between 550°C and 535°C. It will not matter much where you choose, but remember that the closer to the strain point you do the temperature equalisation, the longer the soak should be.  The length of soak at 535°C can be determined by use of the Bullseye chart for Annealing Thick Slabs. This gives the times and rates for the anneal cooling of glass by thickness.  The temperatures need to be changed, but otherwise the information can be applied.

The softening point seems to be 725°C for all the glasses. This is a good low temperature for slumping.

Approximate Temperature Characteristics of Various Glasses

Various glasses have different temperature characteristics. This listing is an attempt to indicate the differences between a variety of popular glasses used in kiln forming. They are not necessarily exact, but do give an indication of differences.

Bullseye Transparents
Full fusing 832C
Tack fusing 777C
Softening 677C
Annealing 532C
Strain point 493C

Bullseye Opalescents
Full fusing 843C
Tack fusing 788C
Softening 688C
Annealing 502C
Strain point 463C

Bullseye Gold Bearing Glasses
Full fusing 788C
Tack fusing 732C
Softening 632C
Annealing 472C
Strain point 438C

Desag GNA
Full fusing 857C
Tack fusing 802C
Softening 718C
Annealing 530C
Strain point 454C

Float Glass
Full fusing 835C
Tack fusing 760C
Softening 720C
Annealing 530C
Strain point 454C

Oceanside
Full fusing 788C
Tack fusing 718C
Softening 677C
Annealing 510C
Strain point 371C

Wasser
Full fusing 816C
Tack fusing 760C
Softening 670C
Annealing 510C
Strain point 343C

Wissmach 90
full fusing  777C
Tack fusing
Softening  688C
Annealing  510C
Strain point

Wissmach 96
Full fusing  777C
Tack fusing
Softening  688C
Annealing  510C
Strain point

Youghiogheny 96
Full fusing  773C
Tack fusing  725C
Softening  662C
Annealing  510C
Strain point