Frit Stretch Crack

Description

A frit stretch in the flattening developed two internal cracks at 4:00 and 8:00 going through colours (but not felt on top or bottom). The flattening was to reduce the centre thickness from about 8m, to be nearer the 6mm of the edges. It was fired in a side element kiln. The maker does not know if there is stress beyond the cracks.

The question is about firing again to conceal the cracks.

Schedules first.

Generally side fired kilns need to have up ramp rates for at least one thickness greater than top fired kilns to counteract uneven heating. The heat input to glass with side fired kilns is directly to the edge of the glass and so more uneven than in top fired kilns. Slower rates for the up ramps are needed than for top firied kilns. Down ramps are not affected in the same way, so down rates can be the same as for top fired kilns.

1st firing (before the break):

277°C/500°F – 594°C/1100°F, 0’ [A reasonable first ramp for small pieces]

28°C/50°F – 691°C/1275°F, 60’ [At 691°C the glass has begun to stick together, so a lower temperature of 677°C/1250°F would continue to allow air out as the glass slumped.]

9999 – 830°C/1525°F, 60’ [This speed allowed only about 15 minutes to get to top temperature. The impossibility of the glass equalising the top to bottom temperature is alleviated by the one hour soak. The hotter top layers of the stretch begin to move toward the centre before the bottom layers. This ASAP 3^rd segment will reduce the time for the flow to the centre. Peeking at intervals will show when, or if, the centre has been filled by the flow.]

9999 – 510°C/950°F, 120’ [It is known a frit stretch will have a variation in thickness from about 9mm to 6mm. This would indicate firing for at least 1.5 times the thickest part, approximately 12mm, which is what was done]

28°C/50°F – 427°C/800°F, 0’ [A two hour soak indicates annealing for 12mm, so the cooling needs to be for that too. 55°C/100°F to 427°C/800°F is adequate. And can be followed by 99°C/180°F to 371°C/700°F. The final cool rate could be as fast as 330°C/600°F to room temperature.]

Off

If there are no more than 5 segments available in the kiln controller, use the slower first rate to 371°C/700°F. If the natural cooling rate of your kiln is more than the slow first rate at that temperature, reduce the target temperature further.

I rarely fire faster than 330C/600F to top temperature. Faster does not allow all the heat to reach the bottom and so meld the glass together. There are two blog posts that help to explain the difficulties with AFAP ramps up in temperature. This difference increases with thickness. The effects on glass temperatures, and the heat control problems

2^nd firing:

111°C/200°F – 594°C/1100°F, 0’ [because of the cracks, I suggest 55°C/100°F is fast enough as the first ramp rate. But it does not need to go to more than 500°C/932°F to be sure of being above the strain point.]

125°C/225°F – 663°C/1225°F, 30’. [I would maintain this rate to top temperature, as any bubbles are already trapped]

9999 – 830°C/1525°F, 60’ [I would use this top temperature and time but at the slower rate of the previous ramp]

9999 – 510°C/950°F, 120’ [I think this is an appropriate anneal soak time]

28°C/50°F – 427°C/800°F, 0’ [The cool rate should be as suggested for the first firing. The elimination of the bubble squeeze segment allows a final segment rate of 100°C/180°F – 371°C/700°F or lower if required – for a controller with fewer segments.]

You need to put the heat into the glass slowly when trying to flatten or thin a piece. With a slow rate it may be possible to reduce the one hour soak at top temperature and still get the same thinning result.

The break goes through colours so it is not a compatibility problem. That moves attention to annealing soak and cooling. The schedules do not set off any alarms for me as causing the crack (although I would have been more cautious). It is clearly highly stressed with the crack essentially going across the whole piece except the edges. It will continue to be delicate because of the stress unless re-fired carefully.

Testing for stress

A stress test is a means of determining how sound the fired piece is. It can also be used on smaller pieces to determine what stresses may be stored in the glass and to determine if the glasses are compatible.

This tests for both stress and compatibility.  These give notes on the use of the polarised filters.

Strain Point

The strain point is the temperature above which the viscosity of the glass is low enough that it is rarely subject to thermal shock, because it is no longer brittle. That temperature is determined by the glass’s viscosity, which is achieved at different temperatures for different glass, such as float glass.

Repair

I suggest placing a dam around the piece before firing for a repair. This can be thick fibre paper backed with kiln furniture, or some other circular material to confine the piece, in case the crack develops into a full break during the firing.

The Ramp Rate I suggested of 55°C/100°F per hour is because of the existing cracks. This is to avoid inducing additional stress to the piece during the temperature increase. Once the strain point is passed, the rate can be increased. I suggest the rest of the firing should be as for 12mm / 0.5". The anneal soak and cooling are given in the Bullseye document Annealing Thick Slabs, Celsius and  Fahrenheit,  which is applicable to "CoE 96" glasses, except for the anneal soak temperature.

Further investigation

I suggest two e-books which will be very helpful in your exploration of kilnforming:

Kilnforming Principles and Practices, by Stephen Richard

Firing Schedules for Kilnformed Glass, by Bob Leatherbarrow

Both are available from Bullseye or the authors.

Anomalous Annealing Schedules

Why do schedules vary so widely?

I frequently observe strange annealing schedules promoted on the internet. These include:

• extremely long anneal times with rapid cools risking thermal shock.

• single slow cool rates, using more time and electricity than necessary

  • to 371°C/700°F.  There are better cool arrangements that fit the needs of cooling glass.

  • to room temperature.  Again using more time and electricity than needed.

  • somewhere between.  These still do not adhere to the requirements of the glass.

• Cool rates with staged soaks. There is no use for soaks below the strain point (about 470°C/800°F) as any annealing stress is locked in by that time.

• Cool rates unrelated anneal soak lengths.  Adequate cool rates are directly (but inversely – longer soak = slower cool rates) related to the length of the anneal soak.

• Cool rates ending at 371°C/700°F which are inadequate for thick glass.

They are strange, because there is a well researched document giving annealing times and cooling rates published by Bullseye for Fahrenheit and Celsius, 

It may be understandable that users of fusing glasses other than Bullseye are cautious about the use of this table.  But it is a well researched table supported by independent academic research, which is applicable to all soda lime glass. [McLellan and Shand (1984), Glass Engineering Handbook, 3rd Edition, New York, McGraw Hill]  Only the annealing temperature needs to be changed, as all the rates and times apply equally to all fusing glasses, and even float glass.

Yes, sometimes changes are required for:

• Profile

  • Contour fuse needs times and rates for 1.5 times the thickest part

  • Tack fuse needs times and rates for 2 times the thickest part

  • Sharp tack or sinter needs times and rates for 2.5 times the thickest part

• Contrasting colours, and differing viscosities need times and rates for at least 3mm thicker than the profile indicates

• Extreme forming such as drops and deep slumps also need times and rates for at least 3mm thicker than the profile indicates

These matters are explained in greater detail in my e-book Annealing Concepts, Principle and Practice available from Bullseye and Etsy

What are the Effects of Firing on the Kiln Floor?

Credit: The Pottery Wheel

There will be differences when firing on the floor of a kiln - the two important ones are working temperature and annealing.

Differential temperatures 

When firing on the floor of the kiln, expect the effective temperature to be a little lower than when on an elevated shelf. The temperature is always lower at the floor of the kiln and hotter higher in the kiln. This effect is often experienced with glass nearer the elements than usual.

This differential temperature between the floor and the top of the kiln is alleviated to some extent by the infrared heating, if the glass is exposed to it. If the glass is shaded, there will be a distinct difference.

Annealing

Annealing and cooling will be affected most. On the floor the glass can only cool from the top surface, as the bottom of the glass can only cool as fast as the cooling of the kiln. The annealing soak needs to be longer and the cooling needs to be slower than with an elevated shelf.

I suggest that firing for one layer thicker than calculated for the profile will cope with both these conditions.

Relieving Existing Stress - How?

Why is the stress not relieved after the strain point when slumping?

The answer relates to whether it is on the cool or on the heat up.

Cooling

The annealing occurs at a higher temperature than the strain point. The aim of the annealing soak is to even out the temperature within the glass to be equal to or less than 5°C/10°F (∆T=5C). When this small differential in temperature is achieved, there is little stress in the glass. In an adequate anneal, stress will be relieved during the soak. This differential needs to be maintained through the first cool, taking the glass temperature to below the strain point.

Relieving stress occurs between the glass transition point to just above the strain point. The viscosity of the glass is so high below the strain point (brittle phase) of the glass that no stress can be relieved.

The more rapid cooling during the brittle phase of the glass needs to be slow enough to avoid creating large contraction differentials within the glass. The reason for progressive cooling stages during the brittle phase of the glass is that it can withstand greater temperature differentials and so the cooling rates can be increased.

Heat up

Any stress on the way up for an already fused piece is induced by uneven heating. This can be across the piece, which is most evident in side fired kilns. The source of the infrared heating is nearest the edge of the glass, so it heats first leaving the centre cooler – sometimes the difference in expansion is great enough to break the glass.

In top fired kilns the differential is usually between top and bottom surfaces. Glass transmits heat slowly so the difference in temperature between the top and the bottom can be enough to cause a break from unequal expansions.

Both these conditions are caused by rapid ramp rates and short anneals on the cool.

Ramp Rates

Breaking of a flat piece on the kiln shelf is from too short a soak or too fast a cool, or both (unless there is an incompatibility). Breaking in a slump most often is a result of too rapid an initial ramp rate. A fused piece needs slower ramp up rates in a slump than in the initial fuse. It is now a single thicker piece, rather than multiple pieces as at the beginning of a fuse. While you might fire a flat 6mm/0.25” piece at 200°C/360°F for the fuse, the ramp rate for the slump needs to be no more than about 100°C/180°F. Tack fused pieces need much slower heat up rates during the slump, usually only half of the rate used to fuse the piece.

Tests have shown that even though the anneal soak for both firings can be the same, a more stress-free piece can be achieved by annealing as for one layer thicker. I do not know why, but I speculate that it is more difficult to achieve the ∆T=5C in the curved piece, than in a flat one.

More information is available in the ebook Annealing Concepts Principles and Practice.

What is the Annealing and Cooling Relationship?

 Annealing Includes Cooling

Often people recommend a long anneal soak for potentially difficult pieces followed by an arbitrary 55C/100F cool rate to 371C/700F or 319C/600F. It is arbitrary because the same rate is frequently recommended regardless of the length of the anneal soak.

It does not have to be guesswork. Bullseye has provided us with the science of the anneal/cool in an accessible form: Annealing Thick Slabs (which covers thicknesses of 6mm/.025” to 200mm/8”). This document provides the annealing time for the chosen thickness and the directly linked cooling rates based on scientific principles.

The anneal soak is determined by the profile and thickness of the piece. Work for the e-book Low Temperature Kilnforming showed a relationship between the profile and the annealing time. Annealing for the profiles of sintering, tack, contour and full fuses requires calculation of the thickness to be applied to various profiles:

• Sinter or lamination – 2.5 times the thickest part
• Tack fuse – 2 times the thickest part
• Contour fuse – 1.5 times the thickest part
• Full/flat fuse – 1 times the thickest part

The cool stages are not random either. They have an intimate but inverse relationship to the anneal soak. They are to keep temperature differentials within the glass to acceptable levels. The anneal soak determined by the profile and thickness is to attain and keep the internal temperature within a range of 5°C throughout the glass. This is often referred to as ∆T=5°C.

The cool rates are to maintain acceptable temperature differences within the glass. The first cool rate is to maintain that temperature differential of ∆T=5°C. The second cool rate allows a wider range of temperature differential of 10°C, or ∆T=10°C. This is possible because the glass has become viscous enough to withstand this greater range of different temperature. The final cool needs to maintained at a differential of 20°C, or ∆T=20°C. Again, this is possible because the viscosity is high enough to withstand this amount of differential.

This information about cool rates and an allowable ∆T spread also indicate that turning off the kiln at 371°C/700°F is not always safe. It is almost always safe to do this for anything calculated to be annealed as for 12mm thick, and it may be safe for a piece up to 15mm thick, but remember that a tack fused piece of two base layers and a further decorative layer needs annealing and cooling as for 19mm/0.75”. The Bullseye research shows that the cooling rate for this is less than the unpowered cooling rate of many kilns. If there are additional complicating factors such as strongly contrasting colours, the annealing and cooling needs to be longer and slower than a simple multiplication of thickness.

There seems to be a practice of a single annealing rate to 371°C/700°F or 319C/600F. So the question will arise “Why is it necessary to have multiple cooling stages.” The response is that it will use unnecessary time and power. Attempting to maintain the ∆T=5°C over extended temperature ranges will not provide extra sound annealing. As the glass can withstand a ∆T=10°C from 427°C/800°F to 371°C/700°F, there is less power required at the faster rate than the slow one. This is even more so for the cool to 319C/600F and lower temperatures.

Knowing the safe temperature to turn the kiln off, requires knowing the cooling rate of the unpowered kiln. This blog shows how to determine the natural cooling rate of your kiln.  Knowing this is as important as knowing what effect different fusing temperatures have on the glass in your kiln.

The object of this blog post is to demonstrate that cooling is part of annealing. Just as much attention must be paid to the cooling rates as the length of the annealing soak. They are inseparable for sound kilnforming practices.

Some work that may be of assistance in understanding the importance of knowing the relationship between the annealing soak and the annealing cool are:

Annealing Concepts, Principles, and Practice

Available from: Bullseye and Etsy

Kilnforming Principles and Practice

Available from: Bullseye  and Warm Glass

Low Temperature Kilnforming

Available from: Bullseye and Etsy

Is White a Difficult Glass?

Description of the Project

A white 3mm base with 3mm and 6mm decorations made up of mosaic pieces from previously fused glass (all the same CoE). At the end of the firing three corners had broken and their edges rounded. The fourth corner had sharp edges. The tentative conclusion was that there was incompatibility between the white and the previously fired pieces. There were no other cracks visible on the white or between the mosaic pieces. The author did not indicate what the schedule was for either firing, nor what the profile of the last firing was, but asserts white is a particularly difficult glass which does not work well with a wide variety of colours.

My observations are: 

• Compatibility is not an issue on the heat up. It is only a problem at annealing and cooling.
• Breaks on the ramp up (showing rounded edges at the conclusion of the firing) are normally the results of too fast rates.
• Breaks during cooling (showing sharp edges) are due to annealing, compatibility, cooling rates, or some combination of these.
• Previously fired glass can show some shift in compatibility and so needs slower up ramp rates than normal for the profile and thickness.
• Incompatibility between the base and the mosaic pieces would show up as breaks in the white glass under each top mosaic piece.
• Not all glass of the same CoE from different manufacturers is compatible.

Could this have been from incompatibility?

On the way to top temperature the pieces have not yet combined. The incompatibility will only show up during the cooling, as it is the imbalance of  viscosity and contraction between the fused pieces that cause the breaks.

Only one of the broken corners has those sharp edges, making incompatibility an improbable cause of the breaks. Further, incompatibility between the base and upper layers present either a crazed appearance at the connections, or simple breaks around the base of each decorative piece. Incompatibility would have multiple breaks all over the base, if not the top too. Finally, if the fired mosaic pieces were incompatible with the white glass, there would have been breaks throughout the whole piece, not just at the corners.

A further possibility is that the corners were very close to the sides of the kiln, because only the corners broke away from the piece,. If it was side fired, much slower rates are required. And all kilns tend to be cooler near the sides on the heat up than toward the centre, even if top fired.

My guess, based on the description, is that the up ramps were too fast, and the anneal was too short and the cool too fast. Unless the previously fused pieces were tested for stress it is not possible to know whether those were stressed before the final firing, which could have caused the break off of the three of the corners. The fourth corner break was on the cool down and is most likely to be too short an anneal and/or too quick a cool.

Is white glass especially difficult?

There is nothing in this piece to identify white glass as an extraordinarily difficult glass, or that a multiplicity of colours added to white would provoke breaks. The problems exhibited are most likely related to fast heat up ramp rates, and inadequate annealing and cooling.

How do I Evaluate 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

How Can I Relieve Stress in Fused Glass?

An stress test strip and annealing witness between polarised filters.

If an unbroken fired piece shows stress that is known not to be from incompatibility, it is possible to fire and anneal again to relieve the stress.  If the stress results from incompatibilities, annealing again will not change the compatibility.  The process for stress testing is here. 

Conditions for doing this re-firing are:

• Slower heat up rates than usual for this thickness and profile are required. The glass is more than usually fragile and needs gradual heating. This avoids creating additional stress that may cause a break.

• Take the temperature up to the lower end of slumping temperature range - say 600 - 620C (1100 - 1150F) - and soak for 10 – 30 minutes depending on profile and thickness.  This ensures any existing stress is relieved and the glass is ready for the annealing.

• Reduce the temperature as fast as possible to the existing or new annealing temperature.

• Anneal for longer than previously. This can be for a greater thicknesses than the thickness and profile used for the stressed piece.  Most importantly, the anneal soak for the combination of profile and thickness needs to be followed.

• My experimentation has shown that the profile determines the additional amount of thickness that needs to be allowed for a sound anneal is as follows:

• Full flat fuse - fire for the thickness (i.e. times 1)
• Contour fuse -  fire for 1.5 times the thickest part
• Rounded tack fuse - fire for 2 times the thickest part
• Sharp tack/sinter - fire for 2.5 times the thickest part.

• Use the cool rates related to the anneal soak time. These are available from the Bullseye site for Celsius and Fahrenheit.  Too rapid a cool can induce temporary stress from differential contraction of the glass that is great enough to cause breaks, so follow the rates determined for this thickness and profile. 

• These rates are scientifically determined for all glass and especially for fusing glass and are inversely related to the anneal soak.  That means the longer the anneal soak, the slower the cooling rates need to be, and directly related to the soak length.  It does not matter which manufacturer's glass is being used, all the target times and temperatures should be followed, except the annealing temperature.

More information is available in my e-book Annealing Concepts, Principles, and Practice available from Bullseye, Etsy, and stephen.richard43@gmail.com

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