Observations on Some Suggestions about Annealing

There are writings from a teacher attempting to make glass fusing simple.  Unfortunately, glass physics and chemistry are very complicated.  Attempting to avoid these complications leads to failures and other difficulties as the practitioner progresses. 

Proper annealing is one of the fundamentals to achieving sound kilnforming results.  Some suggestions have been made by a widely followed person to “simplify” the understanding of the annealing process.  Discussion of the meaning and importance of annealing can be found in many places, including here.  

Annealing temperatures

It has been suggested that the annealing temperatures can be inferred from the CoE of the glass that is being used. Discussion of what CoE is and is not can be found here and here.

Annealing temperatures are not directly related to the expansion coefficient (CoE) of the glass.  This can be shown from the published annealing temperatures for different glasses organised by presumed CoE:

·        “CoE96”: Wisssmach 96 - anneal at 482°C;  Oceanside - anneal at 515°C

·        “COE94”: Artista - anneal at 535°C

·        “CoE 93”: Kokomo - anneal between 507°C and 477°C – average 492°C

·        “CoE 90”: Bullseye - anneal at 482°C; Wissmach90 - anneal at 482°C; Uroboros FX90 - anneal at 525°C

·        “CoE 83”:

o   Pilkington (UK) float - anneal at 540°C;

o   typical USA float - anneal at 548°C;

o   Typical Australian float - anneal between 505°C and 525°C, average 515°C

This shows there is no direct relationship between CoE and annealing temperature.  Do not be tempted to use a CoE number to indicate an annealing temperature.  Go to the manufacturer’s web site to get the correct information.

Temperature equalisation soak

Annealing for any glass can occur over a range of temperatures.  The annealing point is the temperature at which the glass can most quickly be annealed.  However, the glass cannot be annealed if it is not all at the same temperature throughout the substance of the glass.  It has been shown through research done at the Bullseye Glass Company that a temperature difference of more than 5°C will leave stress within the glass piece. To ensure good annealing, adequate time must be given to the temperature equalisation process (annealing). 

From the Bullseye research the following times are required for an adequate anneal soak:

6mm /   1/4"            60 minutes

[9mm /  3/8"           90 minutes]

12mm  / 1/2"          120 minutes

[15mm  /   5/8"       150 minutes]

19mm   / 3/4"         180 minutes

[ ] = interpolated from the Bullseye chart for annealing thick slabs

Anneal Cooling

There are suggestions that a “second anneal” can be used on important pieces.  Other than observing that all pieces are important to the maker, the suggestion should be investigated.  On looking into the idea, it is essentially a second soak at 425°C, which is slightly below the strain point, rather than controlled cool from the anneal soak temperature.

It is reported that the Corning Museum of Glass considers 450°C as the lower strain point – the temperature below which no further relief of strain is possible.  This means that any secondary soak must occur above 450°C rather than the suggested 425°C. Such a soak is unnecessary if the appropriate cooling rates are used. 

Cooling Rate

Except in special circumstances, the cooling rate needs to be controlled as part of the annealing process.  Soaking the glass at the anneal is not the completion of the annealing.  Most practitioners follow the practice of choosing a slow rate of cooling from the annealing soak to some point below the strain point rather than a rapid one with a soak at the strain point temperature.

Annealing is not just the soak time (which is there to equalise the temperature), it is about the rate of the annealing cool too. The rate at which you cool is dependent on the thickness of the glass piece and whether it is all of one thickness or of variable thicknesses.

Even thickness

                                         Cooling rate

Dimension      time (mins)     to 427°C to 371°C

6mm              60                 83°C       150°C

9mm              90                 69°C       125°C

12mm            120                55°C       99°C

15mm            150                37°C       63°C

19mm            180                25°C       45°C

                                        Cooling rate

Dimension      time (mins)     to 800°F   to 700°F

0.25"              60                 150°F       270°F

0.375"            90                 124°F       225°F

0.5"               120                100°F       178°F

0.675"           150                67°F         114°F

0.75"             180                45°F         81°F

Tack fused/ uneven thickness

If your piece is tack fused, you need to treat the annealing rate and soak as though it were twice the actual total thickness. This gives the following times and rates:

Tack fused

Dimension (mm)                                Cooling rate

Actual     Calculated       time (mins)    to 427°C   to 371°C

6            12                 120                55°C       99°C

9            18                 150                25°C       45°C

12          25                 180                15°C       27°C

15          30                 300                9°C         18°C

18          38                 360                6.7°C       12°C

Dimension (inches)                                Cooling rate

Actual     Calculated       time (mins)    to 800°F   to 700°F

0.25          0.5                 120                100°F       180°F

0.375        0.75               150                45°F         81°F

0.5            1.0                180                27°F          497°F

0.675        1.25               300                16°F         36°F

0.75          1.5                360                12°F          22°F

Contour fusing requires firing as though the piece is 1.5 times thicker.  Sharp tack or laminating requires 2.5 times the the actual thickness.

Fusing on the floor of the kiln

There is a further possible complication if you are doing your fusing on the kiln floor, or a shelf resting on the floor of the kiln.  In this case you need to use the times and rates for glass that is at least 3mm thicker than the piece actually is. 

Thus, a flat 6mm piece on a shelf on the floor would use the times and rates for 9mm: anneal soak for 90 minutes, anneal cool at 69°C to 427°C and then at 124°C to 371°C.  It would be safest if you continued to control the cooling to room temperature at no more than 400°C per hour.

But if it were a tack fused piece of a total of 6mm you would use the times and rates for 18mm.  This is using the rates for twice the total thickness plus the additional 3mm for being on the base of the kiln.  This gives the times and rates as being an anneal soak of 360 minutes and cooling rates of 7°C to 427°C and 12°C to 370, followed by 40°C per hour to room temperature.  Any quicker rates should be tested for residual stress before use.

Source for the annealing and cooling of fused glass

These times and rates are based on the table derived from Bullseye research, which is published and available on the Bullseye site.   It is applicable to all fusing glass with adjustments for differing annealing soak temperatures.

Annealing over multiple firings

It has been recommended by a widely followed person that the annealing soak should be extended each time subsequent to the first firing.  I am uncertain about the reasoning behind this suggestion. But the reasons for discounting it are related to adequate annealing and what is done between firings.

If the annealing is adequate for the first firing, it will be adequate for subsequent firings unless you have made significant alterations to the piece.  If you have added another layer to a full fused piece, for example and are using a tack fuse, you will need to anneal for longer, because the style and thickness have been altered.  Not because it is a second firing.  If you are slumping a fired piece, the annealing does not need to be any different than the original firing.

The only time the annealing needs to be altered is when you have significantly changed the thickness of the piece, or the style of fusing (mainly tacking additional items to the full fused piece).  This is when you need to look at the schedules you are planning to use to ensure your heat up is slow enough, that your annealing soak is long enough, and the cool slow enough for the altered conditions.

Determining the annealing point of unknown glass

You don’t have to guess at the annealing temperature for an unknown glass.  You can test for it.  It is known as the slump point test.

This test gives the softening point of the glass and from that the annealing point can be calculated.  This test removes the guess work from choosing a temperature at which to perform the anneal soak. The anneal temperature is important to the result of the firing.  This alone makes this test to give certainty about the annealing temperature worthwhile.

You can anneal soak at the calculated temperature, or at 30°C below it to reduce the anneal cool time.  This is because the annealing can occur over a range of temperatures.  The annealing occurs slowly at the top and bottom of the range. But is at least risk of "fixing in" the stress of an uneven distribution of temperature during the cool when the annealing is done at the lower end of the range.

Do not be fooled into thinking that CoE determines annealing temperatures.  Use published tables, especially the Bullseye table Annealing for Thick Slabs to determine soak times and cooling rates.  Use the standard test for determining the softening and annealing points of unknown glasses.

Further information is available in the ebook Low Temperature Kiln Forming and in Annealing Concepts Principles and Practice 

Revised 14.10.25

Shotgun Annealing

 Shotgun annealing is chosen when the annealing temperature is unknown or uncertain. The name comes from the characteristic spread of the shot pellets to include the target.

To follow this process, pick highest relevant anneal temperature. We know soda lime glass has a range from about 540°C/1004°F to 470°C/878°F. Unless you are firing float glass (which anneals between 540°C/1004°F and 520°C/968°F), you can start the anneal cycle at 520°C/968°F and continue it to 470°C/878°F (a 50°C range). The rate to be used is determined by the amount of time required to anneal the piece according to thickness.

To be safe, a shotgun anneal will need double the time to go through the chosen range that a normal anneal soak requires.

• A 6mm/0.25” full fused piece would normally need an hour soak. So the shotgun anneal rate would be 25C/45F per hour over a 50°C/90°F range.
• A 12mm/0.5” full fused piece would normally need a two hour soak. This implies a rate of one quarter of the range or a cool rate of 12°C/22°F over the range.
• A 6mm/0.25” tack fused piece would need to be fired for twice its thickness, so as for 12mm/0.5”.

Annealing times for different profiles and thicknesses are given in this blog post:  and in this ebook.

If the glass is really unknown or older than fusing glass, a wider shotgun anneal range should be used. This gives a temperature range of 540°C/1004°F and goes to 470°C/878°F, or a range of 70°C/126°F. There is still a requirement for the shotgun process to be double the normal anneal soak.

• So for a 6mm/0.25” full fused piece two hours are required to go through the range, or 35°C/63°F per hour.
• A 12mm/0.5” full fused piece and a 6mm/0.25” tack fused piece will need a rate that takes 4 hours to go through the range, or 18°C/32°F per hour.

Once the slow fall of temperature through the range is complete, there should be a one hour soak to ensure the temperature has been equalised throughout the reduction in temperatures. This is applicable to pieces 12mm/0.5” thick. Thicker pieces need a longer soak at this point.

The final part of the anneal is cooling at a rate appropriate for the thickness and profile. E.g.:

• A 6mm/0.25” full fused piece would be cooled at 83°C/150°F to 427°C/800°F, and then at 150°C/270°F to 370°C/700°F or lower.
• A 12mm/0.5” full fused piece needs a two hour soak, so the cooling rates are determined by that, i.e., 55°C/99°F per hour to 427°C/800°F and then at 99°C/178°F per hour to 370°C/700°F or lower.

There is an alternative process which is used to determine the annealing temperature of an unknown glass. Once the anneal temperature is determined for a glass, there is no need for a shotgun anneal process. This is known as the slump point test. 

Much more on the principles and practices of annealing can be found in my e-book. Annealing Concepts, Principles and Practice from Bullseye, Etsy and stephen.richard43@gmail.com

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.

 

Reducing Annealing Time for Float Glass

Credit: Bullseye Glass Co.

Annealing float glass seems take a long time. The annealing point (Tg) is higher than most fusing glasses, although float glass is part of the family of soda lime glass. This group of glasses should be cooled slowly from annealing temperature to 427ºC/800ºF and below to reduce risks of thermal shock.  This makes a greater temperature range over which to  anneal float than fusing glasses, consequently it extends the cooling time and increases energy expenses.

It does not have to be this way.  Annealing of glass takes place over a range.  This range extends below the published annealing point (Tg).  This is the temperature at which equalisation can most quickly take place, but it is not as energy efficient as starting in the lower range.  Annealing points (Tg) vary between manufacturers, but these are some of them:

Pilkington Optiwhite              559ºC/1039ºF

Pilkington Optifloat               548ºC/1019ºF

USA float (typical)                548ºC/1019ºF

Australian float (average)      550ºC/1022ºF

The annealing range extends to a practical 38ºC/68ºF below the Tg temperature.  Annealing at a lower temperature can be as effective at the lower portion of the range as at the Tg.  Using a lower annealing soak temperature reduces the temperature range of the first cooling stage by as much as 38ºC/68ºF, and reduces the cooling time without increasing risks of breaking.  It also creates a denser glass according to scientific research.  Denser glass is arguably a stronger glass.

This means that the annealing of float glass can take place at the following reduced temperatures:

Pilkington Optiwhite              521ºC/971ºF

Pilkington Optifloat               510ºC/900ºF

USA float (typical)                510ºC/900ºF

Australian float (average)      512ºC/954ºF

 

This reduces the first cooling stage for 12mm/0.5” Pilkington Optiwhite from 2 hours 24 minutes to 1 hour 43 minutes.  Forty-one minutes may not seem much but in electricity costs is significant.  Also using the Bullseye concept of a three stage cooling, further savings can be made.  Their research shows the second cooling stage to 371ºC/700ºF can be increased by 1.8 times the first cooling rate, saving further time and energy.  The chart which shows these rates is Annealing Thick Slabs -  Celsius and - Fahrenheit.

More information on annealing is available in the ebook Annealing: Concepts, Principles and Practice

Annealing float glass at the lower part of the annealing range reduces the time and cost of firings.

Scheduling for Thick Landscapes

Thick slabs often involve numerous firings of increasingly thick work.  I am using an existing example, with their permission, of the first stages of a thick landscape.  The initial concern was with bubbles in the first layup, then the strategy for firing the thick slab.

Plans

This is the first part of a landscape with depth.  It will be fired 5-7 more times.  This first piece will be inverted for the next firing with the clear facing up, to avoid reactions between the colours.  It is similar to an open face casting. There is a Bullseye Tip Sheet on open face casting that will give a lot of information.

Layup

Picture credit: Osnat Menshes

This work has a base of clear that is mostly overlaid with one layer of 3mm pieces, although in some places another layer, and there are some pre-fired elements as well.  It is fired on Thinfire shelf paper.

Bubbles 

There is concern about the number and size of the bubbles after the firing, and how to avoid them.  Will they grow over the multiple firings?

The many small bubbles are characteristic of kilnformed glass.  The few larger bubbles may result from the frit that is under the pieces that form the top surface.  And there are some overlaps of clear over colour that may form pockets where air can collect. I advise leaving the scattering of the frit until all the decorative pieces are in place.  The bubbles will migrate toward the top during the multiple firings.  They will not grow in size unless they combine during the upward migration.  A later suggestion about reducing the number of firings will reduce the bubble migration and risk of increasing in size.

Picture credit: Osnat Menshes

Schedule

Proposed Schedule (Temperatures in degrees Celsius)

1: 180 – 560, 30’    I would go to 610 for 30'

2: 25 – 680, 120’    I would use only 30'

3: 220 – 810, 15’    I would set the top temperature at 816, 15’.

4: 9999 – 593, 30’  Eliminate this segment. 

5: 9999 – 482, 120’ I suggest one hour soak

8: 55 – 370, off      83 – 427, 0’

7: 150 – 371, 0’

8: 330 – to room temperature, off.

 

Eliminate segment number 4.  Any temperature equalisation done at this temperature, is undone by the AFAP to the  annealing.  The temperature equalisation occurs at the annealing temperature. No soak at an intermediate temperature is required.  This blog post gives some information about annealing above and below the annealing point (Tg). 

Firing Incremental Layers

The plan is for five to seven more firings.  Continuing to build up the thickness on each firing, may have some problems.

• There is increased risk of compatibility problems when firing a piece to full fuse many times.
• There is a risk of more bubbles and of the existing ones becoming larger as they move upwards and combine with other smaller ones.
• With each firing the thickness is increasing and so becoming a longer firing.  This is because the heat up, annealing, and cooling each need to be longer.  For example - 6mm needs 3hour cooling, 12mm needs 5 hours, 19mm needs 9 hours. 

Multiple Slabs

These are the main reasons that I recommend firing a series of 6mm slabs separately and combining them in one final firing.  Firing a series of 6mm slabs and then combining them in a single long and slow final firing has advantages.

• The individual pieces do not need to go through so many full fuse firings, reducing the risk of compatibility problems.
• The small bubbles in each firing will not have the chance to rise through all the layers to become larger.
• The total time in the kiln for the combined pieces will be less than adding layers to already fired layers.

Examples

It is often difficult to convince people that firing by adding incrementally to an existing slab, longer firing times are required than by firing a group of 6mm slabs and a single combined firing of all the slabs.  I give an example to illustrate the differences.

Annealing

Assume there are to be a total of eight firings (existing 6mm slab and 3mm for each of seven more firings).  Also assume that each additional firing is of 3mm. This makes a total of 28mm.  Compare annealing and cooling times for each firing:

Firing      thickness       anneal and cool (hours minimum)

1            6mm                    3

2            9mm                    4

3            12mm                   5

4            15mm                   7

5            18mm                   9

6            21mm                   11.5

7            25mm                   14

8            28mm                   17

Total                                   70.5 hours annealing time (minimum)

To fire up 5 six millimetre slabs takes less time – 3 hours annealing and cooling time for each firing cumulates to 15 hours.  Add to that the final firing of 17 hours annealing time.  A total of 32 hours.  This is half the time of adding to the existing slab at each firing.  Multiple 6mm slabs can be fired at the one time if there is space in the kiln, which would reduce the kiln time for the 6mm slabs even further. 

An additional advantage of firing 6mm slabs and combining them, is that bubbles can be squeezed out more easily in the final thick slab fring because of the combined weight of the  slabs.  You could make the individual slabs a little thicker, but that would involve damming each slab.  Not an impossible task of course.  And it would change the calculations, by reducing the number of firings.

Heat Up

Another time saving is to use the second cooling rate from the Bullseye document Annealing Thick Slabs as the first up ramp rate. Take this rate up to a minimum of 540˚C. Although, this is an arbitrary temperature above the strain point to ensure all the glass is above the brittle phase.  It is possible to maintain this initial rate to the bubble squeeze.  But with the slow rises in temperature required for thicker slabs, it is sensible to increase the rate from 540 to bubble squeeze to reduce the firing time.  Once past the bubble squeeze a more rapid rate can be used to the top temperature.  

The heat up times could be about half the minimum cooling times.

A worked example (with certain assumptions) would be:

Firing      thickness       time to top temperature total time.

1            6mm             6.3               

2            9mm             7.1

3            12mm            8.4

4            15mm            10.7

5            18mm            15.9

6            21mm            19.4

7            25mm            25.1

8            28mm            29.1              ca.122 hours

But firing five times for 6mm equals 31.5 hours plus the final firing up of 29.1 hours equals a total of 60.6 hours.  Again about one half the time of progressively building up a base slab to the final thickness.

Savings

This example shows that approximately 90 hours of firing time can be saved by making a series of six millimetre slabs and combining them in a final firing.  There is the additional advantage of reducing the occurrence of bubbles between the layers in the final firing because of the weight of the combined slabs.

Slumping and Annealing bottles

"Can a tack fuse schedule for fusing glass can be used to slump bottles?"

It may be that this person does not have the confidence to write a new schedule.  They may wish to use an existing schedule for another purpose. The short answer is “Although a Bullseye or Oceanside tack fuse temperature will be high enough to slump bottles, they are not suitable for annealing”.  There are reasons for this. 

The softening point of float glass, which is similar to bottle glass, is 720ºC/1330ºF.  Slumping would normally be done at about 20ºC/36ºF above this. You also need a slumping hold at this temperature much longer than a tack fuse schedule would use.

if you use a tack fuse schedule for a fusing glass, your annealing will be inadequate. Bottle and float glass tend to have an annealing point of around 540ºC/1005ºF. An annealing for fusing glass will be between 515ºC/960ºF and 482ºC/900ºF.  This is likely to be too low an annealing point for bottles.  Also, the annealing soak is likely to be too short. Slumped bottles are very thick at the base where it folds over the cylinder of the bottle.  This requires a longer anneal soak and slower cool than a schedule for a tack fuse of fusing glass.

Checking for stress in the completed work is normal.  It is essential for your finished bottle if you use a tack fuse to fire it.

 

Schedules should be devised for the glass and layup of each piece. Transferring a schedule for fusing to bottle glass is unlikely to be successful.

Heat Up vs Annealing

I am amazed by the effort put into ramp up rates, bubble squeezes, and top temperatures in comparison to annealing.  The emphasis on social media groups seems to be to get the right ramp rates for tack fuses and slumps, bubble squeezes, etc.  Most of the attention is on the way up to processing temperature.

The treatment of annealing and cooling is almost cavalier by comparison.  The attention seems to be on what temperature, and how long a soak is needed.  Then some arbitrary rate is used to cool to 370ºC/700ºF.

Annealing, in comparison to firing to top temperature, is both more complex and more vital to getting sound, lasting projects completed.  Skimping on annealing is an unsound practice leading to a lot of post-firing difficulties.

Annealing is more than a temperature and a time.  It is also the cooling to avoid inducing temporary stress. That stress during cooling can be large enough to break the glass.  This temporary stress is due to expansion differentials within the glass.

People often cite the saving of electricity as the reason for turning off at 370ºC/700ºF.  My response is that if the kiln is cooling off slower than the rate set, there will be no electricity used.  No electricity demands.  No controller intervention.  No relay operation.

Annealing at the lower end of the range with a three-stage cooling provides good results.  The results of Bullseye research on annealing are shown in their chart for annealing thick items.  It applies to glass 6mm and much larger.  It results from a recommendation to anneal at the lower end of the annealing range to get good anneals.  Other industrial research shows annealing in the lower end gives denser glass, and by implication, more robust glass.  Wissmach have accepted the results of Bullseye research and now recommend 482ºC/900ºF as the annealing temperature for their W96.  The annealing point of course remains at 516ºC/960ºF.

Bullseye research goes on to show that a progressive cooling gives the best results.  They recommend a three-stage cooling process.  The first is for the initial 55ºC/º100F below the annealing temperature, a second 55ºC/100ºF cooling and a final cooling to room temperature.

It is a good practice to schedule all three cooling rates.  It may be considered unnecessary because your kiln cools slower than the chart indicates.  Well, that is fine until you get into tack and contour fusing.  Then you will need the three-stage cooling process as you will be annealing for thicknesses up to 2.5 times actual height.

 

Of course, you can find out all the reasons for careful annealing in my book "Annealing; concepts, principles, practice" Available from Bullseye at

https://classes.bullseyeglass.com/ebooks/ebook-annealing-concepts-principles-practice.html

Or on Etsy in the VerrierStudio shop

https://www.etsy.com/uk/listing/1290856355/annealing-concepts-principles-practice?click_key=d86e32604406a8450fd73c6aabb4af58385cd9bc%3A1290856355&click_sum=9a81876e&ref=shop_home_active_4