Slumping Splits

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

Credit: Maureen Nolan

Observe the piece.

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

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

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

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

Threads and particles of glass are connecting across the split. 

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

History of the Piece

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

The schedule in essence was:

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

83ºC/150ºF to 688ºC/1270ºF for 10’

222ºC/400ºF to 516ºC /960ºF for 60’

111ºC/200ºF to 427ºC/800ºF for 10’

167ºC/300ºF to 38ºC/100ºF, off

 

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

 

Diagnosis

Too fast

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

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

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

20ºC/36ºF to 427ºC/800ºF, 0

36ºC/65ºF to 371ºC/700ºF,0

120ºC/216ºF to room temperature

 
Commentary on the proposed schedule:

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

 

Suspect Tack Fuse

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

 

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

 

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

 

Slump Split

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

 

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

 

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

 

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

 

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

 

Rates

 

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

 

Conclusion

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

 

Remedy

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

 

 

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

Bullseye

and

Etsy

Devitrification

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

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

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

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

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

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

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

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

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

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

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

[entry revised 28.12.24]

Sample Tiles

credit: Tia Murphy

There are advocates for making tiles as references for future work.  

• They show the profiles achieved at different temperatures.  
• They can be stored for easy visual reference when planning a firing.  
• It is a useful practice for any kiln new to the user.  

These tiles are assembled in identical ways to enable comparisons.  They should include black and white, iridised pieces- up and down, transparent and opal, and optionally stringers, confetti, millefiori, frit and enamels.  

The tiles are fired at different top temperatures with the same heat up schedule with the top temperature of each at about 10C or 20F intervals.  These show what effect different temperatures give.  Start the temperature intervals at about 720C or 1330F.

This is a good practice, even if time consuming.  It gets you familiar with your kiln and its operation.  It gives a reference for the profiles that are achieved with different temperatures at the rates used.

Ramp rate and time

But, as with many things in kilnforming, it is a little more complicated.  The effect you achieve is affected by rate and time used as well as the temperature.

The firing rate is almost as important as the temperature.  

• A slow rate to the same top temperature will give a different result than a fast rate.  
• The amount of heat work put into the glass will affect the temperature required.  
• Slow rates increase the time available for the glass to absorb the heat.  
• Glass absorbs heat slowly, so the longer the time used by slower rates, the rounder the profile will be.

Since time is a significant factor in achieving a given profile, any soaks/holds in the schedule will affect the profile at a set temperature.  A schedule without a bubble squeeze will give a different result than one with a bubble squeeze at the same temperature.

To help achieve knowledge of the rate/time effect, make some further test tiles.  Use different rates and soaks for the test tiles of the same nature as the first temperature tests. But vary only one of those factors at a time. Consider the results of these tests when writing the schedule for more complex or thicker layups. 

Mass

Also be aware that more mass takes longer to achieve the same profile.  Slower rates and longer times will help to achieve the desired profile at a lower temperature.  It is probably not practical to make a whole series of test tiles for thicker items.  But, a sample or two of different thicknesses and mass will be helpful to give a guide to the amount of adjustment required to achieve the desired outcome.

The results of sample tiles are due to more than just temperature.  They are a combination of rate, time, and temperature (and sometimes mass).  These factors need to be considered when devising or evaluating a schedule, because without considering those factors, it is not possible to accurately evaluate the relevance of a suggested top temperature.

See also: Low Temperature Kilnforming, available from Bullseye and Etsy

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.

Heat Up Soaks

Photo credit: Bullseye Glass Co.

It is often advocated that there should be a soak at the strain point to even out the temperature throughout the glass.

My question continues to be why? 

The glass has survived whatever rate has been used up to that point during its brittle phase.  So, it already has every chance of surviving a rapid rate during the plastic phase.

Instead of a soak at the strain point, Bob Leatherbarrow indicates a soak during the brittle phase will be more successful in avoiding heat up breaks.  He has observed that heat up breaks are most likely to happen around 260ºC/500ºF.  Therefore, a soak in that region is most likely to be of use in evenly distributing the heat effectively through the glass rather than at a higher temperature.  He recommends up to a half hour soak there before proceeding at the same rate to the strain point (about 540ºC/1004ºF).  The ramp rate to this heat up soak in the brittle phase should be related to the thickness of the glass and the intended profile.

The thickness to be fired for is determined by the profile.  Rates for full and contour fusing can be as for the thickness before firing.  Rounded tack fuse needs to be fired as though twice as thick, and sharp tack or laminated fuse need to be fired as though 2.5 times.  More information on initial ramp rates to the strain point can be found in Low Temperature Kilnforming available from Bullseye and from Etsy

Diagnosing Slump fractures

Once you have an initial idea of the source of the problem, think about it.  Test it against the evidence.  Is there enough evidence to make a call?  Make sure you have considered alternative explanations.  It is just too easy to make a snap decision about causes in low temperature processes.  The source of breaks in slumping are most often complex and stem from interrelated factors.

I give you an example of the difficulties of diagnosing a slumping break.

On a Facebook group a person showed the break of a single layer on a cyclone mould.  Others commented the same had happened to them.

Picture credit: Esther Mulvihill Pickens

Possible causes suggested on Facebook included:

• Thermal shock on the way up
• Thermal shock on the way down
• Too large on the mould and broke due to differential contraction
• Too many holds on the way up
• Too hot
• Too thin
• Follow the CPI programme
• Glass extending over the sides

Some of these suggestions were of general applicability, some in relation to the state of the broken glass.

The suggestions did not include:

• Cause of the rounded dots at the bottom of the mould.
• A cause for the state of the flat piece off the mould (it appears sharp edged.  Does it show some forming already?).
• The cause for the location of the fully formed remaining glass.
• The effect of the location of the mould and glass in the kiln.
• The consequences of a short soak at top temperature. 
• Is the kiln running hotter than most (1290ºF/698ºC for 10 minutes at top temperature was used)?

Of course, it is difficult to diagnose a problem from just one picture. It is difficult even with many pictures. And so, without handling the object, only suggestions can be made.

But….

You must spend enough time examining the piece with whatever other information is available to make specific suggestions.  The first thought may not consider all the factors.  Consider what kinds of causes there are for breaks during or after slumping.

More close inspection reveals the rounded edges of the break.  That supports the idea that the temperature was too high. It also supports the diagnosis that the break occurred on the heat up.

The edges of the piece that has fallen off the mould, and now rests on the shelf, seem to be square or sharp. This shows the extent of the difference of temperature between shelf and top of the mould – less than 100mm/4 inches.  Also, how small the differences in temperature are between slump and tack.  The extent of difference in fusing does depend on how high in the kiln the mould is placed.  That is demonstrated here by the different elevation of the two pieces. 

The conformation of the glass to the mould is complete.  This supports the diagnosis of the break occurring early in the firing, and certainly before the slump was complete.  These pieces will not fit together.  So, even if the edges were sharp the fact they will not fit together shows they conformed independently to the mould surface.  Therefore, the break was before forming temperature was reached.

The glass hangs over the mould edges on only three sides and at an angle.  This indicates the cause of the overhang was the break.  Not the reverse. An overhang at the beginning of the slump is likely to be even.

The piece on the floor of the kiln combined with the movement of the glass toward the back gives an indication that the origin of the break is at the front.  This relates to uneven temperatures and to the placement of the mould.

No one mentioned the placement of the mould and glass at the back of the kiln.  This will have an effect on scheduling.  The mould and glass are very large in relation to the kiln.  There is little space between the glass on the mould and the walls of the kiln.  Also, the mould is placed asymmetrically in the kiln – very close on three sides.  This will cause uneven heating in any kiln.  To have a successful firing of glass on this mould in this kiln will require radically different schedules to that for a centrally placed mould that is moderate for the size of the kiln.

The large size (relative to the kiln) and the asymmetrical placing are the causes of the break, in my opinion.  I admit that it took me several looks to realise the placement was a key cause of the break.

So, the generalised comments about thermal shock are correct, but not as to the cause of that shock.  The kiln will be hotter in the central part and cooler at the corners.  This is true of all rectangular kilns.  The important thing is to learn how to cope with these temperature differences.

Slow firings to low temperatures with long soaks are the three important elements.  These make up the heat work of the kiln. Applying this to a schedule means:

• slow ramp up rates – as little as one half the recommended rates for centrally placed moulds that are moderately sized in relation to the kiln.
• Low temperatures present lesser risks to the control of the outcome of the firing.  Determining the lower temperature possible requires peeking into the kiln to monitor the progress of the firing.
• Long soaks combined with low temperatures get the kilnforming done with minimal marking of the underside.  Low temperature soaks - in excess of 30 minutes - are required to minimise the marking.  Observation of the slump will be necessary to determine when it is complete.

My suggestions for the causes of other elements are:

·        Cause of the rounded dots at the bottom of the mould.

The temperature was too high. 698ºC/1290ºF is much hotter than needed for a slump. It was hot enough to round edges and small shards of glass.  Which shows excessive heat was received by the glass.

·        A cause for the state of the flat piece off the mould (it appears sharp edged. Does it show some forming already?)

The soak of 10 minutes was too short for the temperature in the kiln to equalise from top to bottom.  The glass on the shelf may not have reached 650ºC/1200ºF with such a short soak.

·        The cause for the location of the fully formed remaining glass.

The glass broke and was forced apart by the size of the expansion differences within the glass.  The movement of a piece at the front of the mould combined with the rearward and side movement of the glass indicate the origin of the break was at the front.  The distance apart shows the amount of force, and so the degree of reduction in the ramp rate required to fire this successfully.

·        The effect of the location of the mould and glass in the back of the kiln has already been discussed.

·         The consequences of a short soak at top temperature.

A high temperature is often considered necessary to pick up all the detail in moulds, whether slump or texture moulds.  The same effect can be achieved at lower temperatures with longer soaks.  The results of this strategy are fewer mould marks on the bottom of the work.

·        Is the kiln running hotter than most (Used 1290F/698C for 10 minutes at top temperature)?

This is one that cannot be answered other than by experiments carried out by the owner of the kiln.  Look at the Bullseye Tech Note #1 Knowing your Kiln for methods of testing temperatures. 

In short:

Diagnosis of slumping breaks is more complex than it appears at first.

More information is available in the eBook Low Temperature Kilnforming, an Evidence Based Approach to Scheduling.

This is available from Bullseye or 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

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.