Observations on Some Suggestions about Annealing

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

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

Annealing temperatures

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

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

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

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

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

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

·        “CoE 83”:

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

o   typical USA float - anneal at 548°C;

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

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

Temperature equalisation soak

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

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

6mm /   1/4"            60 minutes

[9mm /  3/8"           90 minutes]

12mm  / 1/2"          120 minutes

[15mm  /   5/8"       150 minutes]

19mm   / 3/4"         180 minutes

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

Anneal Cooling

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

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

Cooling Rate

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

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

Even thickness

                                         Cooling rate

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

6mm              60                 83°C       150°C

9mm              90                 69°C       125°C

12mm            120                55°C       99°C

15mm            150                37°C       63°C

19mm            180                25°C       45°C

                                        Cooling rate

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

0.25"              60                 150°F       270°F

0.375"            90                 124°F       225°F

0.5"               120                100°F       178°F

0.675"           150                67°F         114°F

0.75"             180                45°F         81°F

Tack fused/ uneven thickness

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

Tack fused

Dimension (mm)                                Cooling rate

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

6            12                 120                55°C       99°C

9            18                 150                25°C       45°C

12          25                 180                15°C       27°C

15          30                 300                9°C         18°C

18          38                 360                6.7°C       12°C

Dimension (inches)                                Cooling rate

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

0.25          0.5                 120                100°F       180°F

0.375        0.75               150                45°F         81°F

0.5            1.0                180                27°F          497°F

0.675        1.25               300                16°F         36°F

0.75          1.5                360                12°F          22°F

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

Fusing on the floor of the kiln

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

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

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

Source for the annealing and cooling of fused glass

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

Annealing over multiple firings

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

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

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

Determining the annealing point of unknown glass

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

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

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

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

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

Revised 14.10.25

Annealing a Stressed Piece

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

A Sintering Project

Ready for firing

The project is to fire 6mm “balls” stacked 3 high onto a single sheet of clear glass without significant alteration to the base sheet or to the stacked balls. This creates a total thickness of 21mm. The proposal is to sinter the whole in one firing.

Scheduling for a sinter firing needs to be done as though 2.5 times the thickest part – in this case 52mm, or 2 inches

It is slightly more risky to do this in two firings, than one, in my opinion. A suggested schedule for sintering frit using Bullseye was:

• 100ºC /180ºF — 482ºC /900ºF, 60'  =5.8 hrs

• 40ºC /72ºF — 593ºC/1100ºF,10'      =2.8

• 20ºC /36ºF — 665ºC /1230ºF,30     =4.1

• Skip to anneal temperature, soak for 6 hours =6.5

• 6.7ºC /12ºF — 427ºC /800ºF,0'       =8.2

• 12ºC /22ºF — 371ºC /700ºF,0'        =4.7

• 40ºC /72ºF — room temperature,0’ =8.8

• Off                       =40.9 hours total or 1.7 days

This was annealing as for 38mm/1.5 inches thick. Annealing for 50mm/2” thick would need about 112 hours or 4.6 days.

However this schedule was not successful – the pieces were only lightly stuck together. Thinking about why, led to the proposal that the soak time and temperature were not long or high enough to give adhesion between the pieces.

A second attempt used a faster ramp rates to higher temperatures.

• 200°C /360°F – 540°C /1004ºF, 30’ =3.2 hrs

• 60°C /108°F -625°C /1157ºF, 30’     =1.92

• 30ºC /54ºF - 685ºC /1265ºF, 120’    =4.0

• skip to anneal temperature and soak/hold for 4 hours (as for 25mm/1”)

• 15ºC /27ºF – 427ºC /800ºF, 0’        =3.67

• 27ºC /49ºF – 370ºC /700ºF, 0’        =2.11

• 90ºC /162ºF – 50ºC /122ºF, 0’       =3.56

• Off

•  = a minimum total of 18.5 hours plus natural cooling of the kiln

This schedule used a:

• faster first ramp to a higher (540ºC /1004ºF) first soak

• a faster (60ºC /108ºF, which is 150% of the previous) rate to the lower slump temperature (625ºC /1157ºF)

• the same relative reduction (50%) in rate to a higher temperature (685ºC /1265ºF)

• a shorter (120’) anneal soak

• and consequently faster cooling rates, which showed no stress after firing

The whole structure held together and was sound. There was no apparent change in the size of the individual 6mm balls.

This difference in scheduling is an illustration of how time and temperature can be interchanged.

It also shows that size matters when sintering pieces together. Higher temperatures and more time are required for dots and balls than for frit.

More information is available in my e-book Low Temperature Kilnforming, available from Bullseye, Etsy and stephen.richard43@gmail.com

Elevation of Moulds

Is it necessary to elevate slumping moulds above the shelf? 

I first heard of the need to elevate moulds from a Bhole representative about 2007. I ignored it, but didn't get around to testing until working on my e-book Low Temperature Kilnforming.

That work showed there is a larger difference in air temperature above and below the unsupported mould than the supported one. But that difference is much smaller than between the air temperature and the glass.

At 150°C/270°F per hour the maximum difference in the temperature under the mould between the elevated and on-the-shelf mould at top temperature was 41°C/74°F while the air temperature difference was 126°C/227°F higher than under the elevated mould.  Many of the tests showed less difference than the maximums given here.

By reducing the ramp rate from 150°C/270°F per hour to 120°C/216°F, the under mould to above mould differential was reduced by a quarter. I didn't test beyond that. But it would appear that slower rates of 100°C/180°F and less will reduce that differential.

The graph also shows that there is a large difference between what the pyrometer reads than the mould temperature of the slump. Slower ramp rates produce an air temperature much closer to the mould temperatures.

Shortly into the rapid cool towards anneal soak and cool only minor temperature difference showed between elevated and on-the-shelf moulds throughout the anneal soak and anneal cool.

These details make it clear to me that elevating moulds is completely unnecessary with slow ramp rates. This of course, fits with the low and slow mantra that many of us promote. However elevating the mould will not harm the slump.

One caution, though. Damp. Wet, or heavy moulds must be supported to avoid breaking the shelf. So I advocate placing these moulds on the floor of the kiln with 2cm posts, rather than on the shelf. I don't know if it is necessary. I haven't tested it. But I do know that moulds in this condition will break the shelf without significant separation between the two.

Low Temperature Kilnforming e-book is available from Bullseye  and Etsy and is applicable to all fusing glasses.

Draping over steep moulds

 Draping over a narrow or small supporting ridge with large areas of glass is difficult.

One solution might be just to invert the whole piece and let the glass slide down into the mould. However, there rarely is enough height in a glass kiln for deep slumps, especially with a “V” shaped mould. It has to be high enough for the edges of the glass to be supported at its edges. You could also approach this by having a first mould with a shallower angle or broader support at its centre. Drape over this first, then use the steeper mould as the second draping mould. This makes the balance less critical.

The idea of supporting the glass is the key to doing this kind of slump that seems to require an impossible balancing act, if it is to be done in one go. Place kiln washed kiln furniture at the edges of the otherwise unsupported glass. Fire the kiln, but watch until the glass begins to slump. Then reach in with a wet stick and knock the kiln furniture aside to allow the glass to continue its slump and conform to the mould shape.

The lower temperature you use to do the draping and the slower your rate of increase is, the less the glass will be less marked by the mould. Frequent brief visual inspection during the drape is vital.

Also have a look at a suggestion for the kind of firing required for this here.

Draping over steep moulds

 Draping over a narrow or small supporting ridge with large areas of glass is difficult.

One solution might be just to invert the whole piece and let the glass slide down into the mould. However, there rarely is enough height in a glass kiln for deep slumps, especially with a “V” shaped mould. It has to be high enough for the edges of the glass to be supported at its edges. You could also approach this by having a first mould with a shallower angle or broader support at its centre. Drape over this first, then use the steeper mould as the second draping mould. This makes the balance less critical.

The idea of supporting the glass is the key to doing this kind of slump that seems to require an impossible balancing act, if it is to be done in one go. Place kiln washed kiln furniture at the edges of the otherwise unsupported glass. Fire the kiln, but watch until the glass begins to slump. Then reach in with a wet stick and knock the kiln furniture aside to allow the glass to continue its slump and conform to the mould shape.

The lower temperature you use to do the draping and the slower your rate of increase is, the less the glass will be less marked by the mould. Frequent brief visual inspection during the drape is vital.

Also have a look at a suggestion for the kind of firing required for this here.

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.