Slump Point Test

At a time when we are all going to be trying a variety of glass of unknown compositions to reduce costs of kiln working, the knowledge of how to determine the slump point temperature (normally called the softening point in the glass manufacturing circles) and the approximate annealing temperature becomes more important.  The slump point test can be used to determine both the slumping point and the annealing soak temperature.  This was required when the manufacturers did not publish the information, and it continues to be useful for untested glasses.

The method requires the suspension at a defined height of a strip of glass, the inclusion of an annealing test, and the interruption of the schedule to enter the calculated annealing soak temperature.

A strip of 3 mm transparent glass is required.  This does not mean that it has to be clear, but remember that dark glass absorbs heat differently from clear or lightly tinted glass. The CoE characteristics given are normally those of the clear glass for the fusing line concerned.  The strip should be 305 mm x 25 mm.  

Suspend the strip 25 mm above the shelf, leaving a span of 275 mm. This can be done with kiln brick cut to size, kiln furniture, or a stack of fibre paper.   Make sure you coat any kiln furniture with kiln wash to keep the glass from sticking.

The 305mm strip suspended 25mm above the shelf with kiln furniture.

Place some kiln furniture on top of the glass where it is suspended to keep the strip from sliding off the support at each end. Place a piece of wire under the centre of this span to make observation of the point that the glass touches down to the shelf easier.

The strip held down by placing kiln furniture on top of the glass, anchoring it in place while the glass slumps.

If you are testing bottles, you may find it more difficult to get such a long strip.  My suggestion is that you cut a bottle on a tile saw to give you a 25 mm strip through the length of the bottle.  Do not worry about the curves, extra thickness, etc.  Put the strip in the kiln and take it to about 740C to flatten it. Reduce the temperature to about 520C to soak there for 20 minutes.  Then turn the kiln off.  

Also add a two layer stack of the transparent glass near the suspended strip of glass to act as a check on whether the annealing soak temperature is correct. This stack should be of two pieces about 100 mm square. If you are testing bottles, a flattened side will provide about the same thickness.  This process provides a check on the annealing temperature you choose to use.  If the calculated temperature is correct there should be little if any stress showing in the fired piece.

The completed test set up with an annealing test and wire set at the midpoint of the suspended glass to help with determining when the glass touches down.

The schedule will need to be a bit of guess work.  The reasons for the suggested temperatures are given after this sample initial schedule which needs to be modified during the firing.

In Celsius
Ramp 1: 200C per hour to 500C, no soak
Ramp 2: 50C per hour to 720C, no soak
Ramp 3: 300C per hour to 815C or 835C, 10 minute soak
Ramp 4: 9999 to 520C, 30 minute soak
Ramp 5: 80C per hour to 370C, no soak
Ramp 6: off.

In Fahrenheit

Ramp 1: 360F per hour to 932F, no soak
Ramp 2: 90F per hour to 1328F, no soak
Ramp 3: 540F per hour to 1500F or 1535FC, 10 minute soak
Ramp 4: 9999 to 968F, 30 minute soak
Ramp 5: 144F per hour to 700F, no soak
Ramp 6: off.

Fire at the moderate rate initially, and then at 50C/90Fper hour until the strip touches down. This is to be able to accurately record the touch down temperature.  If you fire quickly, the glass temperature will be much less than the air temperature that the pyrometer measures.  Firing slowly allows the glass to be nearly the same temperature as the air.  

Observe the progress of the firing frequently from 500C/932F onward.  If it is float or bottle glass you are testing you can start observing from about 580C. Record the temperature when the middle of the glass strip touches the shelf. The wire at the centre of the span will help you determine when the glass touches down.  This touch down temperature is the slump point of your glass.  You now know the temperature to use for gentle slumps with a half hour soak.  More angular slumps will require a higher temperature or much more time.

Once you have recorded the slump point temperature, you can skip to the next ramp (the fast ramp 3).  This is to proceed to a full fuse for soda lime glasses. Going beyond tack fusing temperatures is advisable, as tack fuses are much more difficult to anneal and so may give an inaccurate assessment of the annealing. Most glasses, except float, bottles and borosillicate will be fully fused by 815C. If it is float, bottles or borosilicate that you are testing, try 835C. If it is a lead bearing glass, lower temperatures than the soda lime glass should be used. In all these cases observation at the top temperature will tell you if you have reached the full fuse temperature. If not add more time or more heat to get the degree of fuse desired.

While the kiln is heating toward the top temperature you can do the arithmetic to determine the annealing point.  To do this, subtract 40C/72F from the recorded touch down temperature to obtain an approximate upper annealing point.  The annealing point will be 33C/60F below the upper point.  This is approximate as the touch down temperature is, by the nature of the observation. approximate.  

The next operation is to set this as the annealing soak temperature in the controller. This will be the point at which it usually possible to interrupt the schedule and change the temperature for the annealing soak that you guessed at previously. Sometimes though, you need to turn the controller off and reset the new program.  Most times the numbers from the last firing are retained, so that all you need to do is to change the annealing soak temperature.

The annealing soak should be for 60 minutes to ensure an adequate anneal. This may be excessive for 3 mm glass, but as the anneal test is for 6 mm, the longer soak is advisable. The annealing cool should be 83C/hr down to 370C. This is a moderate rate which will help to ensure the annealing is done properly. The kiln can be turned off at that temperature, as the cooling of the kiln will be slow enough to avoid any thermal shock to the annealing test piece.

When cooled, check the stack for stress. This is done by using two polarised light filters. See here for the method. 

Squares of glass showing different levels of stress from virtually none to severe
 (no light emanating for no stress to strong light from the corners indicating a high degree of stress.)

If the anneal test piece is stressed, there could be a number of reasons for the inadequate annealing. It could be that the glass has devitrified so much that it is not possible to fuse this glass at all. If you also test the suspended strip for stresses and there is very little or none, it is evidence that you can kiln form single layers of this glass. You now know the slumping temperature and a suitable annealing temperature and soak for it, even though fusing this glass is not going to be successful.

Other reasons for stress due to inadequate annealing could be that the observations or calculations were incorrect.  

• Of course, before doing any other work, you should check your arithmetic to ensure the calculations have been done correctly. I'm sure you did, but it is necessary to check.  If they are not accurate, all the following work will be fruitless.

• The observation of the touch down of the suspended strip can vary by quite a bit - maybe up to 15C.  To check this, you can put other annealing test pieces in the kiln.  This will require multiple firings using temperatures in a range from 10C/18F above to 10C/18F below your calculated annealing soak temperature to find an appropriate annealing soak temperature.

• If stress is still showing in the test pieces after all these tests, you can conduct a slump point test on a strip of glass for which there are known properties. This will show you the look of the glass that has just reached touch down point as you know it will happen at 73C above the published annealing point.  You can then apply this experience to a new observation of the test glass. 

Revised 30.12.24

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

Kilnforming Opalescent Stained Glass

The statement that a sheet of glass can be fused to itself is true in certain circumstances.  It applies to transparent and some streaky glasses best.  These forms of glass are more likely to fuse together successfully although not formulated for fusing.

Transparent and Streaky Glasses

Of course, the best practice is to test for compatibility.  I found in my early days of sticking stained glass together that it was beneficial to test. In doing so, I found Spectrum and Armstrong transparent and streaky glass to be largely consistent across many sheets.  I did not have access to much Kokomo or Wissmach.  I cannot comment on how their glass behaves in terms of compatibility across the production range.  Not all transparent and streaky glass remains stable at fusing temperatures. There are some glasses that opalise, some change colour, some devitrify. This variability makes compatibility testing important - even for the transparent form of stained glass.

Photo credit: Lead and Light

Wispy Glasses

The statement about fusing to itself is less applicable to wispy glass.  Not all the wispy stained glass from the same sheet can be fused.  It seems to be dependent on the amount of opalescence in any one area of the glass.  I found that it is possible - if you are very careful - to fuse certain Spectrum wispies with the clear fusing standard on top, but not on the bottom.  This should be applicable to other manufacturers’ wispy glass too.  There must be a marginal compatibility that is contained by the clear fusing glass on top, but I am not certain.

Photo credit: Lead and Light

Opalescent Glasses

The statement about fusing to itself is almost completely inapplicable to opalescent glass.  Stained glass opalescent glass does not have the compatibility requirements of fusing glasses.  They very often severely devitrify when taken to fusing temperatures.  This devitrification means that opalescent stained glass is often not compatible with itself.  So, no amount of twiddling with schedules will make stained glass opalescent glass fusible, even with itself.

Manufacturers have spent a lot of time and effort to produce fusing compatible opalescent glass.  It is as though there is a minor element of devitrification embodied in the opalising process.  Whether this is so, it becomes very apparent on doing compatibility testing that opalescent stained glass has severe devitrification at fusing temperatures.

Stock photo

Compatibility Testing

It is important to test for compatibility before committing to the main firing.  Some transparent and streaky glass changes colour, devitrifies, and some opalise at fusing temperatures. This applies with even more force to wispies.  They contain a significant proportion of opalescence within them.  Some opalescents are so unstable at fusing temperatures that the devitrification becomes so bad the glass crumbles.

The importance of testing pieces of the sheet for compatibility before committing to a firing is reinforced by these factors.

Slumping

Slumping temperatures are not so high as fusing, and it is often stated that single layers can be slumped.  Again, it is not always true.

Some glasses change colour at slumping temperatures.  A few opalise. It is not always certain what effect moderate temperatures will have on stained glass.  The compatibility testing will show.  Observe the test firing at slumping temperatures.  Also, you will learn if there are changes at moderate temperatures.

One element must be commented upon about slumping.  It is important to have the edges finished to the appearance that you want the final piece to have.  The regularity of the edges without bumps or divots, and the degree of polish need to be showing before the firing starts.  The slumping temperatures are not high enough to alter the shape or appearance of the edges.

Firing of stained glass to itself is normally a low risk activity, but with unpredictable results.  It can teach a lot about behaviour of glass at higher temperatures.  Slumping single layer pieces can give information about the way single layers of glass slump or drape.  But testing is important for fusing.  And can inform about how the glass will react at slumping temperatures too.

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

Compatibility of Glasses with the Same CoE

Questions such as “How compatible are Wissmach W90 and Bullseye?” are asked from time to time.  This does show some awareness that Bullseye may not be Coe 90 and that CoE does not equal compatibility.  The same question may be asked about whether Youghiogheny Y96, Wissmach W96 and Oceanside are compatible with each other.

What is CoE

It is important to know what CoE means before the question can be answered.  It is a measure of average expansion from 20°C to 300°C.  This is suitable for crystalline materials as their low temperature expansion rates can be projected onto the behaviour of the material until near molten temperatures.  However, it is not suitable for non-crystalline materials, such as plastics or glass, as their behaviour is much more unpredictable as the temperature rises.  Measurementsof CoE have been made of glass at the glass transition temperatures which show at least seven times greater expansion near the annealing temperature than at 300°C. 

An extended essay on compatibility written by Lani Mcgregor is here, 

and here

Compatibility Tests

The degree of compatibility is uncertain between different manufacturers.  Each manufacturer will take their own way toward balancing the viscosity with the CoE.  While they can say their glass has similar characteristics to another manufacturer’s glass, they cannot guarantee compatibility.

When using glass from different manufacturers together, the best advice is to test the glasses yourself for compatibility. Do this before you commit to the project.  Bullseye notes how they do their stress tests on the education section.  I have been unable to ascertain how other manufacturers test for compatibility within their range of fusing glasses.  Another simple method of testing for stress is here.

There are reports that W90 and Bullseye work together and others that say they don’t.  There are those that say the 96 CoEs work with Oceanside, and those who say they don’t. Testing for yourself is the only way to know what works.

Scale

It seems that combining different manufacturers’ glasses may work at smaller scales, but less well at larger.  Since very few people test for compatibility before, or after, when combining different manufacturer's glasses, they don't know whether their pieces are showing signs of stress. Just because the pieces do not break immediately does not mean they are compatible or stress free. 

Size, Shape and Quantity

You should also note that the relative sizes and shapes of the combined glasses effect the survivability (rather than compatibility) of the piece.

Shape

The shape of the main piece has an effect.  Circular or broad ovals can contain the stress much more easily than a long rectangle or a wedge-shaped piece.

The same applies to the pieces added.  Pointed pieces concentrate the stress more than rectangular ones.  The stress from circular additions are easier than rectangles for the base piece to hold.

Placing

Where you place the additions is important.  Anything placed near the edge of the base is more likely to cause enough stress that it can not be contained and so the piece breaks.

Mass

How much of another manufacturers’ glass are you putting on the base?  The bigger the area or the thicker the piece(s) the less well the base will be able to hold the stress before breaking.

CoE Useage

Does anyone know what CoE means?

·         First the proper abbreviation is CoLE.

·         This means Coefficient of Linear Expansion.

·         A coefficient is an average.  This number may be exact at a given temperature, or an average over a range.

·         Linear is the length.  

·         Expansion is measured in fractions of a metre e.g., 0.0000096 metre.

·         The coefficient is given as the average amount of expansion per each degree Celsius.
     The temperature range used is 20C to 300C.  Expansion characteristics vary greatly at higher temperatures.

So CoE is the average amount (in metres) that glass expands for each degree (Celsius) increase in temperature from 20C to 300C. 

Whether you call it CoE or CoLE is immaterial, as it still does not equal compatibility.

It does not measure viscosity. Viscosity is a (possibly the major) element in making a range of compatible fusing glasses.

It does measure expansion rates, but up to 300C only.  It does not tell you how glass expands above that temperature.  Note: it does not behave in a linear pattern as crystalline materials do.

The CoE must be adjusted to match the viscosity to achieve compatible glass.  Spectrum has stated that their glass has a range of CoE of at least ten points to make compatible fusing glass.  Bullseye have stated their range to be 5 points. They also have indicated their base glass is nearer to 91 than 90.  

The only constant required in fusing glass is compatibility. 

CoE varies within each manufacturer’s range of fusing compatible glass to match the viscosity. And remember the CoE of glass at the critical annealing point is  higher than the low temperature expansion rate. See this post for details.

Viscosity varies according to the materials used in the colouration of the glass and their proportions, requiring the glass manufacturer to make adjustments in CoE to get compatible fusing glass.  More information here.

CoE does not mean compatibility.  It does not measure volume expansion at the glass transition point.  It does not measure the most important element – viscosity.  It is not even the correct term for the measure – CoLE is.

Since CoE does not mean a fusing compatible glass, its continued use can lead people (especially novices) to believe the simple number means any glass labelled with that number will be compatible with others so labelled.  This leads to unexpected incompatibilities for newcomers to the field.

My plea is: STOP USING COE TO MEAN COMPATIBILITY.

What can you use instead? It is easy – use the manufacturer’s name.  Where the manufacturer is making more than one range of fusing compatible glass use the manufacturer’s nomenclature.

Please: STOP USING COE TO MEAN COMPATIBILITY.

"CoE Equals Compatibility" - Kiln Forming Myths 10

CoE equals compatibility.

This is as persistent myth.  CoE is an abbreviation for Coefficient of Linear Expansion.  It is not an abbreviation for Compatibility.  

Apparently, CoE is used by manufacturers of glass that is being marketed to capitalise on the popularity of fused glass without the necessity of carrying out the testing and quality control required to ensure compatibility.  It is also used as a marketing device by wholesalers and retailers possibly to make greater sales.  It is used by individuals who have been lead into sloppy thinking about the materials they are using.

There are several facts to reinforce the assertion that CoE does not equal, nor is a shorthand for, compatibility.

·         Glass marketed as CoE90 or CoE96 has to be tested by the user.  Many users have often found that the compatibility with their other glass is suspect and inconsistent. This comes from breakages that occur with one sheet of glass but not another.

·         The System 96 range was made by two glass manufacturers who had testing and quality control to ensure their whole range is compatible.

·         Uroboros makes fusing compatible glass that many claim to be compatible with Bullseye.  In general, that is the case.  But many have found that it is important to test the compatibility of the glasses from Uroboros and Bullseye against each other before committing to a project, as the compatibility is not (and cannot) be guaranteed.

·         Not all float (window) glass is compatible between manufacturers.  Even the coloured glass is marketed with a range of 6 CoE points.  And some float glass is not compatible with the accessory glass. There is even a float glass that has a CoE of 96, but it is nowhere near compatible with System 96 glass.

·         There are physical reasons too.  Coefficient of Linear Expansion is tested as the average expansion between 20°C and 300°C.  This is the brittle range for glass.  We are much more interested in what happens at the glass transition point – the small range of temperature where the glass changes from a viscous liquid to a solid – generally between 480°C and 530°C. 

·         At the glass transition there is a surprising (to me) reduction in the CoE before a rapid rise.  This variation is influenced by the viscosity of the glass.  Also, at this temperature the CoE is much higher than at the measured region and cannot be taken as a guide to what is happening at the transition point.

·         In the early attempts to make compatible glass for fusing, it was discovered that the closer to the same CoE the glass was made, the less compatible it became.

·         Viscosity is the important element in the making of compatible glass.  The change in viscosity at the glass transition point must be balanced with the expansion characteristics of the glass.  A more viscous glass requires to be balanced by a different CoE glass than a less viscous one. Thus the CoE is being adjusted – not the viscosity – to balance the forces within the glass.

·         Finally, I believe the CoE of Bullseye’s clear glass is actually 90.6 rather than 90, so if we are rounding, Bullseye might be called CoE91. 

Whether the clear CoE90 or CoE96 of other manufacturers is the same as the Bullseye, System96, or Uroboros is not the relevant point.  The relevant point is whether it is compatible.  Whether these other companies have the quality control to ensure all their glass is compatible with the claimed fusing glass without further user testing is the essential point.  At this time, it appears that they do not have that capacity.  So, those using glass marketed as CoE90 or CoE96 will need to continue to test for compatibility with each sheet they use.

Other posts on Compatibility are here:
Is Coe Important?
What is Viscosity?
CoE varies with temperature
Defining the glass transition stage

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Mixing COE

Our use of Coe as an equivalent for compatibility can lead to difficulties. The only compatibility that can be relied on is that given by the manufacturer. No manufacturer can attest to the compatibility of another manufacturer's glass. They can only verify their own.

So, if you mix manufacturers' glass even though advertised as the same COE, it does not make them compatible. There is much more than expansion rates that goes into compatibility. You need to test different manufacturers' glass against each other before you use it.

These are notes on aspects of compatibility.

COE

Importance of COE

COE variations

Viscosity

Annealing

Is CoE Important?

CoE is more important to the manufacturer (in combination with viscosity) than to the kiln worker. It has gained a heightened profile, as it has been used as a shorthand for compatibility. So it is important to know what CoE is and what the numbers mean.

“During heat transfer, the energy that is stored in the intermolecular bonds between atoms changes. When the stored energy increases, so does the length of the molecular bond. As a result, solids typically expand in response to heating and contract on cooling; this response to temperature change is expressed as its coefficient of … expansion. 





“The ... expansion coefficient is a thermodynamic property of a substance. It relates the change in temperature to the change in a material's linear dimensions. It is the fractional change in length [metres] per degree [C] of temperature change [expressed as a two digit whole number]. 





“Most solids expand when heated. The reason for this is that this gives atoms more room to bounce about with the large amount of kinetic energy they have at high temperatures. Thermal expansion is a relatively small effect which is approximately linear in the [absolute] temperature range.”

Source

What does CoE mean?

There are at least two types of expansion with increasing temperature. One is volume expansion and the other that we are more interested in, is the linear expansion. “The Coefficient of Linear Expansion of a substance is the fraction of its original length by which a rod [or sheet] of the substance expands per degree rise in temperature.” Source 

What do the numbers mean?

The numbers attached to a CoLE -usually referred to as CoE – are an expression of the average amount that a material expands per degree over a given temperature range. The standard temperature range is 0ºC to 300ºC and the unit of length is one metre. They are expressed as a two digit number times 10 to the power of -6. That means the two digit number really has 6 decimal points in front of the whole number. So a CoLE of 85 means the same as an expansion rate of .000085 metres per degree C; or .0085mm/ºC.

However the rate of expansion is not a straight line when graphed against higher temperatures. The ranges in which kiln formers work show an erratic and much higher rate of expansion. Have a look at the CoE ranges at different temperatures to see how variable the expansion rates are at elevated temperatures.  Other examples are:

Graph showing the change in the CoLE of aluminium between 0ºC and 527ºC (Kelvin being about 273 degrees lower than Celsius)

This graph shows a material that actually contracts briefly as it warms.  Its CoLE would be between 20 and 35 - an extremely low rate of expansion.

This shows an idealised material that has a CoLE of  about 40 at 0ºC and around 60 at 300ºC, remaining thereabouts as the temperature rises toward 1200ºC

Should We use CoE?

CoLE is “a meaningless number unless defined by the temperature range in which the measurement is taken. Calling any glass or glass combination “compatible” without specifying under what conditions is no more useful than identifying a glass by its COE without specifying the relevant temperature range.” [L. MacGreggor]