Float Annealing Temperatures

Float glass annealing temperatures vary quite a bit from one manufacturer to another; and even within one manufacturer’s product line.

Comparisons of various float glasses

Some companies are more informative that others.  Pilkington are one of the more open of European glass manufacturers on various bits of information.

Pilkington Float

CoLE 83 *^10-5

Softening point:  715°C

annealing point:  548°C

strain point: 511C

Pilkington Optiwhite ™

Softening point:  ca. 732°C

annealing point:  ca. 559°C

strain point:  ca. 526°C

There is a difference of 11C between two of the Pilkington product lines for the annealing points.  The softening and strain points are slightly wider.

Glaverbel, a Belgian company, restricts their information to CoLE and the softening point.

CoLE 91 * 10^-5

Softening point: 600°C

Saint-Gobain, a French company, shows some more of the variation in the product lines, although they do not give specific annealing points for the different products.

CoLE 90 * 10^-5

annealing range:  520 - 550°C

Low E glass

softening – 840°C

strain - 617°C

R glass (sound reducing)

softening – 986°C

strain - 736°C

D glass (decorative)

softening point – 769°C

Compatibility

Even this small sample of float glasses shows there is a significant difference between manufacturers for the softening, annealing and strain points.  This means that, unless you are sure of the glass merchant’s source of glass, you will need to test each batch of glass for compatibility with previous batches, if you are combining from different suppliers.

I included the CoLE numbers (which all the manufacturers specified as an average change in length for each degree C increase in temperature from 0 to 300°C) to show the variation and to challenge anyone to find Bullseye and Saint-Gobain or Glaverbel compatible with each other.  My experience has shown that the Optul coloured frit and confetti is more likely to be compatible with Pilkington than the other two.

Annealing

I have been beginning my annealing of float glass at 525°C.  This little bit of literature research shows that my annealing soak should be starting higher, possibly at 540°C, certainly no lower than 530°C.  Other areas of the world may find their float glass has significantly different annealing ranges.

CoE Varies with Temperature

Information from Bullseye shows that the Coefficient (average) of Linear Expansion changes rapidly around the annealing range.

The following is from results of a laboratory test of Bullseye clear (1101F)
Temperature range.......................COE
20C-300C (68F -­ 572F).................90.6
300C-400C (572F - ­752F).............102.9
400C-450C (752F - 842F).............107.5
570C-580C (1058F-1076F)............502.0

Bullseye glass is probably typical of soda lime glasses designed for fusing.

The change of CoE by temperature is further illustrated by Kugler (a blowing glass) who state their CoE by temperature range. Remember CoE is an average expansion over a stated range of temperatures)
CoE 93 for the range 0C-300C
CoE 96 for the range 20C - 300C
CoE 100 for the range 20C - 400C

The extension of the range by 100C beyond the brittle phase of glass has a distinct effect on the average expansion over the (larger) range. 

This shows why it is not helpful to refer to CoE without also mentioning the range of temperature.

In addition, here is an illustration of the effect. 

Image credit: Kerwin and Fenton, Pate de Verre and Kiln Casting of Glass,2000, p.32

It is understandable and common sense that as the temperature increases, so the rate of expansion increases and this applies to most solids.  Glass behaves differently as the graph above shows.  The change in expansion of Bullseye glass shows a relatively consistent average expansion until the strain point is reached.  Once out of the brittle phase, glass expansion rates change very much more rapidly.  It is not be coincidence that viscosity of glass changes at almost the same rates.  It is the viscosity that is controlling the CoE, not the other way around.   

Revised  5.1.25

Breaks after the Piece is Cool

People sometimes fire a piece only to have it break after it is cool.  They decide to re-fire with additional decoration to conceal the break.  But it breaks again a day after it has cooled.  Their questions centre around thermal shock and annealing. They used the same CoE from different suppliers, so it must be one of these elements that caused the breakage.

Thermal Shock

This is an effect of a too rapid heat changes.  Its can occur on the way up in temperature or on the way down.  If it occurred on the way up to a fuse, the edges will be rounded.  If it occurred on the way up to a slump the edges may be sharp still, but the pieces will not fit together because the slump occurred before the slump.  It the break occurs on the way down the pieces will be sharp.  The break will be visible when you open the kiln.  More information is here.

If the break occurs after the piece is cool, it is not thermal shock.

Annealing

Another possible cause of delayed breakage is inadequate annealing.  Most guidelines on annealing assume a flat uniform thickness.  The popularity of tack fused elements, means these are inadequate guides on the annealing soak and annealing cool.  Tack fused items generally need double the temperature equalisation soak and half the annealing cool rate. This post gives information on how the annealing needs modification on tack fused items. 

The annealing break usually crosses through the applied pieces and typically has a hook at each end of the break.  If the piece has significant differences in thicknesses, the break may follow the edge of the thicker pieces for some distance before it crosses it toward an edge. This kind of break makes it difficult to tell from an incompatibility break.

Compatibility

The user indicated all the glass was of the same CoE.  

This is not necessarily helpful. 

Coefficient of Linear Expansion (CoE) is usually measured between 20°C and 300°C. The amount of expansion over this temperature range is measured and averaged. The result is expressed as a fraction of a metre per degree Celsius. CoE90 means that the glass will expand 9 one-thousandths of a millimetre for each degree Celsius.  If this were to hold true for higher temperatures, the movement at 800C would be 7.2mm in length over the starting size.  However, the CoE rises with temperature in glass and is variable in different glasses, so this does not tell us how much the expansion at the annealing point will be.  It is the annealing point expansion rate that is more important.  More information is here.

• Compatibility is much more than the rate of expansion of glass at any given temperature.  
• It involves the balance of the forces caused by viscosity and expansion rates around the annealing point.

Viscosity is probably the most important force in creating compatible glasses. There is information on viscosity here.  To make a range of compatible glass the forces of expansion and viscosity need to be balanced.  Each manufacturer will do this in subtly different ways.  Therefore, not all glass that is claimed by one manufacturer to compatible with another’s will be so. 

All is not lost.  It does not need to be left to chance.

Testing glass from different sources is required, as you can see from the above comments.  It is possible to test the compatibility of glass from different sources in your own kiln.  The test is based on the principle that glass compatible with a base sheet will be compatible with other glasses that are also compatible with that same base sheet.  There are several methods to do this testing, but this is the one I use, based on Shar Moorman’s methods.  

If you are buying by CoE you must test what you buy against what you have.

If you are investing considerable effort and expense in a piece which will use glass from different sources or manufacturers, and which is simply labelled CoE90, or CoE96, you need to use these tests before you start putting the glass together.  The more you deviate from one manufacturer’s glass in a piece, the more testing is vital. 

In the past, people found ways of combining glass that was not necessarily compatible, by different layering, various volume relationships, etc.  But the advent of manufacturers’ developing compatible lines of glass eliminated the need to do all that testing and experimenting.  While the fused glass market was small, there were only a few companies producing fusing glass.  When the market increased, the commercial environment led to others developing glass said to be compatible with one or other of the main producers of fusing compatible glass.

An incompatibility break may occur in the kiln, or it may occur days, months or years later.  Typically, the break or crack will be around the incompatible glass.  The break or crack may follow one edge of the incompatible glass before it jumps to an edge.  The greater the incompatibility, the more likely it is to break apart.  Smaller levels of incompatibility lead to fractures around the incompatible glass pieces, but not complete breaks.

If the break occurs some length of time after the piece is cool, it can be an annealing or a compatibility problem.  They are difficult to distinguish apart sometimes.  There is more information about the diagnosis of the causes of cracks and breaks here.

The discussion above shows that even with the best intentions, different manufacturers will have differences that may be small, but can be large enough to destroy your project.  This means that unless you are willing to do the testing, you should stick with one manufacturer of fusing compatible glass. 

Do not get sucked into the belief that CoE tells you much of importance about compatibility.

Revised 30.12.24

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 

Clear – nominal CoE 96 +/- 2  (94-98)

softening point: - 694C/1281F

Annealing point: - 508C/946F

Strain point: - 485C/904F

Colours - nominal CoE 96 +/- 4 (92-100)

Annealing point: - 500C/932F

Strain point: - 460C-500C/860 -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: 493C – 498C / 920F - 929F

Softening Point: 565C/1050F

Pemco Pb83 – nominal CoE 108

Annealing point – 415C/780F

Reichenbach - 

nominal CoE 96 +/-2 (94 -98)

Annealing range; - 470C-530F/878F-986F; Ave 510C/950F

nominal CoE 104  no further information at present.

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+

• 
  

CoE as the Determinant of Temperature Characteristics

Credit:: Ryan Rutherford

Many people are under the impression that CoE can tell you a wide number of things about fusing glass. 

What does CoE really mean?

The first thing to note is the meaning of CoE.  Its proper name is the coefficient of linear expansion.  It tells you nothing certain about the expansion in volume, which can be as or more important than the horizontal expansion. 

It is an average determined between 20°C and 300°C.  This is fine for materials that have a crystalline structure. Glass does not.  Glass behaves quite differently at higher temperatures. 

It may have an average expansion of 96 from 20°C-300°C – although there is no information on the variation within that range – but may have an expansion of 500 just above the annealing point. 

The critical temperatures for glass are between the annealing and strain points.  One curious aspect to the expansion of glass is that the rate of expansion decreases around the annealing point.  The amount of this change is variable from one glass composition to another.

credit: ScienceDirect

The CoE of a manufacturer’s glass is an average of the range which is produced.  Spectrum has stated that their CoE of their fusing compatible glass is a 10 point range.  Bullseye has indicated that their CoE range is up to 5 points. These kind of ranges can be expected in every manufacturer’s compatible glass.

CoE does not tell us anything about viscosity, which has a bigger influence on compatibility than CoE alone. 

Comparison of CoE and Temperature

Among the things people assume CoE determines is the critical temperatures of the strain, annealing and softening points of various glasses.

Unfortunately, CoE does not necessarily tell you fusing or annealing temperatures. 

“CoE 83”

Most float glass is assumed to be around CoE 83.  The characteristics depend on which company is making the glass and where it is being made.  These are the annealing points and softening points:

Pilkington Optiwhite              559ºC/1039ºF    720°C/1328°F

Pilkington Optifloat               548ºC/1019ºF    720°C/1328°F

USA float (typical)                548ºC/1019ºF    615°C/1139

Typical Australian float has a CoE of 84 and anneals in the range 505°C -525°C/941°F - 977°F.

“CoE 90”

Uroboros FX90 has an annealing point of 525°C compared to Bullseye at 482°C, and Wissmach 90 anneal of 510°C. 

Wissmach 90 has a full fuse temperature of 777°C compared to Bullseye's 804 - 816°C.   

There is a float glass with a CoE of 90 that anneals at 540°C and fuses at 835°C.

Bullseye has a slump temperature of 630°C-677°C and Wissmach’s 90 slumps between 649°C and 677°C, slightly higher.

“CoE 93”

Kokomo with an average CoE of 93 has an annealing range of 507°C to 477°C. Kokomo slumps around 565°C

“CoE 94”

Artista with a CoE of 94 has an annealing point of 535°C and a full

fuse of 835°C, almost the same as float with a Coe of 83. 

“CoE96”

Wissmach 96 anneals at 482°C with a full fuse of 777°C and a slump temperature of 688°C.

Spectrum96 and its successor Oceanside Compatible anneals at 510°C and full fuses at 796°C.

Conclusion

In short, CoE does not tell you the temperature characteristics of the glass. These are determined by several factors of which viscosity is the most important. More information can be gained from this post or from your own testing and observation as noted in this post.

Revised 23.12.24

Sealing MEMS Devices with Glass

 

Krista Grayson

•

December 5, 2023

Micro-electromechanical systems (MEMS) have revolutionized various industries by integrating miniaturized mechanical and electrical components onto a single substrate, enabling the creation of highly sensitive and efficient devices. They are key elements in an array of medical, automotive, industrial, and defense applications.

a

However, the success of MEMS devices often hinges on maintaining a hermetic environment to protect their delicate internal components. This is where glass frit sealing technology comes into play, providing a superior solution for achieving reliable hermetic seals in precise applications like MEMS manufacturing and packaging.²

Understanding Hermetic Sealing and its Importance

Hermetic sealing involves creating an airtight barrier around a device to prevent the entry of contaminants, moisture, and other external elements. This sealing technique is crucial for MEMS devices as even minute environmental influences can alter their performance or lead to premature failure. In applications where stability, precision, and reliability are paramount, such as in the aerospace, medical, and telecommunications industries, achieving a hermetic seal is essential.²

Glass Frit Sealing: The Ideal Solution for MEMS

Among the various methods available to achieve hermetic seals, glass frit sealing stands out as a versatile and high-yield approach, particularly suited for MEMS applications. This technique leverages the unique properties of glass to create a reliable, robust, and precise encapsulation for MEMS devices while imposing minimal stress on the bonding surface. In a three-step process, a glass paste is screen-printed on a capping wafer, which is then bonded to the subject device through thermocompression for 10 minutes. During this process, 1000 mBar of force and 440 °C are applied to the material under a vacuum. Capable of bonding both hydrophobic and hydrophilic surfaces, this technique can be applied to almost all commonly used microsystem surface materials, such as aluminum, silicon, and glass.^3,4

Tailoring Precision Using the Coefficient of Thermal Expansion (CTE)

As the name implies, glass frit sealing makes use of glass particles, known as frit, which can be precisely formulated to match the coefficient of thermal expansion (CTE) of different materials.⁴ The CTE of a material refers to how its dimensions change with temperature fluctuations. By tailoring the glass frit’s composition, its CTE can be adjusted to closely match that of the MEMS device and the encapsulating material. This compatibility ensures that, when subjected to temperature variations, the seal remains intact without compromising the structural integrity of the device.²

Mo-Sci, a pioneering glass technology company, has been at the forefront of developing and perfecting glass frit sealing solutions for various high-tech applications, including MEMS devices. Its expertise lies in creating sealing glasses with customizable thermal expansion coefficients. With a diverse range of glass-metal and glass-ceramic seals that are meticulously matched in terms of CTE and are capable of enduring temperatures as high as 1600°C, Mo-Sci is an ideal partner for MEMS manufacturers seeking reliable hermetic sealing solutions.^2,5

The Versatility of Glass Frit Sealing

The applications of glass frit sealing extend beyond MEMS devices and encompass a range of cutting-edge technologies:

1. Solar Cells

Sealing glasses find utility in encapsulating perovskite photovoltaic elements. These elements are promising alternatives to traditional silicon solar cells due to their high efficiency and lower production costs. However, perovskite cells are highly sensitive to moisture, whereby even small amounts can completely prevent function. Laser-assisted bonding of glass frit sealing guarantees a durable hermetic barrier, shielding perovskite cells from moisture exposure and locking in lead-containing chemicals.²

2. Metal Ion and Thermal Batteries

In the evolving landscape of energy storage solutions, glass frit sealing plays a pivotal role in enhancing the reliability and longevity of metal ion batteries, including lithium-ion and sodium-ion batteries. These batteries require seals that can withstand high temperatures and resist chemical corrosion. Sealing glasses provide a resilient barrier that enables the efficient operation of these advanced battery technologies.

Sealing glass is also a viable solution for molten salt batteries. These batteries are highly dependent on sodium salts, including sodium-nickel and sodium-sulfur chloride, to achieve remarkable energy and power densities. For this reason, they are an appealing option for large-scale industrial and energy storage applications.

Sealing glasses are classed as a high-energy alternative to conventional polymeric or metal seals as they exhibit excellent resilience against demanding chemical environments but also against the rigorous operating temperatures inherent to molten salt batteries, which can range from 300 °C to 350 °C.²

3. High Temperature Sensors

Glass frit sealing also finds applications in high-temperature environments, such as automotive systems and chemical processing plants. The predictable thermal expansion and corrosion-resistant nature of sealing glass ensure the longevity and stability of sensors operating in extreme conditions.²

4. Solid Oxide Fuel Cells (SOFCs)

SOFCs hold tremendous promise for clean and efficient power generation, but their high operating temperatures present engineering challenges. To create high-temperature sealant materials for SOFCs, Mo-Sci currently utilizes two methods. One relies on a traditional glass-ceramic seal, wherein the glass undergoes crystallization to establish bonds with the sealing components.

The second approach involves the development of viscous-compliant glass seals. These seals remain vitreous throughout application and can self-heal, mitigating the risks associated with thermal stresses and ensuring the long-term stability of SOFCs.This groundbreaking technology is anticipated to play a pivotal role in facilitating the commercialization of SOFCs and driving their widespread adoption.^2,6

Embracing the Future with Glass Frit Sealing

Glass frit sealing technology has emerged as a transformative solution for achieving hermetic seals in MEMS devices and a wide array of other advanced applications. By precisely engineering the properties of sealing glasses, companies like Mo-Sci enable manufacturers to create highly reliable and robust encapsulation systems.

As industries continue to push the boundaries of technological innovation, the role of glass frit sealing in safeguarding sensitive components and ensuring optimal device performance becomes increasingly vital.

References and Further Reading

1. Forbes. Why Timing Must Be Tough Enough For Our Digital World. Available at: https://www.forbes.com/sites/forbestechcouncil/2021/09/02/why-timing-must-be-tough-enough-for-our-digital-world/ (Accessed on 10 August 2023).
2. Mo-Sci. Sealing Glass Applications. Available at: https://mo-sci.com/sealing-glass-applications/ (Accessed on 10 August 2023).
3. Chang H-D, et al. (2010). High hermetic performance of glass frit for MEMS package. 2010 5th International Microsystems Packaging Assembly and Circuits Technology Conference. https://doi.org/10.1109/IMPACT.2010.5699539
4. Knechtel R. (2015). Chapter 31 – Glass Frit Bonding. Handbook of Silicon Based MEMS Materials and Technologies (Second Edition). https://doi.org/10.1016/B978-0-323-29965-7.00031-2
5. Mo-Sci. Matching Coefficient of Thermal Expansion in Glass Seals. Available at: https://mo-sci.com/matching-cte-in-glass-seals/ (Accessed 10 August 2023).
6. Mo-Sci. Sealing Glass. Available at: https://mo-sci.com/products/sealing-glass/ (Accessed on 10 August 2023).

source:https://mo-sci.com/sealing-mems-devices-with-glass/?utm_source=Mo-Sci+Newsletter&utm_campaign=b5090c88ed-EMAIL_CAMPAIGN_2023_09_28_06_45_COPY_01&utm_medium=email&utm_term=0_-cf8dcfb60f-%5BLIST_EMAIL_ID%5D&mc_cid=b5090c88ed&mc_eid=0ab94327fb

Coe and compatibility

From time to time you will see the statement:

“CoE is the determinant of compatibility”

This is Not True!  

I wish I could come up with something simple to counteract this CoE fallacy, but glass is complicated and I can’t think of a snappy phrase to help.  To understand why the statement above is false, some background on what CoE does mean and what range of temperature it applies to is important.

What CoE measures

The coefficient of expansion can be a measure of either linear or volumetric expansion.  It is most often conducted over the range of 20°C to 300°C.  The result is expressed as an average over this range.  If there are variations in rates of expansion, they are absorbed in this coefficient, ie., average.  The measure is of the part of one metre the material expands for each degree Celsius increase in temperature.  In the glass community this coefficient is expressed as two digits such as 83 which represents the expansion of glass by 0.0000083 of a metre for each degree Celsius change in the measured temperature range.

Relevant temperature range

Note the temperature range over which this is measured – up to 300°C.  This coefficient works well for crystalline solids, but not for glass.  Amorphous solids do not have linear expansion rates throughout the working range of temperatures. Room temperature to 300°C is not a critical temperature range for glass.  After all, many of us turn the kiln off around 370°C.  This means that the CoE measured up to 300°C is not really relevant to us, as we have discovered that the expansion rates for 6mm or less thick glass are not critical below 370°C.

Annealing range

The CoEs at annealing temperatures – the critical range for glass -  are in the 400 to 500 range.  It is in the annealing range – generally about 45°C above and below the annealing point of the glass – that CoE is most important.  The annealing point is above the now popular, but lower, annealing soak temperature. This is where the glass is soaked to obtain a temperature with a differential of no more that 5°C throughout the glass.  The practice has become to do this temperature equalisation at the lower portion of the annealing range.  Often this is only 10°C above the lower boundary of the annealing range. This gives a shorter cool and increases the density of the glass. Do not confuse annealing point with the annealing soak. They are not the same.

Critical temperature range for CoE

The Coefficient of Expansion is more important at the glass transition point. This is the temperature at which the molten material becomes a slightly flexible solid. The CoE and the viscosity interact in this range.  It is critical, as the opposing forces of viscosity and CoE must balance.  The CoE is adjusted by the manufacturer to create this balance.  It shows that CoE is dependent on the viscosity of the glass.  And the characteristics of each colour must also match all the other glass in the range of tested compatible fusing glass. This is not a simple thing to do.  If it were, there would be lots of companies doing it.

Experience of moving to a single CoE for fusing glass

The Bullseye experience of attempting to achieve compatibility across a range of glass in the early days of making fusing compatible glass showed that less compatibility was experienced when the colours had matching CoEs. Lani Macgreggor describes this experience well in this blog, “Eclipse of the Fun”. 

An expert’s explanation

A Bullseye article by Dan Schwoerer - possibly the major expert on making compatible glass - on achieving compatibility through compensating differences is the key to understanding the balancing of CoE with the viscosity.  It is on the Bullseye site as Tech Note #3.

There is a more impassioned description of matters relating to compatibility in five linked blogs by Lani Macgregor in the To BE or not BE blog.

Manufacturing to a range of CoE

Spectrum long ago stated that the CoE of their glass ranges up to 10 points  to achieve a compatible range of fusing glass.  This is probably true for every manufacturer of fusing compatible glass. 

Why CoE is NOT the determinant of fusing compatible glass

The things that mean CoE cannot be the determinant of compatible glass are:

• ·        The coefficient is for an inappropriate temperature range for glass.
• ·        The critical temperatures for expansion are in the annealing range, for which there are no widely published figures.
• ·        The expansion rates need to be adjusted to match the viscosity in this annealing range.
• ·        A major manufacturer has indicated their glass, known by the CoE of its fusing standard glass, has a 10-point range of CoEs within their fusing range.

It is not true that CoE is a determinant of compatibility.

CoE is an inappropriate number to indicate compatibility.  It does not guarantee compatibility.  It is a suspiciously accurate number leading people to erroneously believe any glass labelled with a given number will be compatible with any other with the same number. 

Other blog posts on CoE:

CoE does not determine critical temperatures: 

https://glasstips.blogspot.com/2017/08/coe-as-determinant-of-temperature.html

Demonstration that CoE does not determine annealing or fusing temperatures:

https://glasstips.blogspot.com/2017/08/comparisons-of-coe-and-temperatures.html

Note on the physical changes at annealing

https://glasstips.blogspot.com/2014/03/annealing-physical-changes.html

Absence of any correlation between specific gravity and CoE:

https://glasstips.blogspot.com/2018/11/specific-gravity-cole-and-colourants-of.html