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.

Slow Rates to Annealing

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

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

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

credit: ww.protolabs.com

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

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

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

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

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

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

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

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

Slumping contrasting colours and styles

 A question about why a tack fused 6mm/0.25” piece of combined dense white and black in a slump firing broke has been raised.  Other pieces of black and other whites also tack fused in the same firing did not break.

"Living in the Grey" Stephen Richard

Contrasting colours

Combining the most viscous and the least viscous of bullseye glasses - dense white and black - is a challenge.  The survival of other pieces in the firing with slightly less viscous white give an indication.  Their survival shows that the anneal and cooling conditions were too short and fast for the broken piece. 

It may be worth checking how much stress is in the surviving pieces.  It may not be possible directly on these fired pieces. There is a way.  Mock up the black and white in the same way as the surviving pieces.  Put this on a larger clear piece and fire in the normal way. This enables you to see stress in opalescent layups. If there is any, it is revealed on the clear by using polarising filters. 

The usual recommendation is to anneal and cool as for twice the thickness was followed in this firing.  It is important to anneal and cool more conservatively in cases of contrasting colours. Strongly contrasting colours and styles (low viscosity transparent and high viscosity white opalescent) require more time at annealing and need slower cooling.  I do that by using a schedule for one layer thicker than calculated.  In this case, as for 15mm/0.61” (two tack layers needs firing as for four tack layers, plus one extra for the high and low viscosity combination).

Viscosity

The reasons for this are viscosity:  

·        Annealing is done at a temperature that achieves a viscosity of around 10^13.4 poise. It can be done in a range from there toward the strain point of 10^14.5 poise.  Below the strain point temperature (which is determined by the viscosity), no annealing can occur.  The glass is too stiff.  The closer to the strain point that the annealing is done, the more time is required at the annealing temperature.

·        The annealing of Bullseye is already being done in the lower range of viscosities. It is possible the viscosity of the white is so high as to be difficult to anneal with the usual length of soak.

·        Although I do not know the exact viscosities of dense white and soft black at the annealing temperature, it is known white has a higher viscosity than the black.  The means to achieve less stress in the glass is to hold at the annealing temperature longer than normal.  A cooling schedule related to the length of the anneal hold is needed.  This information can be obtained from the Bullseye chart for annealing thick glass.  The rates and times apply to all soda lime glass, which is what fusing glass is. Only the temperatures need to be changed to suit the characteristics of your glass.

Slumping

The slumping of this combination of high and low viscosity glasses requires more care too.  My research has shown that the most stress-free result in slumping is achieved by firing as for one layer thicker than that used for the fuse firing.  For a tack fuse, this means firing for twice the thickness, plus one more layer for contrasting colour and style.  Then schedule the slump by adding another layer to the thickness.  This means scheduling as for 19mm/0.75"instead of as for 12mm/0.5”.  This is to account for profile, contrasting colours, and stress from slumping.  This is about three times the actual height of the piece.  

Slumping tack fused pieces of contrasting colours requires very cautious firing schedules.  These longer schedules need to have a justification.  It is not enough to add more time or slow the cooling just in case.  Excessively long anneal soaks, and slow cools can create another set of problems. 

More viscous glass needs more time at the annealing soak to an even distribution of temperature between the more and less viscous glasses.

More information about other low temperature processes can be found in my eBook Low Temperature Kilnforming.  Available from Bullseye and Etsy  

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

Slumping Strategy

A schedule was presented for a slumping problem of a 6mm/0.25” blank.  It consisted of three segments each of a rate of 277C/500F with short holds up to 399C/750F and then a rapid rise to 745C/1375F.  The cool was done with two long holds at 537C/1000F and 482C/900F followed by cooling rates for 12mm/0.5”

My response was that, yes it was fired too high.  Not only that, but the firing strategy, as shown by the schedule, is odd. 

Strategy

The general strategy for slumping follows these ideas.

·        Glass is slow to absorb heat, and in one sense, this schedule accepts that by having short soaks at intervals.  As glass is slow to absorb heat, it is necessary to use slow ramp rates and without pauses and changes in rates.  This should be applied all the way to the slumping temperature.

·        Holds of short durations are not effective at any stage in a slumping firing.  The objective is to allow the glass time to form to the mould with as little marking as possible.  This implies slow rates to low temperatures with significant holds at appropriate stages.  This about putting enough heat work into the glass that higher temperatures are not needed.

·        This kind of firing requires observation for new moulds and new arrangements of glass to ensure the slump is complete.  Once you know the mould requirements and are repeating the layup of the glass, the firing records will tell you what rates and times to use to get a complete slump with minimum marking.

·        The hold at annealing temperature is to equalise the temperature throughout the glass to produce a stress-free result.  Any soaks above are negated or repeated by the necessary soak at the annealing temperature.  The hold there must be long enough to complete the temperature equalisation that is the annealing.

·        My work has shown that annealing for one (3mm/0.125”) layer thicker produces a piece with less stress.  This indicates that a 6mm/0.25” piece should be annealed as for 9mm/0.35” to get the best result.

The summary of the firing strategy for slumping is:

• ·        A single ramp of a slow rate to the slumping temperature.
• ·        Observation of the progress of the slump to determine the lowest practical temperature and hold time.
• ·        Annealing for one layer thicker that being slumped.
• ·        Three stage cooling of the piece at rates related to the annealing hold.

Critique

This is a critique of the schedule. For comparison, my schedule for a full fused 6mm blank would be different.

• ·        140ºC/250ºF to 677º/1250ºF for 30 to 45 minutes.
• ·        9999 to 482ºC/900ºF for 1.5 hours
• ·        69ºC/124ºF to 427ºC/800ºF, no hold
• ·        125ºC/225ºF to 371ºC/700ºF, no hold
• ·        330ºC/600ºF to room temperature, off.

The rate of the published schedule is fast for a full fused blank and extremely fast for a tack fused blank. This needs to be slowed.  The schedule provides a single (fast) rate of heating, but with unnecessary holds.  The holds are so short as to be ineffective, anyway. There is no need for the holds on the way up to the slumping temperature.  In general slumping schedules are of fewer segments.   This is because glass behaves well with steady slow inputs of heat.

Then strangely, the schedule increases the rate to top temperature.  It does so with a brief soak at 593ºC/1100ºF.  This fast rate of 333ºC/ 600ºF begins at 400ºC/750ºF.  This is still in the brittle phase of the glass and risks breaking the glass.  The brittle stage ends around 540ºC/ 1005ºF.

This rapid rate softens the surface and edges of the glass without allowing time for the underside to catch up.  This explains uneven edges.  It also risks breaking the glass from too great expansion of the top before the bottom.

Additionally, the schedule uses a temperature more than 55ºC/100ºF above what is a reasonable highest slumping temperature.  The top temperature of this schedule is in the tack fusing range.

There is no need for a hold 55ºC/100ºF above annealing soak. It is the annealing soak that equalises the temperature before the cool begins.  The higher temperature equalisation is negated by the cooler soak at annealing temperature. So, the hold at the higher temperature and slow cool to the annealing temperature only delays the firing by about two hours.  It does not have any effect on the final piece.

The schedule is cooling for a piece of 12mm/0.5”.  This is slower than necessary.  As noted above, cooling for one layer thicker than the piece is advisable to get the most stress free result.  The annealing soak could be 1.5 hours following this idea.  Cooling with a three stage schedule reduces the risk of inducing temporary stresses that might break the glass.  Although the initial cooling rate I recommend is very similar to this schedule, it safely reduces the total cooling time.

• ·        69ºC/124ºF to 427ºC/800ºF, no hold
• ·        125ºC/225ºF to 371ºC/700ºF, no hold
• ·        330ºC/600ºF to room temperature, off.

Using my kind of schedule for the first time will require peeking once top temperature is reached to determine when the slump is complete. It may take as much as an hour. Be prepared to either extend the hold, or to skip to the next segment if complete earlier. The controller manual will explain how.

 More information is given in Low Temperature Kilnforming, An Evidence-based guide to scheduling.  Available from Etsy and Bullseye

Scheduling with the Bullseye Annealing Chart

This post is about adapting the Bullseye chart Annealing Thick Slabs to write a schedule for any soda lime glass as used in kilnforming.

I frequently recommend that people should use the Bullseye chart for Annealing Thick Slabs in Celsius  and Fahrenheit.  This chart applies to glass from 6mm to 200mm (0.25” to 8”).

“Why should the Bullseye annealing chart be used instead of some other source?  I don’t use Bullseye.”

My answer is that the information in the chart is the most thoroughly researched set of tables for fusing compatible glass that is currently available.  This means that the soak times and rates for the thicknesses can be relied upon.

“How can it be used for glass other than Bullseye?”  

The rates and times given in the chart work for any soda lime glass, even float. It is only some of the temperatures that need to be changed.

"How do I do that?"  

My usual response is: substitute the annealing temperature for your glass into the one given in the Bullseye table.

 "It’s only half a schedule."

That is so.  The heating of glass is so dependent on layup, size, style, process, and purpose of the piece.  This makes it exceedingly difficult to suggest a generally applicable firing schedule.  People find this out after using already set schedules for a while. What works for one layup does not for another.

Devising a Schedule for the Heat Up

There is no recommendation from the chart on heat up.  You have to write your own schedule for the first ramps.  I can give some general advice on some of the things you need to be aware of while composing your schedule.

The essential element to note is that the Bullseye chart is based on evenly thick pieces of glass.  Tack fusing different thicknesses of glass across the piece, requires more caution. The practical process is to fire as for thicker pieces.  The amount of additional thickness is determined by the profile being used.  The calculation for addition depends on the final profile.  The calculation for thickness is as follows:

• Contour fusing - multiply the thickest part by 1.5. 
• Tack fusing - multiply the thickest part by 2. 
• Sharp tack or sinter - multiply the thickest part by 2.5.

The end cooling rate for the appropriate thickness is a guide for the first ramp rate of your schedule.  For example, the final rate for an evenly thick piece 19mm/0.75” is 150ºC/270ºF.  This could be used as the rate for the first ramp. 

Bob Leatherbarrow has noted that most breaks occur below 260ºC/500ºF.  If there are multiple concerns, more caution can be used for the starting ramp rate.  My testing shows that using a rate of two thirds the final rate of cooling with a 20 minute soak is cautious.  In this example of a 19mm piece it would be 100ºC/180ºF per hour.

Even though for thinner pieces the rates given are much faster, be careful.  It is not advisable to raise the temperature faster than 330ºC/600ºF per hour to care for both the glass and the kiln shelf.

Once the soak at 260ºC//500ºF is finished, the ramp to the bubble squeeze should maintain the previous rate.  It should not be speeded up.  The glass is still in the brittle phase.

After the bubble squeeze you can use a ramp rate to the top temperature of up to 330C/600F.   AFAP rates to top temperature are not advisable.  It is difficult to maintain control of the overshoots in temperature that are created by rapid rates.  

The top temperature should be such as to achieve the result in 10 minutes to avoid problems that can occur with extended soaks at top temperature.

In the example of an evenly thick 19mm/0.75” piece a heat up full fuse schedule like this could be used:

• 150ºC/270ºF to 566ºC/1052ºF for 0 minutes
• 50C/90F to 643C/1191F for 30 minutes
• 333ºC/600ºF to 804ºC/1479ºF for 10 minutes

 

If a more cautious approach to the heat up is desired, this might be the kind of schedule used:

 

• 100ºC/180ºF to 260ºC/500ºF for 20 minutes
• 100ºC/180ºF to 566ºC/1052ºF for 0 minutes
• 50C/90F to 643ºC/1191ºF for 30 minutes
• 333ºC/600ºF to 804ºC/1479ºF for 10 minutes

This approach is applicable to all fusing glasses.

 

Adapting the Bullseye Annealing Chart

After writing the first part of the schedule, you can continue to apply the annealing information from the Bullseye chart.  The first part of the anneal cooling starts with dropping the temperature as fast as possible to the annealing temperature.

The method for making the chart applicable to the annealing is a matter of substitution of the temperature.  All the other temperatures and rates apply to all fusing glasses.

Use the annealing temperature from your source as the target annealing  temperature in place of the Bullseye one.  The annealing soak times are important to equalise the temperature within the glass to an acceptable level (ΔT=5ºC).  The annealing soak time is related to the calculated thickness of the piece.  This measurement is done in the same way as devising the appropriate rate for heat up. 

Applying the Cooing Rates

Then apply the rates and temperatures as given in the chart.  The three stage cooling is important.  The gradually increasing rates keep the temperature differentials within acceptable bounds with the most rapid and safe rates.

The temperatures and rates remain the same for all soda lime glasses – the range of glass currently used in fusing, including float glass.  The soak time for the calculated thickness of your glass piece will be the same as in the Bullseye chart.  

This means that the first cooling stage will be to 427ºC/800ºF.  The second stage will be from 427ºC/800ºF to 371ºC/700˚F.  And the final stage will be from 371ºC/700˚F to room temperature.

I will repeat, because it is so important, that the thickness to be used for the anneal soak and cooling rates for your schedule relates to the profile you desire.  A fuse with even thickness across the whole piece can use the times, temperatures, and rates as given in the chart as adapted for your glass.  The thicknesses to use are for:

Contour fusing - multiply the thickest part by 1.5. 

Tack fusing - multiply the thickest part by 2. 

Sharp tack or sinter - multiply the thickest part by 2.5.

An annealing cool schedule for 19mm/0.75" Oceanside glass is like this:

• AFAP to 510˚C/ 951˚F for 3:00 hours
• 25˚C/45˚F to 427˚C/800˚F for 0 time
• 45˚C/81˚F to 371˚C/700˚F for 0 time
• 150˚C/270˚F to room temperature, off.

Many will wish to turn off the kiln as early as possible.  This is not part of best kilnforming practice.  If you still wish to do this, the turn off temperature must be related to the thickness and nature of the piece.  To turn off safely, you need to know the cooling characteristics of your kiln.  This can be determined by observing the temperature against time and then calculating the kiln’s natural cooling rate. And then applying that information to cooling the kiln.

 

The best source for devising schedules is the Bullseye chart for Annealing Thick Slabs.  It is well researched and is applicable with little work to develop appropriate schedules for all the fusing glasses currently in use.

 

 

Anneal and Cool Relationship

Annealing and cooling are directly related. You cannot extend the anneal soak without also slowing the cooling rates and expect to have a sound piece. What I am seeing on the internet groups about annealing breaks is comments saying the anneal soak is not long enough. So, people add time to the hold at the annealing temperature and still get breaks. They get breaks because the cooling rates are not slowed when the annealing soak time is extended.

A recording of an anneal soak and cool

If you need 3 hours anneal soak, you cannot cool at a rate of 83C°/150°F to 371C°/700°F. An anneal of 3 hours implies you are firing a piece of effectively* 19mm/0.75”. This needs a cool rate of :

• ·        25°C to 427°C. (45°F/hr to 800°F),
• ·        45°C/hour to 371°C (81°F/hr to 700°F),
• ·        150°C/hour (270°F) to room temperature.

Put the other way around, if you can use a first cool rate of 55°C (100°F)/hr you can use a two-hour soak at anneal. That means that you are firing a piece effectively* 12mm/0.5” thick.

But you cannot expect to maintain the required small temperature differential of 5°C/10°F (achieved at the anneal) with a single cool rate. Tests by Bullseye and confirmed by my own recorded tests show that a three-stage cooling is necessary to maintain that small difference of temperature throughout the cooling without using excessive firing times.

A two-hour soak requires cooling in three stages of:

• ·        55°C /100°F to 427°C/ 800°F
• ·        99°C/179°F to 371°C/700°F
• ·        330°C/595°F to room temperature.

If that small 5°C/10°F temperature differential is not maintained in the first stage cooling, temporary stresses can be induced.  Slightly higher levels of temperature differentials can be withstood during the next stages. The stresses induced by larger temperature differences can be great enough to break the glass. In many schedules published online by kilnformers, very long soaks are being used in relation the effective* thickness. But the cool soaks are too rapid in relation to the anneal hold to avoid inducing excessive (although temporary) stress.

This practice presumes the anneal soak is all there is to the production of a sound piece of glass. It is not. The cool rates from annealing to room temperature are important. To repeat, a long annealing soak with fast cool rates can lead to breaks - breaks that are not related to the annealing time. The cooling rates must be related to the amount of time needed for the anneal soak. A fast cool can induce temporary stresses that are great enough to break the glass. The appearance of the break will often be similar to an anneal break.

Don’t worry about using additional electricity with the slower rates of cooling. If the kiln cools more slowly than the scheduled rate, no power will be used. The relays will not have to operate.

Annealing times and cool rates are intimately related. And must be scheduled in relation to one another to avoid unnecessary breaks.

A more extensive discussion of this issue can be found in the ebook Low Temperature Kilnforming.

*”Effectively” in this context means a flat piece of the given dimension. The “thickness” of piece that is of uneven levels - as for a tack fuse - can be calculated to need firing as though it was a multiple of the actual total thickness. The multiple is based on the tack fusing profile.

Long Anneal Soaks

Credit: Bullseye https://www.youtube.com/watch?v=-8hK9Klprvc

Long anneal times seem to be becoming popular. At least they are being recommended frequently by people in Facebook groups. They are recommending very long anneals to solve stress and breakage problems.

Are they effective?

This Bullseye video on some results from annealing research shows (at 13:00 minutes) that excessively long soaks can cause more stress than the recommended length does. The video shows a 1” slab annealed for 4 hours (the recommended time) has less stress than one annealed for 16 hours.

The thoughts are that this effect results from the cooler space under the shelf and glass than the top during long soaks. This induces temperature differences between the bottom and the top of the glass, if not across the surface. The recordings show that during the anneal soak the temperature at the bottom of the kiln is less than above the glass.  This difference on a long anneal soak is larger than the ΔT=5˚C required for a good anneal.

The remedy shown by the video is to introduce heating elements under the shelf, which are separately controlled. This is impractical and is not needed in smaller kilns. The solution for these smaller kilns is to use slower and graduated cooling rates from the end of the annealing soak – not longer annealing soaks. 

The slower rate can be selected from the table. Whether you choose the rates for one or two layers thicker, relates to your perception of risk. Do not extend the length of the annealing soak when you use the slower cooling rates.

This cooling process has been researched by Bullseye and is effective to keep the glass within the temperature distribution requirements. It is a three-stage process. Stage one is to 427˚C/800˚F. The rate for the actual or calculated thickness is given in the Bullseye table (see below).

Stage two is to 371˚C/700˚F. This is normally 1.8 times the rate of the first cool.

Stage three is the cooling from 371˚C/700˚F to room temperature. This can be up to 6 times the first stage cooling rate. However, I find that a final cooling rate of 330˚C/600˚F is faster than most kilns can achieve.

I do programme this final cooling into the schedule. 

• It does not use more electricity unless the kiln is cooling faster than programmed. 
• It does not cause the relays it click in and out if the cooling rate of the kiln is slower than programmed. 
• It does protect the glass from too rapid cooling, or peeking. This is so especially from 100˚C/212˚F, when we are inclined to want the glass to cool faster than the closed kiln allows. 
• The sound of the relays operating indicates the kiln is open too much for the safety of the glass.

 

The Bullseye table Annealing Thick Slabs shows the recommended soak times and cool rates for each cooling stage. It is applicable to all fusing glasses. After annealing for the appropriate time at the temperature for your glass, use the rates and temperatures from this table.

Kilnforming with 3mm Glass

 A power point presentation I made a few months ago to the group Lunch with a Glass Artist.

It is 33 slides long.

Kilnforming with 3mm Glass.pptx

Fixing a Broken Piece

This conversation is reproduced by permission (with some editing out of extraneous information). It is presented as an example of how conducting a critique of your schedule can have dramatic effects on the results of your firing. 

This is the piece as it came out of the kiln.

Picture credit: Ike Garson

You may have seen the photo I posted of a large copper blue streaky piece that has cracked right across. …  I’m wondering if it would be better trying to bring the 2 pieces together instead of opening up the 2 pieces and inserting frit. I was thinking of firing it with a tack or contour schedule.

This is the crack that developed later through the frit and single layer centre.

Picture credit: Ike Garson

I have 4 questions:

A.   Even if I manage to fix it, do you think that fissure line will always be too weak and liable to break off at any point?

The strength of the joint will be dependent on the firing conditions.  To make it strong, the temperature should go to full fuse.  Tack fusing will leave the joint more visible and weaker.  To stop the joint rounding during heat up, you will need to dam the piece tightly to stop the normal expansion of the glass and ensure the glass is forced together during the higher temperatures.

B.     I have some large pieces of clear confetti. Would it benefit using them to bridge the 2 sections from below?

Anything you put on the bottom will have distinct outlines and visibility.  The temperature on the bottom can be 10C or more different from the top surface, which is why you can get crisp lines with the flip and fire technique.

C.    Would clear powder hide the crack or would it always be visible after firing?

Any additions to the top may be less visible, but adding clear powder makes the join more obvious.  You need to use powder of the same colour as the sheet glass.  Since you are using a streaky glass, you can’t use coloured power either as it is very difficult to imitate the steaks even with powders of the same colours. 

More information was given indicating the first contour fuse schedule in Celsius:

1. 260 730 00.20
2. FULL 515 00.60
3. 260 150 End

This is the contour schedule I have used many times successfully but never for a piece during this week.

My critique of the schedule. 

Segment 1.

• ·    It is too fast for the small distance to the side of the kiln. 
• ·    It is too fast for a piece of varying thicknesses. Most expansion breaks occur above 300˚C, so a soak at ca.260˚C will help ensure the glass maintains an even temperature, especially with large differences in thickness. Then you can advance more quickly. 
• ·   There is no bubble squeeze.
• ·   The top temperature seems low for a good tack, or the soak is a bit short.  Long soaks allow the glass molecules to bind at the atomic level firmly. This is the principle used in pate de verre.
• ·   It definitely needs to be on fibre paper covered with thinfire to allow air out.

Segment 2.

• ·   The soak at 515˚C is better done at 482˚C for Bullseye.
• ·   My tests have shown that contour firing a piece like this at rates and holds for 1.5 times the height of the piece is necessary for good results.

Segment 3.

• ·   Also, my tests have shown that a three-stage cooling provides the best result.  Slow cooling keeps the glass within the 5°C difference required for avoiding stress.
• ·   Annealing at the bottom end of the range combined with an appropriate length of soak and slow cooling gives a denser glass than soaking at the middle of the annealing range. 
• ·   The best cooling comes from a three-stage cooling process.  This involves a slow rate for the first 55C, a rate of 1.8 times this for the second 55C, and a rate of 3 times this for the final cool to room temperature.

These points mean that I would recommend you fire for at least 10mm thick.  This recommendation is for a new piece, not a repair. In this repair case and for the conditions, I would choose 12mm as being more cautious. My schedule would look something like:

1. 120˚C to 260˚C, 20’
2. 300˚C to top temperature, 10’
3. Full to 482˚C, 120’
4. 20˚C to 427˚C,0’
5. 36˚C to 370˚C, 0’
6. 120˚C to room temperature, off

The anneal soak is for a piece 12mm thick.  The cool rates are for 21mm thick.  This is to compensate for the nearness of the glass to the edge of the kiln.  It will help to ensure the glass does not have excess stress locked into the piece during the cooling.

D. Do you think this schedule would work [for a repair]? It's adapted from a standard tack schedule.. 

1. 222 677 00.30
2. 222 515 00.40
3. FULL 482 01.30
4. 63 371 ENDS

Critique of the schedule.

Segment 1. 

• ·   Too fast given earlier difficulties. 
• ·   Too low for good adhesion unless you use about 10 hours soak. 
• ·   Even at sintering temperature (690°C) you would need 2 hours.  But at sintering temperature you do not alter the surface 

Segment 2. 

• ·   Too slow a cool from top temperature and risks devitrification. Should be FULL.
• ·   You do not need the soak at 515˚C.  It only delays the annealing process.  It seems this idea of soaking at the upper portion of the annealing range was introduced by Spectrum over 2 decades ago. 
• ·   Any advantage that might be achieved by the higher soak is cancelled by the FULL rate to the annealing soak. 
• ·   Go straight to the anneal soak. 

Segment 3. 

• ·   You need a more controlled 3 stage cooling to get the best result.

My schedule for repair would look something like this:

1. 120˚C to 540˚C, 10’
2. 300˚C to 780˚C, 10’
3. Full to 482˚C, 210’
4. 20˚C to 427˚C,0’
5. 36˚C to 370˚C, 0’
6. 120˚C to room temperature, off

I am making the assumption that 780˚C is full fuse in your kiln.  Anything less than full fuse will certainly show the crack.

 

A Look at Causes.

• ·  The piece is far enough away from the elements.  It is not on the floor. These are not the causes.
• ·  It is very near the sides of the kiln.  These are always cooler than the centre. There is always a risk of breaking in this case.  Very slow rates are needed. 
• ·  There is a 3.5 times difference in thickness within the piece. This also requires slow rates.
• ·  If the break were to have been on the heat up these elements of uneven heating, and rapid rates are a problem.  But the break occurred after the cool down. So, the annealing soak and cool is a problem. 
• ·  I have suggested some alterations to the schedules to address these things.

 

Fixing for Yourself

• ·   Dam it tightly to avoid expansion within the glass as it heats.  This holds the join together and causes the glass to gain a little height. 
• ·   Place the piece on 1mm or thicker fibre paper topped with thinfire.  This will help avoid a bubble forming in the clear.
• ·   I have suggested a schedule which is slower to ensure no further breaks.  It is slow to the strain point and fast after that. 
• ·   It needs to be a full fuse to fully join the two pieces and ensure it is sound.
• ·   The cool to annealing should be FULL.  Eliminate the soak in the upper annealing range. The effects of the time spent there is nullified by the rapid rate to the main annealing soak. 
• ·   Anneal as for 12mm, but with slower cool rates (for 21mm) to ensure there are no stresses built into the piece by the nearness of the glass to the edge of the kiln.
• ·   These methods and schedules will make it a strong whole.  But the join will still show on the bottom. 
• ·   After fixing, if you are still not satisfied, break it up for incorporation in other projects.

Finally, and unfortunately, I do not think it can be satisfactorily repaired for a client.  The crack will show on the back. You will know it is a repair, rather than a whole. And that will reflect on your feeling about the piece, and possibly your reputation.

 

Conclusion

The commission was successfully re-made from scratch by the artist using some of my suggestions on scheduling. This is the resulting piece.

 

Picture credit: Ike Garson

 

Careful analysis of the conditions around a break are important to making a successful piece in the future. Many factors were considered, but the focus became the schedule. Analysis of each step of the schedule led to changes that resulted in a successful piece with the original vision and new materials.