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”.

CoE and Temperatures

CoE as a Determinant of Temperature Characteristics

What CoE Really Tells Us

The wide spread and erroneous use of CoE to indicate compatibility (it does not) seems to have led to the belief that CoE tells us about other things relating to the characteristics of fusing glasses.  It is important to know what CoE means.  

First it is an average of linear expansion for each °C change between 0°C and 300°C.  This is fine for metals with regular behaviour, but not for glasseous materials where we are more interested in the 400°C to 600°C range.  Measurements there have shown very different results than at the lower temperatures at which CoLE (coefficient of linear expansion) are measured.  In kiln forming we are also interested in volume changes and CoE tells us nothing about that.

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

And not even relative temperatures.  

Some examples: 

• Uroboros FX90 has an annealing point of 525C compared to Bullseye (516/482C), and to the Wissmach 90 anneal of 510C. 
• Wissmach 90 has a fuse temperature of 777C compared to Bullseye's 804C.  
• Another example is Kokomo with an average CoE of 93 which has an annealing range of 507-477C and slumps around 565C. 
• There is a float glass of a CoE of 90 that anneals at 540C and fuses at 835C.  
• Artista (which is no longer made, except in clear) had a Coe of 94 with an annealing point of 535C and fuse of 835C, almost the same as float with a Coe of 83. 

These examples show that CoE can not tell you the temperature characteristics of the glass. These are determined by a number of factors of which viscosity is the most important. More information can be gained from this post on the characteristics of some glasses, or from testing and observation as noted in this post .

CoE does not tell you much about compatibility either, since viscosity is more important in determining compatibility.  CoE needs to be adjusted and varied in the glass making process to balance the viscosity of the glass.  Viscosity is described here .

This post and its links describes why Coe is not a synonym for compatibility. 


What CoE REALLY tells us is that we look for simple answers, even when the conditions are complex.  

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]

Coe and Annealing

If you have changed CoE (i.e., the manufacturer), then the annealing temperature is different. If you don't correct that, it's never going to work quite right.

 

I have several problems with this statement.

CoE does not determine the manufacturer. There are several manufacturers who claim to manufacture fusing glass to the same CoE.

No manufacturer makes to one CoE. All manufacturers have to vary the CoE of a particular glass to balance its viscosity. The CoE is a dependent variable. It depends on what the viscosity of the colour is. Spectrum at one point stated their System96 glass had a 10-point variation in CoE number. Oceanside will be no different. Bullseye have stated a 5-point difference. Other manufacturers have not stated their variations.

No manufacturer can guarantee compatibility with another’s. This is because the ingredients to make a fusing range of glass varies from one manufacturer to another. These variations can make the glass incompatible. To determine if you can combine two glasses from different manufacturers you need to do the compatibility testing yourself. The CoE number does not determine the temperature characteristics of the glass either. 

Annealing 

Having got my disagreements with the statement out of the way, I can go on to looking at differing annealing temperatures. There is a difference between annealing point and annealing temperature.

Annealing Range

Annealing occurs over a relatively small range between the softening point at the higher end to the strain point at the lower end of the range. The softening point is the temperature, above which the glass is so plastic that it cannot be annealed. The strain point is the temperature at which the glass becomes so solid than no annealing can occur below it.

Annealing Point

The annealing point is mathematically determined as the point at which the glass most quickly relieves the stresses within it. That temperature is determined by the viscosity of the glass. It is known as the glass transition point, and is expressed as Tg. In practice there are advantages in annealing at or below the published annealing point.

A soak above the annealing point is of no effect. Any equalisation of temperature that occurs on that soak is negated by the drop to the annealing point. It is better to spend the cumulative soak/hold time at the (lower) annealing temperature.

Annealing Temperature

The average annealing point for Bullseye is 516°C/962°F. Different formulations of their fusing compatible glass have different Tg temperatures. Research showed the best results for their thick glass is 482°C/900°F. Other research in academic institutions has shown that annealing at the lower part of the range provides a denser and stronger finished glass piece. This applies to thick as well as thin glass.

Bullseye has chosen to use a temperature 34°C/61°F below the average annealing point, based on their research. This is still about 7°C/13°F above the strain point. This approach can be applied to any fusing glass.

The strain point is approximately 43°C/78°F below the mathematically determined annealing point. If you know the annealing point you can choose to anneal – i.e., equalise the temperature of your glass – up to 30°C/54°F below that. 

This has a practical demonstration. Wissmach for some years designated 510°C/950°F as the annealing point for W96. A few years ago, they changed their recommended annealing temperature to be 482°C/900°F. The annealing results are good at both temperatures. The difference is that the annealing soak is for a in longer time at the lower than at the higher temperature. But it still provides a shorter annealing cool.

Firing with different anneal points

This apparent diversion - into annealing ranges - shows that it is possible to anneal glass with slightly different glass transition points at the same temperature. You may compromise a little for one glass or the other. You will also use longer times at the annealing temperature.

The annealing soak of Oceanside and Wissmach96 could both be at 482°C/900°F. Or, if it felt safer, it can be an average of the two. The average of the difference would make the annealing soak at 496°C/926°F. You would use a longer soak at this temperature than at the higher one. The safest would be to hold for an hour instead of 30 minutes for 6mm/0.25” of glass.

However, if the annealing point differs greatly, it is much more difficult. For example, float glass with an annealing point of 540°C/1005°F would be difficult to fit in the same firing with most fusing glass because of the wide range of official annealing points.

 

It is possible to anneal different glass at the same time if the annealing points are not widely different. Compromises need to be made.

 

Notes on Polarised Light Filters

Polarised light filters are used to detect stress in a non-destructive testing method in kilnforming.  The use of the filters is described in this blog. To produce consistent reliable results, there are certain conditions.

 The light source needs to be diffused in such a way that it is even across the viewing area.  An intense, single point light makes it difficult to determine the relative intensity of apparent stress. Another tip is that you can use your phone or tablet as a source of diffused light and as the bottom filter.  It emits polarised light, meaning only a top filter is needed.

Stress halos from broken and fused bottles

 It is important that the glass being tested is of the same temperature throughout to get a meaningful result.  This was emphasised to me when I was running a series of tests. I got in a hurry to test for stress to be able to start the next trial quickly.  I began to notice inconsistencies in the amount of stress I recorded for results of the series of tests.  Going back to the stressed test pieces, showed different stress levels when they were cold from when they were warm.

 The conclusion is that the glass to be tested for stress must be the same temperature throughout.  Even if it is only slightly warm, the apparent stress will be exaggerated.  It may be that the testing can only be done 24 hours after removed from the kiln.

 Stress will be more evident at points and corners.  The light will be brighter at highly stressed points, and even at extreme stress exhibit a rainbow effect.  More generalised stress is evident in a lighter halo.

Stress points in a drawing square illustrating the concentrated stress at corners

 It is much more difficult to check for stress in opaque areas of a piece.  If there are transparent areas, the stress will show there, although the stress may originate in the opaque ones. To be aware of potential stress in the combination of opaque glass, strip tests must be conducted on samples of the glasses. 

 Remember to include an annealing test too, as the stress test does not distinguish the type of stress.  If the annealing test shows stress, the annealing was inadequate. It is of course, possible that the glass is stressed because of incompatibility.  But the only way to determine that is to fire another test with a longer soak at annealing.