Float Annealing Temperatures

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

Comparisons of various float glasses

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

Pilkington Float

CoLE 83 *^10-5

Softening point:  715°C

annealing point:  548°C

strain point: 511C

Pilkington Optiwhite ™

Softening point:  ca. 732°C

annealing point:  ca. 559°C

strain point:  ca. 526°C

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

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

CoLE 91 * 10^-5

Softening point: 600°C

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

CoLE 90 * 10^-5

annealing range:  520 - 550°C

Low E glass

softening – 840°C

strain - 617°C

R glass (sound reducing)

softening – 986°C

strain - 736°C

D glass (decorative)

softening point – 769°C

Compatibility

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

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

Annealing

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

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

Over Annealing

 I hear the comment "you can't over anneal" all the time. Is it true?

My response to this may be controversial, and I do expect there will be some dispute with aspects of what follows.  My view of the statement “you can’t over anneal” is that it results from a lazy approach to thinking about the process.

The short answer is, in my view “yes, you can over anneal”.

• ·         Lengthy anneal soaks can induce stress in certain circumstances. More later.
• ·         Excessive annealing soaks waste time and money.
• ·         Annealing is more than the soak.  It is a combination of equalisation of the heat within the glass (not just temperature) and the gradual cooling of the glass to below the lower strain point to ensure the glass does not incorporate differences of temperature of plus or minus 5°C.

There is both tradition and research to assist in determining the length of the anneal soak.  The tradition seems to embrace 30 minutes anneal soak for each 3mm-layer of glass. The research has been done by Bullseye and they have developed a table to assist in accurately determining annealing soaks for thick glass. 

It informs users of the relationship between thickness and annealing soaks and cooling.  The table starts at 6mm/0.25" thick, and goes up to 200mm/8" thick.  The annealing soak temperature used needs to be altered for glass other than Bullseye, but the soaks, rates, and temperatures remain valid for all fusing glasses. Use the research, rather than tradition.

Other considerations include the nature of the kiln.  If your kiln has significant temperature differentials across the shelf, long annealing soaks will incorporate those differences during the annealing cool and result in a stressed piece. You do know the temperature distribution within your kiln, don’t you?  This Tech Note #1 from Bullseye will give you the information to test for the temperature distribution. Using this information will enable you to avoid the cool spots when placing your pieces and utilise the areas where the heat is even.

Economy is another reason that it is possible to over anneal.  Soaking at the annealing temperature uses a significant proportion of the electricity consumed in a firing.  This means an overly long temperature equalisation soak will use more electricity than necessary.  It also uses more kiln time than necessary, by delaying the anneal cooling and the following natural cooling rate of the kiln.

It is possible to under anneal, of course.

You need to learn about the effects of your project on annealing requirements, because it is possible to under anneal.  The research on annealing is based glass of uniform thickness. The most popular style of kilnforming appears to be tack fusing of one degree or another.  This is unfortunate for the novice, as it is the most difficult of styles to anneal adequately. There are a lot of factors to consider when setting the annealing schedule for tack fusing. 

I feel this is the origin of “can’t over anneal” thinking.  Instead of thinking about the specific annealing difficulties, many seem to just add more time in a generally random manner.  The post on tack fusing considerations (the link above) is designed to help in thinking about the requirements of the lay-up of your piece. The cumulation of factors can easily triple the annealing soak time and slow the rates by three times. 

What is the anneal?

Another problem is that most often annealing is thought of as merely a soak at the annealing point of the glass.  It is much more than that.  The annealing point is usually the temperature at which the heat within the glass is equalised in preparation for the anneal cool.  This is because the annealing temperature is that at which the glass will most quickly anneal.  Since the anneal is temperature sensitive, the equalisation of the temperatures within the glass will be most successful at getting a good anneal throughout the cool.

For two-layer flat fused items, the annealing point can be used as the heat equalisation temperature.  The soak is to get the glass within 5°C/10°F throughout the piece.  The annealing, especially with thicker or more difficult pieces, is done closer to the lower strain point. The reasons for this is to save time in the annealing cool, it is easier to maintain the small difference in temperature, and it has been shown to produce a more dense (therefore stronger) glass.  If you look at the Bullseye annealing chart, you will see how slowly thick pieces need to be cooled, so starting 35°C/ below the annealing point can save many hours of cooling.

Once the glass has equalised in temperature, the object is to cool the glass at a rate that ensures the internal temperatures do not vary more than plus or minus 5°C/10°F across and through the piece.  The rate can increase by 1.8 times the initial cool rate after the lower strain point has been reached.  This second stage rate should take the glass to around 370°C/700°F, where the rate to room temperature can be doubled as much as six times the initial cool rate.

Difficult pieces

Tack fused and other pieces with uneven thicknesses require more care in the annealing to ensure even cooling of the whole without a greater variation in temperature than +/- 5°C.  As said above, tack fusing is one of the most difficult of styles to anneal adequately and the blog entry indicates some factors requiring more careful annealing.

As an example, a piece 6mm thick, with two layers of rectangular and pointed pieces that are just barely rounded.  This adds five factors of complications for the fusing - two levels of tack fusing, rectangular pieces, pointed pieces, laminated tack fusing.  This number of complications increases the practical thickness to 21mm – 6mm of flat base, 3mm each layer of tack (6mm), 3mm for rectangles, 3mm for pointed pieces, 3mm for laminated fuse.  Because this is tack fused, the next practical step up in the table needs to be used. That is the one for 25mm, which requires a four-hour temperature equalisation soak, and 15°C per hour initial anneal cool rate.

Of course, a simpler method can be used, as it has been researched and practiced by many people.  That is simply double the thickest part of the piece and use that thickness for the anneal soak and cool.  Sharp tack profiles need to be annealed as though 2.5 times thicker, and contour profiles only need 1.5 times the thickness.

Glass other than Bullseye

It is possible to apply these times and rates to any glass of which you know the annealing point.  The annealing soak can be set above the lower strain point, which to be safe, can be taken as a point 35°C/63°F below the annealing point.

E.g., if you are annealing a 12mm slab of float glass, the annealing point of which (in the UK) is 540°C, you chose a temperature of 505°C to do your two-hour soak, followed by a cool rate of 55°C/100°F to 427°C and then 99°C/178°F per hour for the second stage cool to 370°C/700°F.  The final cool of 330°C/595°F.  So, you can see the soak times, rates and target temperatures remain the same regardless of the glass type.  

More information on long annealing soaks in a video at 13:00 minutes.

More discussion on annealing and cooling is given in the ebook Low Temperature Kilnforming from Bullseye and Etsy.

Long Annealing Soaks

You Can’t Anneal Too Long.

Can you anneal too long?

Yes, you can.

It’s not just the possible temperature differences in the kiln.  If you have temperature differentials across your kiln, any piece that crosses those boundaries will have temperature differences locked into the glass.  If you know you have temperature differentials and your glass by circumstance must be in both the cooler and the hotter regions, you need to do a standard length of soak only.  Then reduce the rate of cooling a little more than normal, so that a slower cool occurs.  This should avoid most of the stress that can be induced by very long soaks in a kiln with hot and cool spots.

The other factor against annealing too long has been revealed by Bullseye research on annealing.  This video at about 13:00 minutes into the film explains.  This complicating factor in annealing is about the difference in temperatures of the surfaces of the glass.  The research shows that the longer you anneal the greater the differential in temperature becomes between the upper and lower surfaces of the glass.  This means that you can introduce stress across the whole piece, rather than just a section as in an unevenly heated kiln.

This comes from the recording of a typical long annealing cool.

What is more, the longer you soak, the cooler the bottom becomes in relation to the top.  The reported research is described in this video at about 13:00 minutes.  It can be assumed that the air temperature differences are the cause.  Even during cooling the air is hotter on top of the shelf than under.  This would allow the bottom surface to cool more than the top. This assumption is borne out by the fact that the effect is reduced or eliminated by having elements under the shelf.

There are two reasons to avoid long soaks. Uneven temperatures across the surface are locked into the glass.  And long soaks at annealing induce an unwanted temperature differential between the top and the bottom of the piece.

More information is available in the ebook Low Temperature Kilnforming; an evidence based approach to scheduling.

Revised 29.1.25

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 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

Effects of Annealing at the Top End of the Range

It is possible to begin your annealing at any point in the annealing range.

The annealing point is the temperature at which the glass most quickly relieves the stress within.  This occurs at the glass transition point. 

The  annealing range is between the softening point and the strain point of the glass.  No annealing can be achieved above the softening point, nor below the strain point.  This range, for practical purposes can be taken to be 55°C above and below the published annealing point.  For thick slabs, Bullseye has chosen to start the anneal 34°C below the published annealing point of 516°C.

High Annealing Point

A high annealing temperature, even up to 571°C, the approximate strain point of the glass could have been chosen, but this is impractical.  The effect of this is a greater slow cool range and so an extended anneal cool.  The reasons are as follows:  

• The anneal cool range is greater as the first rate of cool needs to be maintained to the strain point.
• The anneal cool has to extend to at least just below the strain point.
• The highest practical annealing temperature is determined by the viscosity of the glass.  Any soaks above that temperature are ineffective in production of soundly annealed glass.
• The purpose is to get all the glass at the same temperature in preparation for  cooling.  It is more difficult to maintain the small differentials in temperature achieved by the annealing soak over a large range of temperature.

Low Annealing Point

Starting the anneal cool closer to the strain point requires a slightly longer soak to ensure the glass is all at the same temperature (+/- 2.5°C, often called the Delta T=5C) before the anneal cool begins.  Typically, this initial soak would be for an hour before the initial cool begins (for a 6mm/0.25" thick piece).

Effect of the Differences in Approach

The advantages and disadvantages centre around the need to:

• soak long enough to get all the glass to the same temperature, and to 
• cool slowly enough to maintain the delta T throughout the glass.

Example

If you think of an example of a piece of Bullseye glass 12mm/0.5" thick, it will show the differences in approach.

High temperature soak

A soak of 120 minutes at 571°C/1060°F (the highest possible start for an annealing soak) is still required to even the temperature.  To ensure the temperature differentials in the glass do not deviate from the Delta T, the cool needs to be at 18°C/32°F per hour down to 427°C/800°F.  It is possible then to increase the speed to 36°C/65°F per hour down to 370°C/700°F.  This gives you a total annealing cool of just over 11.5 hours.

Low temperature soak

Starting the anneal at 482°C still requires a two hour soak followed by a decrease in temperature of 18°C/32°F per hour to 427°C, and an increased rate of 36°C/65°F to 370°C/700°F.  This gives an anneal cool time of just over 6.6 hours.

The example shows how, although the annealing result may be the same, there is considerable time saved (and especially for thicker pieces) in using the lower part of the annealing range to begin the annealing.  It also will save some electricity.

However, an anneal of two hours at 516°C with a cool of 18°C/32°F per hour to 427°C/800°F and 36°C/65°F to 370°C/700°F will still give a perfectly adequate anneal for 12mm thick pieces even though it will take about 2 hours longer.

Revised 30.12.24

Annealing Strategies

This is a power point presentation I gave a few weeks ago to a group.  It may be of interest to others.  There is no commentary.

 

Devitrification

What is it? When does it happen? Why does it happen? These are frequent questions.

Dr. Jane Cook states that devitrification is not a category (noun), but a verb that describes a process. Glass wants to go toward devitrification; a movement toward crystallisation.*

Mild devitrification is the beginning of crystallisation on the surface of the glass. It can look like a dirty film over the whole piece or dirty patches. At its worst, the corners begin to turn up or a crackling can appear on a granular surface.  This is distinct from the effects from an unstable glass or the crizzling as in a ceramic glaze. Devitrification can occur within the glass, but normally is a surface effect as oxygen is required.

Differences in the surface of glass promotes precipitation of the crystal formation of silica molecules.  This fact means that two defences against the formation of crystals are smooth and clean surfaces. There are other factors at play also.  The composition of the glass has an effect on the probability of devitrification.  Opaque glass, lime, opalising agents, and certain colouring agents can create microcrystalline areas to "seed" the devitrification process.  One part of the composition of glass that resists devitrification is the inclusion of boron in the composition of the glass, acting as a flux.

Visible devitrification generally occurs in the range of approximately 720°C – 830°C/1330F - 1525F, depending to some extent on the type of glass.  This means that the project needs to be cooled as quickly as possible from the working (or top) temperature to the annealing point, which is, of course significantly below this range.

There is evidence to show that devitrification can occur on the heat up by spending too long in this devitrification range, and that it will be retained in the cooling. Normally this is not a problem as the practice in kilnforming is for a quick advance on the heat up through this range, causing movement in the glass and so working against any crystallisation.  The quick advance does not (and should not for a variety of reasons) need to be as fast as possible.  A rate of 300°C per hour will be sufficient, as time is required for devitrification to develop.

Medical research into using a glass matrix to grow bone has shown that devitrification begins around 650C/1200F, but only becomes visible after 700C/1290F.  This has implications for multiple slumps.  Devitrification is cumulative, so the devitrification that may have begun on the flat piece will be added to in the slumping process and may become visible.  For me this has appeared as a haze on the edge of the slumped piece.  Avoidance of this effect is by thorough cleaning of the piece before placing it in the mould.

The devitrification seen in typical studio practice results more often from inadequately cleaned glass than from excessive time at a particular temperature, up or down, through the devitrification range.  It is often seen as a result of grinding edges to fit.  Even though the ground edge is cleaned, it may still be rough enough to promote devitrification.  The edge must be prepared for fusing by grinding to at least 400 grit (600 is better).  Alternatively, use a fine coating of clear powder to give a new surface to the whole piece.

Dr. Cook suggests three approaches to devitrification:*
Resistance through:
 - Schedules
 - Flux
Dealing with it:
 - Cold work
 - Acids
Embrace it:
 - Allow it
 - Use it

Other sources of information:
Temperature range for devitrification
Homemade devitrification solution
Frit to fill gaps
Low Temperature Kilnforming at Etsy and Bullseye

* From a lecture given by Dr. Jane Cook at the 2017 BECON

[entry revised 28.12.24]