Testing for Stress

Testing for stress is one of the most important elements in kilnforming.  It may not look like there is stress when there is considerable amounts.  The non-destructive tests are outlined in this Power Point presentation, prepared some time ago, to describe why and how stress testing can be conducted.  There is no commentary.

 

Stress Analysis of Broken Glass

Will stress still show with polarised filters on cracked and broken glass?

It's not a straightforward answer.

I was looking at some broken fused float glass a few years ago.  I had always subscribed to the idea that a fracture relieves the stress. Not always. The broken float glass had been slumped, and the pieces still showed stress.  This turned out to be a compatibility problem, although both layers were float.  

The stress of inadequately annealed glass is likely to remain visible through the filters, because inadequately annealed glass will have stress distributed across the whole piece.  But glass that has been cooled too quickly and suffered thermal shock, is more likely to show minimum stress because the break relieved most of it.

It is likely stress will show on the tree piece pictured because it has not completely broken a[art. And even when it does break, it may still show a residue of stress.

It is sensible when trying to diagnose the problem to perform a strip test of the glasses for compatibility of the glasses concerned to be sure what is happening. If no stress shows on the test strip, the stress showing on the cracked piece is unlikely to be from incompatible glass, and other factors need to be considered.

Photo credit:  Debi Frock-Lyons 

Tack Fusing Considerations

Initial Rate of Advance

Tack fuses look easier than full fusing, but they are really one of the most difficult types of kiln forming. Tack fusing requires much more care than full fusing.
On heat up, the pieces on top shade the heat from the base glass leading to uneven heating. So you need a slower heat up. You can get some assistance in determining this by looking at what the annealing cool rate for the piece is. A very conservative approach is needed when you have a number of pieces stacked over the base layer.  One way of thinking about this is to set your initial rate of advance at approximately twice the anneal cool rate. 

Annealing 

The tacked glass us loosely attached rather than fully formed together.  So, the glass pieces are still able, partially, to act as separate entities, meaning excellent annealing is required.

Effects of thicknesses, shapes, degree of tack

1. Tack fusing of a single additional layer on a six millimetre base

1. Rectangular pieces to be tack fused

1. Sharp, pointed pieces to be tack fused

1. Multiple layers to be tack fused

1. Degree of tack – the closer to lamination, the more time required

Glass contracts when it's cooling, and so tends to pull into itself. In a flat, symmetrical fuse this isn't much of a problem. In tack fuses where the glass components are still distinct from their neighbours, they will try to shrink into themselves and away from each other.  If there is not enough time for the glass to settle into balance, a lot of stress will be locked into the piece that either cause it to crack on cool down or to be remarkably fragile after firing.  In tack fusing there also are very uneven thicknesses, making it is hard to maintain equal temperatures across the glass.  The tack fused pieces shield the heat from the base, leading to localised hot spots during the cool down.

On difficult tack fuses it's not unusual to anneal for a thickness of two to three times greater than the thickest part of the glass.  That extended cool helps ensure that the glass has time to shift and relax as it's becoming stiffer, and keeps the temperature more even throughout.

In general, tack fused pieces should be annealed as though they are thicker pieces. Recommendations range from the rate for glass that is one thickness greater to at least twice the maximum thickness of the whole item.  Where there are right angles - squares, rectangles - or more acutely angled shapes, even more time in the annealing cool is required.

It must be remembered, especially in tack fusing, that annealing is much more than the annealing soak.  The soak is to ensure all the glass is at the same temperature, but it is the anneal cool that ensures the different thicknesses will all react together. That means tack fusing takes a lot longer than regular fusing.

  

The more rectangular or pointed the pieces there are in the piece, the greater the care in annealing is required.  Decisions on the schedule to use varies - some go up two or even four times the total thickness of the piece to choose a firing schedule.

A simple way to determine the schedule is to subtract the difference between the thickest and the thinnest part of the piece and add that number to the thickest part. If you have a 3mm section and a 12mm section, the difference is 9mm. So, add 9 to 12 and get 17mm that needs to be annealed for. This thickness applies to the heat up segments too.

Another way to estimate the schedule required is to increase the length the annealing schedule for any and each of the following factors:

The annealing schedule to be considered is the one for at least the next step up in thickness for each of the factors. If you have all five factors the annealing schedule that should be used is one for at least 21mm thick pieces according to this way of thinking about the firing.

 

4 – Testing/Experimentation

The only way you will have certainty about which to schedule to choose is to make a mock-up of the configuration you intend in clear.  You can then check for the stresses.  If you have chosen twice the thickness, and stress is showing, you need to try 3 times the thickness, etc., which can be done on the same piece.  You can reduce time by having your annealing soak at the lower end of the annealing range (for Bullseye this is 482C, rather than 516C).

You will need to do some experimentation on what works best for you. You also need to have a pair of polarisation filters to help you with determining whether you have any stress in your piece or not. If your piece is to be in opaque glasses, The mock-up in clear will be useful.

First published 18.12.2013

Revised 29.01.25

Slumping Splits

 This is a description of the analysis process to determine the possible causes of a split during a slump.     

Credit: Maureen Nolan

Observe the piece.

It is a tack fused piece, about 20cm (8") square, which has been slumped. 

The base layer is of clear. The piece has three additional layers, but the fourth layer is only of small glass dots and rectangles.  The central, heart, area is made of three layers.

A split has appeared during the slump. It is split irregularly through pieces rather than around them.  It is split through the thickness but only partially across the piece.

In one area the (brown) third of four layers spans the split.  Further to the left a brown second layer seems to have broken, but still spans the split.

Threads and particles of glass are connecting across the split. 

The edges are probably sharp, although only so much can be deduced from a description and one photograph.

History of the Piece

The tack fused piece has been put in a mould to form a platter and has split during the slump.

The schedule in essence was:

139ºC/250ºF to 565ºC/1050ºF for a 30’ soak (some pauses but all at a ramp rate of 139ºC/250ºF)

83ºC/150ºF to 688ºC/1270ºF for 10’

222ºC/400ºF to 516ºC /960ºF for 60’

111ºC/200ºF to 427ºC/800ºF for 10’

167ºC/300ºF to 38ºC/100ºF, off

 

The assumption is that the tack fused piece received a similar annealing soak and cool.

 

Diagnosis

Too fast

Slumping a tack fused piece of three layers plus decorative elements on top needs to be fired as for 19mm (6 layers) minimum (twice the actual).  My work for the Low Temperature Kilnforming* eBook showed best results are achieved by slumping as for one more layer (21 mm/0.825” in this case).  This gives a proposed schedule of:  

120ºC/216ºF to 630ºC/1166ºF (not 688ºC/1270ºF) but for 30 to 45 minutes

AFAP (not 400ºF) to anneal 516ºC/960ºF for 3.5 hours (not 1 hour)

20ºC/36ºF to 427ºC/800ºF, 0

36ºC/65ºF to 371ºC/700ºF,0

120ºC/216ºF to room temperature

 
Commentary on the proposed schedule:

The slump is relatively shallow, so a low temperature with a long soak is the most suitable schedule for this piece.  The drop to anneal is at a sedate rate of 222ºC/400ºF.  This is inappropriate, generally.  Just as there is a rapid rate to top temperature to avoid devitrification, so there needs to be an AFAP drop to anneal, also to avoid devitrification.  The anneal soak was not the cause of the break, but it is worthwhile noting the recommended anneal soak and cool rates are longer and slower than that used.  This is a note for the future.

 

Suspect Tack Fuse

If the tack fuse schedule was like the slump schedule, the slump was started with inadequate annealing in the previous firing.  More importantly, the evidence for an inadequate tack fuse is that the split under the brown rectangle was through the clear and red on top, but the split left the brown intact.  This means it was not securely fixed to the red below it. 

 

If the condition of the tack fuse is not sound, it is probable that difficulties will be experienced in the slump.  The poster commented “… why do [these splits] happen only when slumping – it came through tack just fine.”    It is probable the tack fuse was not “just fine”.  The way to be sure the previous firing was just fine, is to test for stress.

 

There is enough clear in this piece that an inspection for stress could be conducted by use of polarising filters before the slump.  Testing for stress is a simple viewing of the piece between two sheets of polarised light filters.  Doing this test will give information on the amount of stress, if any, in the flat tack fused blank.

 

Slump Split

During slumping the glass is subjected to more movement and therefore stress than while being fired flat.  The glass is often only barely out of the brittle zone when being slumped and that makes the stress more evident during the early part of the slump. This requires careful inspection of the failed piece.

 

Look at the glass surrounding the split.  My opinion is that the edges are sharp.  If rounded, the threads of glass from the edges of white would have melted to the edges of the split rather than spanning it. 

 

It appears the top layers were hot enough for less viscous glass on top to form stringers that span the break as the underlying layers split.  It is probable that the split was during the plastic phase of the slump for the upper glass, but  the lower layers were not as hot and suffered thermal shock. 

 

This split of lower layers, while the overlying ones are whole, is often seen in tack fuses, although the top ones do slump into the gap as the firing proceeds.  In a slump there is not enough heat, time or space, for the brown piece to slump into the gap.  Both splits appear to be a result of too rapid firing.  In the flat fusing work, the split results from too fast a ramp rate during the brittle phase of the glass.  But the slumping splits appear to occur after the brittle phase, almost as a slow tear in the glass. This may result from the differential heating of the layers if not fully combined.  It may also indicate the split developed slowly. 

 

One other observation is that these splits seem to be more frequent during the slumping of tack fused pieces.  As speculated above, it may be the inadequate tacking together of the pieces of glass during the first firing, which can form a discontinuity in transmitting heat.  And it may be that the different thicknesses across the tack fused piece allow stress to build from differential heating of the glass.

 

Rates

 

Whichever of these speculative effects may be true, it appears the ramp rates are suspect.  As mentioned elsewhere* (and in Kilnforming Principles and Practice to be published soon), the reasons for these splits are not fully known.  Even microscopic examination by Ted Sawyer has not produced a satisfactory explanation.  The only practical approach that has been successful is to slow the ramp rates.  However, the appearance of these splits is essentially random (with our current understanding), so prevention is difficult.

 

Conclusion

The probable cause of the split in the slump has been that the ramp rates were too fast.  This may have been made worse by the too short anneal soak, and the too fast cool of the tack fused blank.

 

Remedy

There is no practical rescue for this piece.  Prevention in the future is to use ramp rates that are for at least one layer thicker, if it is full fused.  If it is tack fused, firing as for twice the thickest part plus one additional layer is advisable to slow the ramp rates, allowing all the glass to heat and form at the same rate.

 

 

*Low Temperature Kilnforming; an Evidence-Based Approach to Scheduling.  Available from:

Bullseye

and

Etsy

Relative stress in Tack and Full Fused Glass

There is a view that there will be less stress in the glass after a full fuse than a tack fuse firing.

This view may have its origin in the difficulties in getting an adequate anneal of tack fused pieces and the uncritical use of already programmed schedules. There are more difficulties in annealing a tack fused piece than one that has all its elements fully incorporated by a flat fuse. This does not mean that by nature the tack fused piece will include more stress. Only that more care is required.

Simply put, a full fuse has all its components fully incorporated and is almost fully flat, meaning that only one thickness exists.  The annealing can be set for that thickness without difficulty or concern about the adequacy of the anneal due to unevenness, although there are some other factors that affect the annealing such as widely different viscosities, exemplified by black and white.

Tack fused annealing is much more complicated than contour or full fusing.  You need to compensate for the fact that the pieces which are not fully fused tend to react to heat changes in differently, rather than as a single unit.  Square, angled and pointed pieces can accumulate a lot of stress at the points and corners. This needs to be relieved through the lengthening of the annealing process.

The uneven levels need to be taken into consideration too.  Glass is an inefficient conductor of heat and uneven layers need longer for the temperature to be equal throughout the piece.  The overlying layers shade the heat from the lower layers, making for an uneven temperature distribution across the lower layer.

The degree of tack has a significant effect on annealing too.  The less incorporated the tacked glass is, the greater care is needed in the anneal soak and cool.  This is because the less strong the tack, the more the individual pieces react separately, although they are joined at the edges.

If you have taken all these factors into account, there will be no difference in the amount of stress in a flat fused piece and a tack fused one.  The only time you will get more stress in tack fused pieces is when the annealing is inadequate (assuming compatible glass is being used).

More information is given on these factors and how to deal with them in this post on annealing tack fused glass and in the eBook Low Temperature Kilnforming available from Bullseye and Etsy.

Revised 5.1.25

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 in Slumping - diagnosis

Diagnosis of breaks during slumping processes is often difficult because the temperature is not high enough to be able to apply the usual rule of 

• sharp edges indicate breaks on the cool down; 
• rounded edges indicate breaks on the heat up.

www.warm-glass.co.uk

This not a universally applicable diagnosis.

At low slump temperatures, the edges will be sharp in both a break on heating up, and on the way down in temperature.

The best test to determine when the break occurs is to observe periodically during the heat up.  You will be able to see if the piece breaks before the top temperature.  If it is whole at top temperature, the break occurred on the way down.

If you have been unable to observe the progress of the firing, you will need to diagnose when the break occurred from the clues left.  The test here is not whether the edges are rounded or sharp, because at normal slumping temperatures, the break will be sharp in both cases. 

If the break occurred before the top temperature, the pieces will shape separately as the will be on different parts of the mould. Therefore, If the pieces no longer fit together, the break was on the rise. If they do fit exactly, the break was on the way down.  Place the pieces very carefully together to see if they form part of a continuous curve.  If they do, the break was on the cool down.  If they almost  match, or do not match at all, then the break was on the rise in temperature.

In general, when the break is on the cool down, there has been an overhang of the flat glass onto the mould which causes the break.  But the most common break of a slumping piece is caused by a too quick rise in temperature.  The distance the pieces are apart will give an indication of the force of the break.  The farther apart the pieces are, the slower the ramp should be - either up or down.

For a flat 6mm piece, the slump temperature rise should be less than twp thirds as quick as the rise for the fusing.  If you have a tack fused piece to be slumped you should reduce the rate of advance to at least half of that for a smooth, flat piece of 6mm.  Thicker glass with tack fused elements will need to be even slower.

Draping is another story.

Revised 28.12.24

Wire in glass

 

The cracks around the wire imbedded in the glass in the above image are not incompatibility cracks. They do not surround the square piece that traps the wire into the glass. These are from differential expansion/contraction stress between the wire and the glass. 

 

Picture credit: Charmaine Maw

This picture shows the stress that a single strand of wire will induce in glass (the bright light around the wire).  Wire is never going to have similar characteristics to the glass, so the glass must be strong enough to contain the resulting stress.  Anything that increases the mass of the wire, such as twisting or spirals, will increase the stress. 

 

Kanthal and nichrome wires are best as included wire hangers. They are designed for high temperature work and so do not weaken from the heat. This means that high temperature wire as thin as 0.5mm/22 gauge can hold a lot of weight.  Much greater weight than is used in most glass objects to be hung rather than fixed.

Keep the wire as a single strand and as thin as possible, consistent with sufficient strength.  Hammering wire flat can also help reduce the stress by thinning it.

Profile

A sharp tacked piece needs to be fired as though thicker. This example is a single layer base and a square of glass to trap the wire fired to a sharp tack.  It needs to be fired as though 2.5 times the thickest part - 15mm.  A rounded tack fuse of the same layup would need to be fired as for 12mm.

Layup

The use of wire in glass needs to consider how the air will escape from around the wire.  Yes, if the wire exits the glass, there is a channel for it to dissipate.  But air tends to collect along the length of the wire.  If the wire is fully enclosed in the glass, the layup must accommodate the need for air escape routes.  This might be with a fine layer of powder, design elements, chips of glass to hold the outer edges of the glass up for longer, or other devices.

 

Scheduling

The example shown at the start of this blog, is a sharp tack and needed the 2.5 times scheduling.  That probably would have avoided the crack in the single layer base.  That single layer cools faster than the wire with the added piece of glass.  A bubble squeeze is a good idea, even though it would not normally be considered.  This gives the best chance of reducing the bubbles that form around the inclusion.

 

You need to be careful about increasing the ramp rate until the glass has passed out of the brittle phase.  This is about 540˚C/1005˚F. The increase in the ramp rate during the brittle phase may cause cracks. It is, of course, more likely to occur during cooling because the metal will be contracting more than the glass during the brittle phase.  This contrast in contraction rates induces stress that may be great enough to crack or break the glass.

 

 

Slow Rates to Annealing

"I have seen recommendations for slower than ASAP rates from the top temperature, but most schedules say 9999 or ASAP.  Which is right?"

Slow drops in temperature from top to annealing temperatures risk devitrification. Accepted advice is to go ASAP to annealing temperature to avoid devitrification forming.

Breaks do not occur because of a too rapid drop from top temperature to annealing. The glass is too plastic until the strain point has been passed to be brittle enough to break. On the way down that will be below an air temperature of 500˚C/933˚F.

credit: ww.protolabs.com

Different kilns cool from top temperature at different rates. Ceramic kilns are designed to cool more slowly and may need assistance to cool quickly.  This is usually by opening vents or even the door or lid a little. Glass kilns are designed to lose temperature relatively quickly from high temperatures. They do not need a crash cooling as ceramic kilns may need in certain circumstances.  Of course, crash cooling may be necessary for some free drops and drapes.

The length of the soak at annealing is determined by the effective thickness of the piece.  Tack fusing needs to be annealed for thickness as a factor of 1.5 to 2.5, depending on profile.

The extent to which you control the cooling to room temperature after the anneal soak is dependent on the calculated thickness of the piece you are cooling. The objective is to keep the internal temperature differential to 5˚C/10˚F or less to avoid expansion/ contraction differences that are great enough to break the piece. Those rates are directly related to the required length of the anneal soak.  Those rates can be taken from the Bullseye chart for Annealing Thick Slabs. The Fahrenheit version is is available too.

An example.  If you have a 2 layer base with 3 layers (=15mm) stacked on top for a rounded tack fuse, you need to fire as for at least 30mm. This will require controlled cooling all the way to room temperature.

• ·        The rate to 427˚C /800˚F will be19˚C /34˚F
• ·        The rate to 370˚C /700˚F will be 36˚C /65˚F
• ·        The final rate 120˚C /216˚F to room temperature.

You may need to wait a day before any coldworking. An example from my experience shows the necessity.  I checked a piece for stress a few hours after removing the piece from the kiln when it felt cool to the touch. It puzzled me that stress showed, although it didn't on similar pieces.  The next morning, I went to check if I misunderstood the reading. Now, a full 15 hours after coming out of the kiln, there was no stress.  The example shows that the glass internally is hotter than we think. And certainly, hotter than the air temperature.

In the temperature regions above the strain point, the glass needs to be cooled quickly. In the annealing region and below the glass needs to be cooled slowly.

More information is available in the eBook Low temperature Kilnforming.  This is available from Bullseye or Etsy