Rapid Ramp Rates with Soaks

I have seen many schedules with initial rates of advance interrupted by soaks.  These kinds of schedules that are written something like this:

250°C/450°F to 200°C/482°F, soak for 10 (or 20 or 30) minutes

250°C/450°F to 500°C/933°F, soak for 10 (or 20 or 30) minutes

300°C/540°F to 595°C/1100°F, soak for 10 (or 20 or 30) minutes

300°C/540°F to 677°C/1250°F, soak for 10 (or 20 or 30) minutes

330°C/600°F to working temperature (1450°, 1500° etc.)

When I have asked, I’m usually told that these are catch up pauses to allow all the glass to have an even temperature.  There are occasions when that may be a good idea, but I will come to those later.  For normal fusing, draping and slumping these soaks are not needed.

To understand why, needs a little information on the characteristics of glass.  Glass is a good insulator, and therefore a poor transmitter of heat.  Glass behaves better with a moderate steady input of heat to ensure it is distributed evenly throughout the glass.  To advance the temperature quickly during the initial heat up stages where the glass is brittle risks thermal shock. 

The soaks at intervals do not protect against a too rapid increase in temperature.  It is the rate of heat input that causes thermal shock.  Rapid heat inputs cause uneven temperatures through and across the glass.  When these temperatures are more than 5°C different across the glass, stress is not relieved.  As the temperature differential increases, so does the stress until the glass is not strong enough to contain those stresses and breaks.  At higher temperatures these stresses do not exist as the glass is less viscous.

If, as is common and illustrated in the schedule above, you advance at the same rate on both sides of the soak, the soak really does not serve any purpose – other than to make writing schedules more complicated.  If the glass survived the rate of heat input between the soaks, it will survive without the soaks.

But you may wish to be a little more careful. The same heating effect can be achieved by slowing the rate of advance.  Just consider the time used in the soak and then slow the rate by the appropriate amount.  Take the example above using 30-minute soaks:

250°C/450°F to 200°C/482°F, soak for 30 minutes

250°C/450°F to 500°C/933°F, soak for 30 minutes

This part of the schedule will take three hours.  You can achieve the same heat work by going at 167°C/300°F per hour to 500°C/933°F.  This will add the heat to the glass in a steady manner and the result will be rather like the hare and tortoise.  If you have to pause periodically because you have gone too quickly, you can reach the same end point by steady but slower input of heat without the pauses.

But, you may argue, “the periodic soaks on the way up have always worked for me.”  As you work with thicker than 6mm glass, this “quick heat, soak; quick heat, soak” cycle will not continue to work.  Each layer insulates the lower layer from the heat above.  As the number of layers increase, the greater the risk of thermal shock. Enough time needs to be given for the heat to gradually penetrate from the top to the bottom layer and across the whole area in a steady manner.

To be safest in the initial rate of advance, you should put heat into the glass in a moderate, controlled fashion.  This means a steady input of heat with no quick changes in temperature.  How do you calculate that rate?  Contrary as it may seem, start by writing out your cooling phases of the schedule.  The cooling rate to room temperature is the safe cooling rate for the final and now thicker piece.  If that final cool rate is 300°C/540°F, the appropriate heat up rate is one third of that or 100°C/180°F. 

This “one third speed” rate of advance will allow the heat to penetrate the layers in an even manner during the brittle phase of the glass.  This rate needs to be maintained until the upper end of the annealing range is passed.  This is normally around 55°C/100°F above the annealing point.

Then you can begin to write the rate of advance portion of your schedule.  It could be something like:

100°C/180°F to 540°C, no soak

225°C/405°F to bubble squeeze, soak

330°C/600°F to working temperature, soak 10 minutes

Proceed to cool segments 

I like simple schedules, so I normally stick to one rate of advance all the way to the bubble squeeze.  This could be at the softening point of the glass or start at 50°C below with a one hour rise to the softening point with a 30-minute soak there before proceeding more quickly to the working temperature.

Exceptions.

I did say I would come back to an exception about soaks on the first ramp rates  segment of the schedules.  When the glass is supported – usually in a drape – with a lot of the glass unsupported you do need to have soaks.  The kind of suspension is when draping over a cylinder or doing a handkerchief drop.  This is where a small portion of the glass is supported by a point or a long line while the rest of the glass is suspended in the air.  It also occurs when supported by steel or thick ceramic.

The soaks are not to equalise the temperature in the glass primarily.  They are to equalise the temperature between the supports and the glass.  A thick ceramic form supporting glass takes longer to heat up than the glass.  The steel of a cocktail shaker takes the heat away from the glass as it heats faster. 

The second element in this may already be obvious.  The glass in the air on a ceramic mould can heat faster than that on the mould.  The glass on a steel mould can heat faster over the steel than the suspended glass.  Both these cases mean that you need to be careful with the temperature rises.

Now, according to my arguments above, you should be able to slow the rate of advance enough to avoid breakage.  However, my experience has shown that periodic soaks in combination with gradual increases in the rates of advance are important, because it is more successful. 

An example of my rates of advance for 6mm glass supported on a steel cylinder is:

100°C/180°F to 100°C/212°F, soak 20 minutes

125°C/225°F to 200°C/392°F, soak 20 minutes

150°C/270°F to 400°C/753°F, soak 20 minutes

200°C/360°F to draping temperature

Call me inconsistent, but this has proved to be more effective than dramatically slowing the rates of advance.  This exception does not apply to slumps where the glass is supported all around by the edge of a circular or oval mould, or where it is supported at the corners of a rectangular or square one.

Another exception is where you have a lot of moisture in a mould, for example. You need to soak just under the boiling point of water to dry the mould or drive out water from other elements of your work before proceeding.  This also applies to situations where you need a burn out, of for example vegetable matter at around 500°C/933°F for several hours.

In both these cases, these are about the materials holding or contained in the glass, rather than the glass itself.

Revised 23.2.25

Tack Fusing Considerations

Initial Rate of Advance

Tack fuses look easier than full fusing, but they are really one of the most difficult types of kiln forming. Tack fusing requires much more care than full fusing.
On heat up, the pieces on top shade the heat from the base glass leading to uneven heating. So you need a slower heat up. You can get some assistance in determining this by looking at what the annealing cool rate for the piece is. A very conservative approach is needed when you have a number of pieces stacked over the base layer.  One way of thinking about this is to set your initial rate of advance at approximately twice the anneal cool rate. 

Annealing 

The tacked glass us loosely attached rather than fully formed together.  So, the glass pieces are still able, partially, to act as separate entities, meaning excellent annealing is required.

Effects of thicknesses, shapes, degree of tack

1. Tack fusing of a single additional layer on a six millimetre base

1. Rectangular pieces to be tack fused

1. Sharp, pointed pieces to be tack fused

1. Multiple layers to be tack fused

1. Degree of tack – the closer to lamination, the more time required

Glass contracts when it's cooling, and so tends to pull into itself. In a flat, symmetrical fuse this isn't much of a problem. In tack fuses where the glass components are still distinct from their neighbours, they will try to shrink into themselves and away from each other.  If there is not enough time for the glass to settle into balance, a lot of stress will be locked into the piece that either cause it to crack on cool down or to be remarkably fragile after firing.  In tack fusing there also are very uneven thicknesses, making it is hard to maintain equal temperatures across the glass.  The tack fused pieces shield the heat from the base, leading to localised hot spots during the cool down.

On difficult tack fuses it's not unusual to anneal for a thickness of two to three times greater than the thickest part of the glass.  That extended cool helps ensure that the glass has time to shift and relax as it's becoming stiffer, and keeps the temperature more even throughout.

In general, tack fused pieces should be annealed as though they are thicker pieces. Recommendations range from the rate for glass that is one thickness greater to at least twice the maximum thickness of the whole item.  Where there are right angles - squares, rectangles - or more acutely angled shapes, even more time in the annealing cool is required.

It must be remembered, especially in tack fusing, that annealing is much more than the annealing soak.  The soak is to ensure all the glass is at the same temperature, but it is the anneal cool that ensures the different thicknesses will all react together. That means tack fusing takes a lot longer than regular fusing.

  

The more rectangular or pointed the pieces there are in the piece, the greater the care in annealing is required.  Decisions on the schedule to use varies - some go up two or even four times the total thickness of the piece to choose a firing schedule.

A simple way to determine the schedule is to subtract the difference between the thickest and the thinnest part of the piece and add that number to the thickest part. If you have a 3mm section and a 12mm section, the difference is 9mm. So, add 9 to 12 and get 17mm that needs to be annealed for. This thickness applies to the heat up segments too.

Another way to estimate the schedule required is to increase the length the annealing schedule for any and each of the following factors:

The annealing schedule to be considered is the one for at least the next step up in thickness for each of the factors. If you have all five factors the annealing schedule that should be used is one for at least 21mm thick pieces according to this way of thinking about the firing.

 

4 – Testing/Experimentation

The only way you will have certainty about which to schedule to choose is to make a mock-up of the configuration you intend in clear.  You can then check for the stresses.  If you have chosen twice the thickness, and stress is showing, you need to try 3 times the thickness, etc., which can be done on the same piece.  You can reduce time by having your annealing soak at the lower end of the annealing range (for Bullseye this is 482C, rather than 516C).

You will need to do some experimentation on what works best for you. You also need to have a pair of polarisation filters to help you with determining whether you have any stress in your piece or not. If your piece is to be in opaque glasses, The mock-up in clear will be useful.

First published 18.12.2013

Revised 29.01.25

Over Annealing

 I hear the comment "you can't over anneal" all the time. Is it true?

My response to this may be controversial, and I do expect there will be some dispute with aspects of what follows.  My view of the statement “you can’t over anneal” is that it results from a lazy approach to thinking about the process.

The short answer is, in my view “yes, you can over anneal”.

• ·         Lengthy anneal soaks can induce stress in certain circumstances. More later.
• ·         Excessive annealing soaks waste time and money.
• ·         Annealing is more than the soak.  It is a combination of equalisation of the heat within the glass (not just temperature) and the gradual cooling of the glass to below the lower strain point to ensure the glass does not incorporate differences of temperature of plus or minus 5°C.

There is both tradition and research to assist in determining the length of the anneal soak.  The tradition seems to embrace 30 minutes anneal soak for each 3mm-layer of glass. The research has been done by Bullseye and they have developed a table to assist in accurately determining annealing soaks for thick glass. 

It informs users of the relationship between thickness and annealing soaks and cooling.  The table starts at 6mm/0.25" thick, and goes up to 200mm/8" thick.  The annealing soak temperature used needs to be altered for glass other than Bullseye, but the soaks, rates, and temperatures remain valid for all fusing glasses. Use the research, rather than tradition.

Other considerations include the nature of the kiln.  If your kiln has significant temperature differentials across the shelf, long annealing soaks will incorporate those differences during the annealing cool and result in a stressed piece. You do know the temperature distribution within your kiln, don’t you?  This Tech Note #1 from Bullseye will give you the information to test for the temperature distribution. Using this information will enable you to avoid the cool spots when placing your pieces and utilise the areas where the heat is even.

Economy is another reason that it is possible to over anneal.  Soaking at the annealing temperature uses a significant proportion of the electricity consumed in a firing.  This means an overly long temperature equalisation soak will use more electricity than necessary.  It also uses more kiln time than necessary, by delaying the anneal cooling and the following natural cooling rate of the kiln.

It is possible to under anneal, of course.

You need to learn about the effects of your project on annealing requirements, because it is possible to under anneal.  The research on annealing is based glass of uniform thickness. The most popular style of kilnforming appears to be tack fusing of one degree or another.  This is unfortunate for the novice, as it is the most difficult of styles to anneal adequately. There are a lot of factors to consider when setting the annealing schedule for tack fusing. 

I feel this is the origin of “can’t over anneal” thinking.  Instead of thinking about the specific annealing difficulties, many seem to just add more time in a generally random manner.  The post on tack fusing considerations (the link above) is designed to help in thinking about the requirements of the lay-up of your piece. The cumulation of factors can easily triple the annealing soak time and slow the rates by three times. 

What is the anneal?

Another problem is that most often annealing is thought of as merely a soak at the annealing point of the glass.  It is much more than that.  The annealing point is usually the temperature at which the heat within the glass is equalised in preparation for the anneal cool.  This is because the annealing temperature is that at which the glass will most quickly anneal.  Since the anneal is temperature sensitive, the equalisation of the temperatures within the glass will be most successful at getting a good anneal throughout the cool.

For two-layer flat fused items, the annealing point can be used as the heat equalisation temperature.  The soak is to get the glass within 5°C/10°F throughout the piece.  The annealing, especially with thicker or more difficult pieces, is done closer to the lower strain point. The reasons for this is to save time in the annealing cool, it is easier to maintain the small difference in temperature, and it has been shown to produce a more dense (therefore stronger) glass.  If you look at the Bullseye annealing chart, you will see how slowly thick pieces need to be cooled, so starting 35°C/ below the annealing point can save many hours of cooling.

Once the glass has equalised in temperature, the object is to cool the glass at a rate that ensures the internal temperatures do not vary more than plus or minus 5°C/10°F across and through the piece.  The rate can increase by 1.8 times the initial cool rate after the lower strain point has been reached.  This second stage rate should take the glass to around 370°C/700°F, where the rate to room temperature can be doubled as much as six times the initial cool rate.

Difficult pieces

Tack fused and other pieces with uneven thicknesses require more care in the annealing to ensure even cooling of the whole without a greater variation in temperature than +/- 5°C.  As said above, tack fusing is one of the most difficult of styles to anneal adequately and the blog entry indicates some factors requiring more careful annealing.

As an example, a piece 6mm thick, with two layers of rectangular and pointed pieces that are just barely rounded.  This adds five factors of complications for the fusing - two levels of tack fusing, rectangular pieces, pointed pieces, laminated tack fusing.  This number of complications increases the practical thickness to 21mm – 6mm of flat base, 3mm each layer of tack (6mm), 3mm for rectangles, 3mm for pointed pieces, 3mm for laminated fuse.  Because this is tack fused, the next practical step up in the table needs to be used. That is the one for 25mm, which requires a four-hour temperature equalisation soak, and 15°C per hour initial anneal cool rate.

Of course, a simpler method can be used, as it has been researched and practiced by many people.  That is simply double the thickest part of the piece and use that thickness for the anneal soak and cool.  Sharp tack profiles need to be annealed as though 2.5 times thicker, and contour profiles only need 1.5 times the thickness.

Glass other than Bullseye

It is possible to apply these times and rates to any glass of which you know the annealing point.  The annealing soak can be set above the lower strain point, which to be safe, can be taken as a point 35°C/63°F below the annealing point.

E.g., if you are annealing a 12mm slab of float glass, the annealing point of which (in the UK) is 540°C, you chose a temperature of 505°C to do your two-hour soak, followed by a cool rate of 55°C/100°F to 427°C and then 99°C/178°F per hour for the second stage cool to 370°C/700°F.  The final cool of 330°C/595°F.  So, you can see the soak times, rates and target temperatures remain the same regardless of the glass type.  

More information on long annealing soaks in a video at 13:00 minutes.

More discussion on annealing and cooling is given in the ebook Low Temperature Kilnforming from Bullseye and Etsy.

Long Annealing Soaks

You Can’t Anneal Too Long.

Can you anneal too long?

Yes, you can.

It’s not just the possible temperature differences in the kiln.  If you have temperature differentials across your kiln, any piece that crosses those boundaries will have temperature differences locked into the glass.  If you know you have temperature differentials and your glass by circumstance must be in both the cooler and the hotter regions, you need to do a standard length of soak only.  Then reduce the rate of cooling a little more than normal, so that a slower cool occurs.  This should avoid most of the stress that can be induced by very long soaks in a kiln with hot and cool spots.

The other factor against annealing too long has been revealed by Bullseye research on annealing.  This video at about 13:00 minutes into the film explains.  This complicating factor in annealing is about the difference in temperatures of the surfaces of the glass.  The research shows that the longer you anneal the greater the differential in temperature becomes between the upper and lower surfaces of the glass.  This means that you can introduce stress across the whole piece, rather than just a section as in an unevenly heated kiln.

This comes from the recording of a typical long annealing cool.

What is more, the longer you soak, the cooler the bottom becomes in relation to the top.  The reported research is described in this video at about 13:00 minutes.  It can be assumed that the air temperature differences are the cause.  Even during cooling the air is hotter on top of the shelf than under.  This would allow the bottom surface to cool more than the top. This assumption is borne out by the fact that the effect is reduced or eliminated by having elements under the shelf.

There are two reasons to avoid long soaks. Uneven temperatures across the surface are locked into the glass.  And long soaks at annealing induce an unwanted temperature differential between the top and the bottom of the piece.

More information is available in the ebook Low Temperature Kilnforming; an evidence based approach to scheduling.

Revised 29.1.25

How Much Frit is Too Much

Scheduling for powder and frit.

 

Bullseye  pumpkin orange medium frit 00321.0002
Cedit: Bullsye Glass Company

“How much frit is too much for thickness calculation?”

There are differences between powder and frit effects on calculations for scheduling.

Powder needs to be about 2mm thick to provide strong colour, and will thin to 1mm or less during firing, so there is unlikely to be any significant effect for scheduling.

Fine frit sizes for Bullseye are between 0.2 and 1.2mm, so a single thickness layer will not affect the firing.  However, several layers thick over a portion of the area will make up to a 3mm layer and will need consideration in the scheduling.

Medium (Bullseye) frit is 1.2 to 2.7mm, So, a concentrated layer of medium and larger frits needs to be treated as an additional layer when they cover significant areas of the glass.

Scattered frits of any size with proportionate spaces between the frit will not need separate consideration in the scheduling.  Frits used to fill spaces between pieces of glass will have no effect on the scheduling either.

 

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

Heat Work

“Heat work” is a term applied to help understand how the glass reacts to various ways of applying of heat to the glass. In its simple form, it is the amount of heat the glass has absorbed during the kiln forming heat-up process.

There is an relationship between how heat is applied and the temperature required to achieve the wanted result.  Heat can be put into the glass quickly, but to achieve the desired result, it will need a higher temperature. If you put the heat into the glass more slowly, the reverse applies.

For example, you may be able to achieve your desired result at 816C/1500F with a 400C/hr (720F/hr) rise and 10min soak. But you can also achieve the same result by using 790C/1454F with a 250C/hr (450F/hr) rise and 10min soak. The same amount of heat has gone into the glass, as evidenced by the same result, but with different schedules. This can be important with thick glass, or with slumps where you want the minimum of mould marks. Of course, you can achieve the same results with the a rise and a longer soak at the lower temperature, e.g. a 400C/hr (720F/hr) to 790C with a 30 min soak, but you will have more marking and difficulty with sticking separators.

In short, this means that heat work is a combination of time and temperature.  The same effect can be achieved with: 
- fast rates of advance and high temperatures.
- slow rates of advance and low temperatures.

- long soaks at low temperatures.

You obtain greater control over the processes when firing at slower rates with lower temperatures.  There is less marking of the back of the piece.  There is less sticking of the separators to the back and so less cleanup.  There is less needling with the lower temperature.  More information on heat work is here.

The adage “slow and low” comes from this concept of heat work. The best results come from lower temperature processing, rather than fast processing of the kiln forming.

More information is available in the eBook Low Temperature Kilnforming available from Bullseye and Etsy.

Revised 1.1.25

Slump Point Test

At a time when we are all going to be trying a variety of glass of unknown compositions to reduce costs of kiln working, the knowledge of how to determine the slump point temperature (normally called the softening point in the glass manufacturing circles) and the approximate annealing temperature becomes more important.  The slump point test can be used to determine both the slumping point and the annealing soak temperature.  This was required when the manufacturers did not publish the information, and it continues to be useful for untested glasses.

The method requires the suspension at a defined height of a strip of glass, the inclusion of an annealing test, and the interruption of the schedule to enter the calculated annealing soak temperature.

A strip of 3 mm transparent glass is required.  This does not mean that it has to be clear, but remember that dark glass absorbs heat differently from clear or lightly tinted glass. The CoE characteristics given are normally those of the clear glass for the fusing line concerned.  The strip should be 305 mm x 25 mm.  

Suspend the strip 25 mm above the shelf, leaving a span of 275 mm. This can be done with kiln brick cut to size, kiln furniture, or a stack of fibre paper.   Make sure you coat any kiln furniture with kiln wash to keep the glass from sticking.

The 305mm strip suspended 25mm above the shelf with kiln furniture.

Place some kiln furniture on top of the glass where it is suspended to keep the strip from sliding off the support at each end. Place a piece of wire under the centre of this span to make observation of the point that the glass touches down to the shelf easier.

The strip held down by placing kiln furniture on top of the glass, anchoring it in place while the glass slumps.

If you are testing bottles, you may find it more difficult to get such a long strip.  My suggestion is that you cut a bottle on a tile saw to give you a 25 mm strip through the length of the bottle.  Do not worry about the curves, extra thickness, etc.  Put the strip in the kiln and take it to about 740C to flatten it. Reduce the temperature to about 520C to soak there for 20 minutes.  Then turn the kiln off.  

Also add a two layer stack of the transparent glass near the suspended strip of glass to act as a check on whether the annealing soak temperature is correct. This stack should be of two pieces about 100 mm square. If you are testing bottles, a flattened side will provide about the same thickness.  This process provides a check on the annealing temperature you choose to use.  If the calculated temperature is correct there should be little if any stress showing in the fired piece.

The completed test set up with an annealing test and wire set at the midpoint of the suspended glass to help with determining when the glass touches down.

The schedule will need to be a bit of guess work.  The reasons for the suggested temperatures are given after this sample initial schedule which needs to be modified during the firing.

In Celsius
Ramp 1: 200C per hour to 500C, no soak
Ramp 2: 50C per hour to 720C, no soak
Ramp 3: 300C per hour to 815C or 835C, 10 minute soak
Ramp 4: 9999 to 520C, 30 minute soak
Ramp 5: 80C per hour to 370C, no soak
Ramp 6: off.

In Fahrenheit

Ramp 1: 360F per hour to 932F, no soak
Ramp 2: 90F per hour to 1328F, no soak
Ramp 3: 540F per hour to 1500F or 1535FC, 10 minute soak
Ramp 4: 9999 to 968F, 30 minute soak
Ramp 5: 144F per hour to 700F, no soak
Ramp 6: off.

Fire at the moderate rate initially, and then at 50C/90Fper hour until the strip touches down. This is to be able to accurately record the touch down temperature.  If you fire quickly, the glass temperature will be much less than the air temperature that the pyrometer measures.  Firing slowly allows the glass to be nearly the same temperature as the air.  

Observe the progress of the firing frequently from 500C/932F onward.  If it is float or bottle glass you are testing you can start observing from about 580C. Record the temperature when the middle of the glass strip touches the shelf. The wire at the centre of the span will help you determine when the glass touches down.  This touch down temperature is the slump point of your glass.  You now know the temperature to use for gentle slumps with a half hour soak.  More angular slumps will require a higher temperature or much more time.

Once you have recorded the slump point temperature, you can skip to the next ramp (the fast ramp 3).  This is to proceed to a full fuse for soda lime glasses. Going beyond tack fusing temperatures is advisable, as tack fuses are much more difficult to anneal and so may give an inaccurate assessment of the annealing. Most glasses, except float, bottles and borosillicate will be fully fused by 815C. If it is float, bottles or borosilicate that you are testing, try 835C. If it is a lead bearing glass, lower temperatures than the soda lime glass should be used. In all these cases observation at the top temperature will tell you if you have reached the full fuse temperature. If not add more time or more heat to get the degree of fuse desired.

While the kiln is heating toward the top temperature you can do the arithmetic to determine the annealing point.  To do this, subtract 40C/72F from the recorded touch down temperature to obtain an approximate upper annealing point.  The annealing point will be 33C/60F below the upper point.  This is approximate as the touch down temperature is, by the nature of the observation. approximate.  

The next operation is to set this as the annealing soak temperature in the controller. This will be the point at which it usually possible to interrupt the schedule and change the temperature for the annealing soak that you guessed at previously. Sometimes though, you need to turn the controller off and reset the new program.  Most times the numbers from the last firing are retained, so that all you need to do is to change the annealing soak temperature.

The annealing soak should be for 60 minutes to ensure an adequate anneal. This may be excessive for 3 mm glass, but as the anneal test is for 6 mm, the longer soak is advisable. The annealing cool should be 83C/hr down to 370C. This is a moderate rate which will help to ensure the annealing is done properly. The kiln can be turned off at that temperature, as the cooling of the kiln will be slow enough to avoid any thermal shock to the annealing test piece.

When cooled, check the stack for stress. This is done by using two polarised light filters. See here for the method. 

Squares of glass showing different levels of stress from virtually none to severe
 (no light emanating for no stress to strong light from the corners indicating a high degree of stress.)

If the anneal test piece is stressed, there could be a number of reasons for the inadequate annealing. It could be that the glass has devitrified so much that it is not possible to fuse this glass at all. If you also test the suspended strip for stresses and there is very little or none, it is evidence that you can kiln form single layers of this glass. You now know the slumping temperature and a suitable annealing temperature and soak for it, even though fusing this glass is not going to be successful.

Other reasons for stress due to inadequate annealing could be that the observations or calculations were incorrect.  

• Of course, before doing any other work, you should check your arithmetic to ensure the calculations have been done correctly. I'm sure you did, but it is necessary to check.  If they are not accurate, all the following work will be fruitless.

• The observation of the touch down of the suspended strip can vary by quite a bit - maybe up to 15C.  To check this, you can put other annealing test pieces in the kiln.  This will require multiple firings using temperatures in a range from 10C/18F above to 10C/18F below your calculated annealing soak temperature to find an appropriate annealing soak temperature.

• If stress is still showing in the test pieces after all these tests, you can conduct a slump point test on a strip of glass for which there are known properties. This will show you the look of the glass that has just reached touch down point as you know it will happen at 73C above the published annealing point.  You can then apply this experience to a new observation of the test glass. 

Revised 30.12.24

Effects of Annealing at the Top End of the Range

It is possible to begin your annealing at any point in the annealing range.

The annealing point is the temperature at which the glass most quickly relieves the stress within.  This occurs at the glass transition point. 

The  annealing range is between the softening point and the strain point of the glass.  No annealing can be achieved above the softening point, nor below the strain point.  This range, for practical purposes can be taken to be 55°C above and below the published annealing point.  For thick slabs, Bullseye has chosen to start the anneal 34°C below the published annealing point of 516°C.

High Annealing Point

A high annealing temperature, even up to 571°C, the approximate strain point of the glass could have been chosen, but this is impractical.  The effect of this is a greater slow cool range and so an extended anneal cool.  The reasons are as follows:  

• The anneal cool range is greater as the first rate of cool needs to be maintained to the strain point.
• The anneal cool has to extend to at least just below the strain point.
• The highest practical annealing temperature is determined by the viscosity of the glass.  Any soaks above that temperature are ineffective in production of soundly annealed glass.
• The purpose is to get all the glass at the same temperature in preparation for  cooling.  It is more difficult to maintain the small differentials in temperature achieved by the annealing soak over a large range of temperature.

Low Annealing Point

Starting the anneal cool closer to the strain point requires a slightly longer soak to ensure the glass is all at the same temperature (+/- 2.5°C, often called the Delta T=5C) before the anneal cool begins.  Typically, this initial soak would be for an hour before the initial cool begins (for a 6mm/0.25" thick piece).

Effect of the Differences in Approach

The advantages and disadvantages centre around the need to:

• soak long enough to get all the glass to the same temperature, and to 
• cool slowly enough to maintain the delta T throughout the glass.

Example

If you think of an example of a piece of Bullseye glass 12mm/0.5" thick, it will show the differences in approach.

High temperature soak

A soak of 120 minutes at 571°C/1060°F (the highest possible start for an annealing soak) is still required to even the temperature.  To ensure the temperature differentials in the glass do not deviate from the Delta T, the cool needs to be at 18°C/32°F per hour down to 427°C/800°F.  It is possible then to increase the speed to 36°C/65°F per hour down to 370°C/700°F.  This gives you a total annealing cool of just over 11.5 hours.

Low temperature soak

Starting the anneal at 482°C still requires a two hour soak followed by a decrease in temperature of 18°C/32°F per hour to 427°C, and an increased rate of 36°C/65°F to 370°C/700°F.  This gives an anneal cool time of just over 6.6 hours.

The example shows how, although the annealing result may be the same, there is considerable time saved (and especially for thicker pieces) in using the lower part of the annealing range to begin the annealing.  It also will save some electricity.

However, an anneal of two hours at 516°C with a cool of 18°C/32°F per hour to 427°C/800°F and 36°C/65°F to 370°C/700°F will still give a perfectly adequate anneal for 12mm thick pieces even though it will take about 2 hours longer.

Revised 30.12.24